Repository: lark-parser/lark
Pull request #926 (2021-06-28 18:55:31)

Merge .pyi files
I started merging the stub files into the code. Here are a couple things I noticed so far:

- In `lexer.py` the signatures of some methods of `TraditionalLexer` and `ContextualLexer` do not match the signatures in `lexer.pyi`.
- `lark.pyi` and `tree.pyi` use `typing.Literal` and `typing.Protocol` which are new in Python 3.8. The [mypy docs](https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module) suggest using the `typing_extensions` package, but it may not be installed. How do we want to approach this?

One other question: Would you like me to add type annotations to more functions where possible, or do you only want them on public interfaces?


Patch:
diff --git a/.github/workflows/mypy.yml b/.github/workflows/mypy.yml
index 85039a4..f1c667f 100644
--- a/.github/workflows/mypy.yml
+++ b/.github/workflows/mypy.yml
@@ -16,4 +16,4 @@ jobs:
           python -m pip install --upgrade pip
           pip install mypy
       - name: Lint with mypy
-        run: mypy -p lark-stubs || true
+        run: mypy -p lark || true
diff --git a/lark-stubs/__init__.pyi b/lark-stubs/__init__.pyi
deleted file mode 100644
index c79a6ef..0000000
--- a/lark-stubs/__init__.pyi
+++ /dev/null
@@ -1,12 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from .tree import *
-from .visitors import *
-from .exceptions import *
-from .lexer import *
-from .load_grammar import *
-from .lark import *
-from logging import Logger as _Logger
-
-logger: _Logger
-__version__: str = ...
diff --git a/lark-stubs/ast_utils.pyi b/lark-stubs/ast_utils.pyi
deleted file mode 100644
index 28246cf..0000000
--- a/lark-stubs/ast_utils.pyi
+++ /dev/null
@@ -1,17 +0,0 @@
-import types
-from typing import Optional
-
-from .visitors import Transformer
-
-class Ast(object):
-    pass
-
-class AsList(object):
-    pass
-
-
-def create_transformer(
-        ast_module: types.ModuleType,
-        transformer: Optional[Transformer]=None
-) -> Transformer:
-    ...
\ No newline at end of file
diff --git a/lark-stubs/exceptions.pyi b/lark-stubs/exceptions.pyi
deleted file mode 100644
index 1c04fa8..0000000
--- a/lark-stubs/exceptions.pyi
+++ /dev/null
@@ -1,65 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import Dict, Iterable, Callable, Union, TypeVar, Tuple, Any, List, Set
-from .tree import Tree
-from .lexer import Token
-from .parsers.lalr_interactive_parser import InteractiveParser
-
-class LarkError(Exception):
-    pass
-
-
-class ConfigurationError(LarkError, ValueError):
-    pass
-
-
-class GrammarError(LarkError):
-    pass
-
-
-class ParseError(LarkError):
-    pass
-
-
-class LexError(LarkError):
-    pass
-
-
-T = TypeVar('T')
-
-class UnexpectedEOF(ParseError):
-    expected: List[Token]
-
-class UnexpectedInput(LarkError):
-    line: int
-    column: int
-    pos_in_stream: int
-    state: Any
-
-    def get_context(self, text: str, span: int = ...) -> str:
-        ...
-
-    def match_examples(
-            self,
-            parse_fn: Callable[[str], Tree],
-            examples: Union[Dict[T, Iterable[str]], Iterable[Tuple[T, Iterable[str]]]],
-            token_type_match_fallback: bool = False,
-            use_accepts: bool = False,
-    ) -> T:
-        ...
-
-
-class UnexpectedToken(ParseError, UnexpectedInput):
-    expected: Set[str]
-    considered_rules: Set[str]
-    interactive_parser: InteractiveParser
-    accepts: Set[str]
-
-class UnexpectedCharacters(LexError, UnexpectedInput):
-    allowed: Set[str]
-    considered_tokens: Set[Any]
-
-
-class VisitError(LarkError):
-    obj: Union[Tree, Token]
-    orig_exc: Exception
diff --git a/lark-stubs/grammar.pyi b/lark-stubs/grammar.pyi
deleted file mode 100644
index 3a3d806..0000000
--- a/lark-stubs/grammar.pyi
+++ /dev/null
@@ -1,14 +0,0 @@
-from typing import Optional, Tuple
-
-
-class RuleOptions:
-    keep_all_tokens: bool
-    expand1: bool
-    priority: int
-    template_source: Optional[str]
-    empty_indices: Tuple[bool, ...]
-
-
-class Symbol:
-    name: str
-    is_term: bool
diff --git a/lark-stubs/indenter.pyi b/lark-stubs/indenter.pyi
deleted file mode 100644
index 3a7aa97..0000000
--- a/lark-stubs/indenter.pyi
+++ /dev/null
@@ -1,47 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import Tuple, List, Iterator, Optional
-from abc import ABC, abstractmethod
-from .lexer import Token
-from .lark import PostLex
-
-
-class Indenter(PostLex, ABC):
-    paren_level: Optional[int]
-    indent_level: Optional[List[int]]
-
-    def __init__(self) -> None:
-        ...
-
-    def handle_NL(self, token: Token) -> Iterator[Token]:
-        ...
-
-    @property
-    @abstractmethod
-    def NL_type(self) -> str:
-        ...
-
-    @property
-    @abstractmethod
-    def OPEN_PAREN_types(self) -> List[str]:
-        ...
-
-    @property
-    @abstractmethod
-    def CLOSE_PAREN_types(self) -> List[str]:
-        ...
-
-    @property
-    @abstractmethod
-    def INDENT_type(self) -> str:
-        ...
-
-    @property
-    @abstractmethod
-    def DEDENT_type(self) -> str:
-        ...
-
-    @property
-    @abstractmethod
-    def tab_len(self) -> int:
-        ...
diff --git a/lark-stubs/lark.pyi b/lark-stubs/lark.pyi
deleted file mode 100644
index 27c6863..0000000
--- a/lark-stubs/lark.pyi
+++ /dev/null
@@ -1,109 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import (
-    TypeVar, Type, List, Dict, IO, Iterator, Callable, Union, Optional,
-    Literal, Protocol, Tuple, Iterable,
-)
-
-from .parsers.lalr_interactive_parser import InteractiveParser
-from .visitors import Transformer
-from .lexer import Token, Lexer, TerminalDef
-from .tree import Tree
-from .exceptions import UnexpectedInput
-from .load_grammar import Grammar
-
-_T = TypeVar('_T')
-
-
-class PostLex(Protocol):
-
-    def process(self, stream: Iterator[Token]) -> Iterator[Token]:
-        ...
-
-    always_accept: Iterable[str]
-
-
-class LarkOptions:
-    start: List[str]
-    parser: str
-    lexer: str
-    transformer: Optional[Transformer]
-    postlex: Optional[PostLex]
-    ambiguity: str
-    regex: bool
-    debug: bool
-    keep_all_tokens: bool
-    propagate_positions: Union[bool, str]
-    maybe_placeholders: bool
-    lexer_callbacks: Dict[str, Callable[[Token], Token]]
-    cache: Union[bool, str]
-    g_regex_flags: int
-    use_bytes: bool
-    import_paths: List[Union[str, Callable[[Union[None, str, PackageResource], str], Tuple[str, str]]]]
-    source_path: Optional[str]
-
-
-class PackageResource(object):
-    pkg_name: str
-    path: str
-
-    def __init__(self, pkg_name: str, path: str): ...
-
-
-class FromPackageLoader:
-    def __init__(self, pkg_name: str, search_paths: Tuple[str, ...] = ...): ...
-
-    def __call__(self, base_path: Union[None, str, PackageResource], grammar_path: str) -> Tuple[PackageResource, str]: ...
-
-
-class Lark:
-    source_path: str
-    source_grammar: str
-    grammar: Grammar
-    options: LarkOptions
-    lexer: Lexer
-    terminals: List[TerminalDef]
-
-    def __init__(
-        self,
-        grammar: Union[Grammar, str, IO[str]],
-        *,
-        start: Union[None, str, List[str]] = "start",
-        parser: Literal["earley", "lalr", "cyk", "auto"] = "auto",
-        lexer: Union[Literal["auto", "standard", "contextual", "dynamic", "dynamic_complete"], Type[Lexer]] = "auto",
-        transformer: Optional[Transformer] = None,
-        postlex: Optional[PostLex] = None,
-        ambiguity: Literal["explicit", "resolve"] = "resolve",
-        regex: bool = False,
-        debug: bool = False,
-        keep_all_tokens: bool = False,
-        propagate_positions: Union[bool, str] = False,
-        maybe_placeholders: bool = False,
-        lexer_callbacks: Optional[Dict[str, Callable[[Token], Token]]] = None,
-        cache: Union[bool, str] = False,
-        g_regex_flags: int = ...,
-        use_bytes: bool = False,
-        import_paths: List[Union[str, Callable[[Union[None, str, PackageResource], str], Tuple[str, str]]]] = ...,
-        source_path: Optional[str]=None,
-    ):
-        ...
-
-    def parse(self, text: str, start: Optional[str] = None, on_error: Callable[[UnexpectedInput], bool] = None) -> Tree:
-        ...
-
-    def parse_interactive(self, text: str = None, start: Optional[str] = None) -> InteractiveParser:
-        ...
-
-    @classmethod
-    def open(cls: Type[_T], grammar_filename: str, rel_to: Optional[str] = None, **options) -> _T:
-        ...
-
-    @classmethod
-    def open_from_package(cls: Type[_T], package: str, grammar_path: str, search_paths: Tuple[str, ...] = ..., **options) -> _T:
-        ...
-
-    def lex(self, text: str, dont_ignore: bool = False) -> Iterator[Token]:
-        ...
-
-    def get_terminal(self, name: str) -> TerminalDef:
-        ...
diff --git a/lark-stubs/lexer.pyi b/lark-stubs/lexer.pyi
deleted file mode 100644
index 004865c..0000000
--- a/lark-stubs/lexer.pyi
+++ /dev/null
@@ -1,161 +0,0 @@
-# -*- coding: utf-8 -*-
-from types import ModuleType
-from typing import (
-    TypeVar, Type, Tuple, List, Dict, Iterator, Collection, Callable, Optional, FrozenSet, Any,
-    Pattern as REPattern,
-)
-from abc import abstractmethod, ABC
-
-_T = TypeVar('_T')
-
-
-class Pattern(ABC):
-    value: str
-    flags: Collection[str]
-    raw: str
-    type: str
-
-    def __init__(self, value: str, flags: Collection[str] = (), raw: str = None) -> None:
-        ...
-
-    @abstractmethod
-    def to_regexp(self) -> str:
-        ...
-
-    @property
-    @abstractmethod
-    def min_width(self) -> int:
-        ...
-
-    @property
-    @abstractmethod
-    def max_width(self) -> int:
-        ...
-
-
-class PatternStr(Pattern):
-    type: str = ...
-
-    def to_regexp(self) -> str:
-        ...
-
-    @property
-    def min_width(self) -> int:
-        ...
-
-    @property
-    def max_width(self) -> int:
-        ...
-
-
-class PatternRE(Pattern):
-    type: str = ...
-
-    def to_regexp(self) -> str:
-        ...
-
-    @property
-    def min_width(self) -> int:
-        ...
-
-    @property
-    def max_width(self) -> int:
-        ...
-
-
-class TerminalDef:
-    name: str
-    pattern: Pattern
-    priority: int
-
-    def __init__(self, name: str, pattern: Pattern, priority: int = ...) -> None:
-        ...
-
-    def user_repr(self) -> str: ...
-
-
-class Token(str):
-    type: str
-    start_pos: int
-    value: Any
-    line: int
-    column: int
-    end_line: int
-    end_column: int
-    end_pos: int
-
-    def __init__(self, type_: str, value: Any, start_pos: int = None, line: int = None, column: int = None, end_line: int = None, end_column: int = None, end_pos: int = None) -> None:
-        ...
-
-    def update(self, type_: Optional[str] = None, value: Optional[Any] = None) -> Token:
-        ...
-
-    @classmethod
-    def new_borrow_pos(cls: Type[_T], type_: str, value: Any, borrow_t: Token) -> _T:
-        ...
-
-
-_Callback = Callable[[Token], Token]
-
-
-class Lexer(ABC):
-    lex: Callable[..., Iterator[Token]]
-
-
-class LexerConf:
-     tokens: Collection[TerminalDef]
-     re_module: ModuleType
-     ignore: Collection[str] = ()
-     postlex: Any =None
-     callbacks: Optional[Dict[str, _Callback]] = None
-     g_regex_flags: int = 0
-     skip_validation: bool = False
-     use_bytes: bool = False
-
-
-
-class TraditionalLexer(Lexer):
-    terminals: Collection[TerminalDef]
-    ignore_types: FrozenSet[str]
-    newline_types: FrozenSet[str]
-    user_callbacks: Dict[str, _Callback]
-    callback: Dict[str, _Callback]
-    mres: List[Tuple[REPattern, Dict[int, str]]]
-    re: ModuleType
-
-    def __init__(
-        self,
-        conf: LexerConf
-    ) -> None:
-        ...
-
-    def build(self) -> None:
-        ...
-
-    def match(self, stream: str, pos: int) -> Optional[Tuple[str, str]]:
-        ...
-
-    def lex(self, stream: str) -> Iterator[Token]:
-        ...
-
-    def next_token(self, lex_state: Any, parser_state: Any = None) -> Token:
-        ...
-
-class ContextualLexer(Lexer):
-    lexers: Dict[str, TraditionalLexer]
-    root_lexer: TraditionalLexer
-
-    def __init__(
-        self,
-        terminals: Collection[TerminalDef],
-        states: Dict[str, Collection[str]],
-        re_: ModuleType,
-        ignore: Collection[str] = ...,
-        always_accept: Collection[str] = ...,
-        user_callbacks: Dict[str, _Callback] = ...,
-        g_regex_flags: int = ...
-    ) -> None:
-        ...
-
-    def lex(self, stream: str, get_parser_state: Callable[[], str]) -> Iterator[Token]:
-        ...
diff --git a/lark-stubs/load_grammar.pyi b/lark-stubs/load_grammar.pyi
deleted file mode 100644
index 86a6341..0000000
--- a/lark-stubs/load_grammar.pyi
+++ /dev/null
@@ -1,31 +0,0 @@
-from typing import List, Tuple, Union, Callable, Dict, Optional
-
-from .tree import Tree
-from .grammar import RuleOptions
-from .exceptions import UnexpectedInput
-
-
-class Grammar:
-    rule_defs: List[Tuple[str, Tuple[str, ...], Tree, RuleOptions]]
-    term_defs: List[Tuple[str, Tuple[Tree, int]]]
-    ignore: List[str]
-
-
-class GrammarBuilder:
-    global_keep_all_tokens: bool
-    import_paths: List[Union[str, Callable]]
-    used_files: Dict[str, str]
-
-    def __init__(self, global_keep_all_tokens: bool = False, import_paths: List[Union[str, Callable]] = None, used_files: Dict[str, str]=None) -> None: ...
-
-    def load_grammar(self, grammar_text: str, grammar_name: str = ..., mangle: Callable[[str], str] = None) -> None: ...
-
-    def do_import(self, dotted_path: Tuple[str, ...], base_path: Optional[str], aliases: Dict[str, str],
-                  base_mangle: Callable[[str], str] = None) -> None:  ...
-
-    def validate(self) -> None: ...
-
-    def build(self) -> Grammar: ...
-
-
-def find_grammar_errors(text: str, start: str='start') -> List[Tuple[UnexpectedInput, str]]: ...
\ No newline at end of file
diff --git a/lark-stubs/parsers/__init__.pyi b/lark-stubs/parsers/__init__.pyi
deleted file mode 100644
index e69de29..0000000
diff --git a/lark-stubs/reconstruct.pyi b/lark-stubs/reconstruct.pyi
deleted file mode 100644
index a8d39e3..0000000
--- a/lark-stubs/reconstruct.pyi
+++ /dev/null
@@ -1,39 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import List, Dict, Union, Callable, Iterable
-
-from .grammar import Symbol
-from .lark import Lark
-from .tree import Tree
-from .visitors import Transformer_InPlace
-from .lexer import TerminalDef
-
-
-class WriteTokensTransformer(Transformer_InPlace):
-
-    def __init__(self, tokens: Dict[str, TerminalDef], term_subs: Dict[str, Callable[[Symbol], str]] = ...): ...
-
-
-class MatchTree(Tree):
-    pass
-
-
-class MakeMatchTree:
-    name: str
-    expansion: List[TerminalDef]
-
-    def __init__(self, name: str, expansion: List[TerminalDef]):
-        ...
-
-    def __call__(self, args: List[Union[str, Tree]]):
-        ...
-
-
-class Reconstructor:
-
-    def __init__(self, parser: Lark, term_subs: Dict[str, Callable[[Symbol], str]] = ...):
-        ...
-
-    def reconstruct(self, tree: Tree, postproc: Callable[[Iterable[str]], Iterable[str]]=None,
-                    insert_spaces: bool = True) -> str:
-        ...
diff --git a/lark-stubs/tree.pyi b/lark-stubs/tree.pyi
deleted file mode 100644
index ea99ff6..0000000
--- a/lark-stubs/tree.pyi
+++ /dev/null
@@ -1,72 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import List, Callable, Iterator, Union, Optional, Literal, Any
-from .lexer import TerminalDef
-
-class Meta:
-    empty: bool
-    line: int
-    column: int
-    start_pos: int
-    end_line: int
-    end_column: int
-    end_pos: int
-    orig_expansion: List[TerminalDef]
-    match_tree: bool
-
-
-class Tree:
-    data: str
-    children: List[Union[str, Tree]]
-    meta: Meta
-
-    def __init__(
-        self,
-        data: str,
-        children: List[Union[str, Tree]],
-        meta: Optional[Meta] = None
-    ) -> None:
-        ...
-
-    def pretty(self, indent_str: str = ...) -> str:
-        ...
-
-    def find_pred(self, pred: Callable[[Tree], bool]) -> Iterator[Tree]:
-        ...
-
-    def find_data(self, data: str) -> Iterator[Tree]:
-        ...
-
-    def expand_kids_by_index(self, *indices: int) -> None:
-        ...
-
-    def scan_values(self, pred: Callable[[Union[str, Tree]], bool]) -> Iterator[str]:
-        ...
-
-    def iter_subtrees(self) -> Iterator[Tree]:
-        ...
-
-    def iter_subtrees_topdown(self) -> Iterator[Tree]:
-        ...
-
-    def copy(self) -> Tree:
-        ...
-
-    def set(self, data: str, children: List[Union[str, Tree]]) -> None:
-        ...
-
-    def __hash__(self) -> int:
-        ...
-
-
-class SlottedTree(Tree):
-    pass
-
-
-def pydot__tree_to_png(
-    tree: Tree,
-    filename: str,
-    rankdir: Literal["TB", "LR", "BT", "RL"] = ...,
-    **kwargs
-) -> None:
-    ...
diff --git a/lark-stubs/visitors.pyi b/lark-stubs/visitors.pyi
deleted file mode 100644
index 3a934ee..0000000
--- a/lark-stubs/visitors.pyi
+++ /dev/null
@@ -1,108 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from typing import TypeVar, Tuple, List, Callable, Generic, Type, Union
-from abc import ABC
-from .tree import Tree
-
-_T = TypeVar('_T')
-_R = TypeVar('_R')
-_FUNC = Callable[..., _T]
-_DECORATED = Union[_FUNC, type]
-
-
-class Transformer(ABC, Generic[_T]):
-
-    def __init__(self, visit_tokens: bool = True) -> None:
-        ...
-
-    def transform(self, tree: Tree) -> _T:
-        ...
-
-    def __mul__(self, other: Transformer[_T]) -> TransformerChain[_T]:
-        ...
-
-
-class TransformerChain(Generic[_T]):
-    transformers: Tuple[Transformer[_T], ...]
-
-    def __init__(self, *transformers: Transformer[_T]) -> None:
-        ...
-
-    def transform(self, tree: Tree) -> _T:
-        ...
-
-    def __mul__(self, other: Transformer[_T]) -> TransformerChain[_T]:
-        ...
-
-
-class Transformer_InPlace(Transformer):
-    pass
-
-
-class Transformer_NonRecursive(Transformer):
-    pass
-
-
-class Transformer_InPlaceRecursive(Transformer):
-    pass
-
-
-class VisitorBase:
-    pass
-
-
-class Visitor(VisitorBase, ABC, Generic[_T]):
-
-    def visit(self, tree: Tree) -> Tree:
-        ...
-
-    def visit_topdown(self, tree: Tree) -> Tree:
-        ...
-
-
-class Visitor_Recursive(VisitorBase):
-
-    def visit(self, tree: Tree) -> Tree:
-        ...
-
-    def visit_topdown(self, tree: Tree) -> Tree:
-        ...
-
-
-class Interpreter(ABC, Generic[_T]):
-
-    def visit(self, tree: Tree) -> _T:
-        ...
-
-    def visit_children(self, tree: Tree) -> List[_T]:
-        ...
-
-
-_InterMethod = Callable[[Type[Interpreter], _T], _R]
-
-
-def v_args(
-        inline: bool = False,
-        meta: bool = False,
-        tree: bool = False,
-        wrapper: Callable = None
-) -> Callable[[_DECORATED], _DECORATED]:
-    ...
-
-
-def visit_children_decor(func: _InterMethod) -> _InterMethod:
-    ...
-
-
-class Discard(Exception):
-    pass
-
-
-# Deprecated
-class InlineTransformer:
-    pass
-
-
-# Deprecated
-def inline_args(obj: _FUNC) -> _FUNC:
-    ...
diff --git a/lark/__init__.py b/lark/__init__.py
index aff5683..609cfc7 100644
--- a/lark/__init__.py
+++ b/lark/__init__.py
@@ -6,4 +6,4 @@ from .exceptions import (ParseError, LexError, GrammarError, UnexpectedToken,
 from .lexer import Token
 from .lark import Lark
 
-__version__ = "1.0.0a"
+__version__: str = "1.0.0a"
diff --git a/lark/ast_utils.py b/lark/ast_utils.py
index 0f2e498..c535f11 100644
--- a/lark/ast_utils.py
+++ b/lark/ast_utils.py
@@ -3,6 +3,8 @@
 """
 
 import inspect, re
+import types
+from typing import Optional
 
 from lark import Transformer, v_args
 
@@ -27,7 +29,7 @@ def _call(func, _data, children, _meta):
 
 inline = v_args(wrapper=_call)
 
-def create_transformer(ast_module, transformer=None):
+def create_transformer(ast_module: types.ModuleType, transformer: Optional[Transformer]=None) -> Transformer:
     """Collects `Ast` subclasses from the given module, and creates a Lark transformer that builds the AST.
 
     For each class, we create a corresponding rule in the transformer, with a matching name.
@@ -49,4 +51,4 @@ def create_transformer(ast_module, transformer=None):
 
                 setattr(t, camel_to_snake(name), obj)
 
-    return t
\ No newline at end of file
+    return t
diff --git a/lark/common.py b/lark/common.py
index e2cde6b..6ad38fc 100644
--- a/lark/common.py
+++ b/lark/common.py
@@ -1,14 +1,30 @@
+from types import ModuleType
+
 from .utils import Serialize
-from .lexer import TerminalDef
+from .lexer import TerminalDef, Token
 
 ###{standalone
+from typing import Any, Callable, Collection, Dict, Optional, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from .lark import PostLex
 
+_Callback = Callable[[Token], Token]
 
 class LexerConf(Serialize):
     __serialize_fields__ = 'terminals', 'ignore', 'g_regex_flags', 'use_bytes', 'lexer_type'
     __serialize_namespace__ = TerminalDef,
 
-    def __init__(self, terminals, re_module, ignore=(), postlex=None, callbacks=None, g_regex_flags=0, skip_validation=False, use_bytes=False):
+    terminals: Collection[TerminalDef]
+    re_module: ModuleType
+    ignore: Collection[str]
+    postlex: 'Optional[PostLex]'
+    callbacks: Dict[str, _Callback]
+    g_regex_flags: int
+    skip_validation: bool
+    use_bytes: bool
+
+    def __init__(self, terminals: Collection[TerminalDef], re_module: ModuleType, ignore: Collection[str]=(), postlex: 'Optional[PostLex]'=None, callbacks: Optional[Dict[str, _Callback]]=None, g_regex_flags: int=0, skip_validation: bool=False, use_bytes: bool=False):
         self.terminals = terminals
         self.terminals_by_name = {t.name: t for t in self.terminals}
         assert len(self.terminals) == len(self.terminals_by_name)
diff --git a/lark/exceptions.py b/lark/exceptions.py
index 2ae0859..9c4dc1e 100644
--- a/lark/exceptions.py
+++ b/lark/exceptions.py
@@ -3,6 +3,12 @@ from .utils import logger, NO_VALUE
 
 ###{standalone
 
+from typing import Dict, Iterable, Callable, Union, TypeVar, Tuple, Any, List, Set, Optional, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from .lexer import Token
+    from .parsers.lalr_interactive_parser import InteractiveParser
+    from .tree import Tree
 
 class LarkError(Exception):
     pass
@@ -28,6 +34,7 @@ class ParseError(LarkError):
 class LexError(LarkError):
     pass
 
+T = TypeVar('T')
 
 class UnexpectedInput(LarkError):
     """UnexpectedInput Error.
@@ -39,10 +46,13 @@ class UnexpectedInput(LarkError):
 
     After catching one of these exceptions, you may call the following helper methods to create a nicer error message.
     """
+    line: int
+    column: int
     pos_in_stream = None
+    state: Any
     _terminals_by_name = None
 
-    def get_context(self, text, span=40):
+    def get_context(self, text: str, span: int=40) -> str:
         """Returns a pretty string pinpointing the error in the text,
         with span amount of context characters around it.
 
@@ -63,7 +73,7 @@ class UnexpectedInput(LarkError):
             after = text[pos:end].split(b'\n', 1)[0]
             return (before + after + b'\n' + b' ' * len(before.expandtabs()) + b'^\n').decode("ascii", "backslashreplace")
 
-    def match_examples(self, parse_fn, examples, token_type_match_fallback=False, use_accepts=False):
+    def match_examples(self, parse_fn: 'Callable[[str], Tree]', examples: Union[Dict[T, Iterable[str]], Iterable[Tuple[T, Iterable[str]]]], token_type_match_fallback: bool=False, use_accepts: bool=False) -> Optional[T]:
         """Allows you to detect what's wrong in the input text by matching
         against example errors.
 
@@ -126,6 +136,9 @@ class UnexpectedInput(LarkError):
 
 
 class UnexpectedEOF(ParseError, UnexpectedInput):
+
+    expected: 'List[Token]'
+
     def __init__(self, expected, state=None, terminals_by_name=None):
         self.expected = expected
         self.state = state
@@ -145,6 +158,10 @@ class UnexpectedEOF(ParseError, UnexpectedInput):
 
 
 class UnexpectedCharacters(LexError, UnexpectedInput):
+
+    allowed: Set[str]
+    considered_tokens: Set[Any]
+
     def __init__(self, seq, lex_pos, line, column, allowed=None, considered_tokens=None, state=None, token_history=None,
                  terminals_by_name=None, considered_rules=None):
         # TODO considered_tokens and allowed can be figured out using state
@@ -187,6 +204,10 @@ class UnexpectedToken(ParseError, UnexpectedInput):
     see: ``InteractiveParser``.
     """
 
+    expected: Set[str]
+    considered_rules: Set[str]
+    interactive_parser: 'InteractiveParser'
+
     def __init__(self, token, expected, considered_rules=None, state=None, interactive_parser=None, terminals_by_name=None, token_history=None):
         # TODO considered_rules and expected can be figured out using state
         self.line = getattr(token, 'line', '?')
@@ -205,7 +226,7 @@ class UnexpectedToken(ParseError, UnexpectedInput):
         super(UnexpectedToken, self).__init__()
 
     @property
-    def accepts(self):
+    def accepts(self) -> Set[str]:
         if self._accepts is NO_VALUE:
             self._accepts = self.interactive_parser and self.interactive_parser.accepts()
         return self._accepts
@@ -228,6 +249,9 @@ class VisitError(LarkError):
     - orig_exc: the exception that cause it to fail
     """
 
+    obj: 'Union[Tree, Token]'
+    orig_exc: Exception
+
     def __init__(self, rule, obj, orig_exc):
         self.obj = obj
         self.orig_exc = orig_exc
diff --git a/lark/grammar.py b/lark/grammar.py
index 405086a..25aec17 100644
--- a/lark/grammar.py
+++ b/lark/grammar.py
@@ -1,13 +1,17 @@
+
 from .utils import Serialize
 
 ###{standalone
 
+from typing import Optional, Tuple, ClassVar
+
 class Symbol(Serialize):
     __slots__ = ('name',)
 
-    is_term = NotImplemented
+    name: str
+    is_term: ClassVar[bool] = NotImplemented
 
-    def __init__(self, name):
+    def __init__(self, name: str) -> None:
         self.name = name
 
     def __eq__(self, other):
@@ -29,7 +33,7 @@ class Symbol(Serialize):
 class Terminal(Symbol):
     __serialize_fields__ = 'name', 'filter_out'
 
-    is_term = True
+    is_term: ClassVar[bool] = True
 
     def __init__(self, name, filter_out=False):
         self.name = name
@@ -43,13 +47,19 @@ class Terminal(Symbol):
 class NonTerminal(Symbol):
     __serialize_fields__ = 'name',
 
-    is_term = False
+    is_term: ClassVar[bool] = False
 
 
 class RuleOptions(Serialize):
     __serialize_fields__ = 'keep_all_tokens', 'expand1', 'priority', 'template_source', 'empty_indices'
 
-    def __init__(self, keep_all_tokens=False, expand1=False, priority=None, template_source=None, empty_indices=()):
+    keep_all_tokens: bool
+    expand1: bool
+    priority: Optional[int]
+    template_source: Optional[str]
+    empty_indices: Tuple[bool, ...]
+
+    def __init__(self, keep_all_tokens: bool=False, expand1: bool=False, priority: Optional[int]=None, template_source: Optional[str]=None, empty_indices: Tuple[bool, ...]=()) -> None:
         self.keep_all_tokens = keep_all_tokens
         self.expand1 = expand1
         self.priority = priority
diff --git a/lark/indenter.py b/lark/indenter.py
index 7e1263d..69a7ba4 100644
--- a/lark/indenter.py
+++ b/lark/indenter.py
@@ -1,20 +1,28 @@
 "Provides Indentation services for languages with indentation similar to Python"
 
+from abc import ABC, abstractmethod
+
 from .exceptions import LarkError
 from .lark import PostLex
 from .lexer import Token
 
 ###{standalone
+from typing import Tuple, List, Iterator, Optional
+
 class DedentError(LarkError):
     pass
 
-class Indenter(PostLex):
-    def __init__(self):
-        self.paren_level = None
-        self.indent_level = None
+class Indenter(PostLex, ABC):
+
+    paren_level: int
+    indent_level: List[int]
+
+    def __init__(self) -> None:
+        self.paren_level = 0
+        self.indent_level = [0]
         assert self.tab_len > 0
 
-    def handle_NL(self, token):
+    def handle_NL(self, token: Token) -> Iterator[Token]:
         if self.paren_level > 0:
             return
 
@@ -64,4 +72,34 @@ class Indenter(PostLex):
     def always_accept(self):
         return (self.NL_type,)
 
+    @property
+    @abstractmethod
+    def NL_type(self) -> str:
+        ...
+
+    @property
+    @abstractmethod
+    def OPEN_PAREN_types(self) -> List[str]:
+        ...
+
+    @property
+    @abstractmethod
+    def CLOSE_PAREN_types(self) -> List[str]:
+        ...
+
+    @property
+    @abstractmethod
+    def INDENT_type(self) -> str:
+        ...
+
+    @property
+    @abstractmethod
+    def DEDENT_type(self) -> str:
+        ...
+
+    @property
+    @abstractmethod
+    def tab_len(self) -> int:
+        ...
+
 ###}
diff --git a/lark/lark.py b/lark/lark.py
index a4d223e..e225bad 100644
--- a/lark/lark.py
+++ b/lark/lark.py
@@ -2,31 +2,71 @@ from abc import ABC, abstractmethod
 import sys, os, pickle, hashlib
 import tempfile
 
-from .exceptions import ConfigurationError, assert_config
+from .exceptions import ConfigurationError, assert_config, UnexpectedInput
 from .utils import Serialize, SerializeMemoizer, FS, isascii, logger
-from .load_grammar import load_grammar, FromPackageLoader, Grammar, verify_used_files
+from .load_grammar import load_grammar, FromPackageLoader, Grammar, verify_used_files, PackageResource
 from .tree import Tree
 from .common import LexerConf, ParserConf
 
-from .lexer import Lexer, TraditionalLexer, TerminalDef, LexerThread
+from .lexer import Lexer, TraditionalLexer, TerminalDef, LexerThread, Token
 from .parse_tree_builder import ParseTreeBuilder
 from .parser_frontends import get_frontend, _get_lexer_callbacks
 from .grammar import Rule
 
 import re
 try:
-    import regex
+    import regex  # type: ignore
 except ImportError:
     regex = None
 
 
 ###{standalone
+from typing import (
+    TypeVar, Type, List, Dict, Iterator, Callable, Union, Optional,
+    Tuple, Iterable, IO, Any, TYPE_CHECKING
+)
+
+if TYPE_CHECKING:
+    from .parsers.lalr_interactive_parser import InteractiveParser
+    from .visitors import Transformer
+    if sys.version_info >= (3, 8):
+        from typing import Literal
+    else:
+        from typing_extensions import Literal
 
+class PostLex(ABC):
+    @abstractmethod
+    def process(self, stream: Iterator[Token]) -> Iterator[Token]:
+        return stream
+
+    always_accept: Iterable[str] = ()
 
 class LarkOptions(Serialize):
     """Specifies the options for Lark
 
     """
+
+    start: List[str]
+    debug: bool
+    transformer: 'Optional[Transformer]'
+    propagate_positions: Union[bool, str]
+    maybe_placeholders: bool
+    cache: Union[bool, str]
+    regex: bool
+    g_regex_flags: int
+    keep_all_tokens: bool
+    tree_class: Any
+    parser: 'Literal["earley", "lalr", "cyk", "auto"]'
+    lexer: 'Union[Literal["auto", "standard", "contextual", "dynamic", "dynamic_complete"], Type[Lexer]]'
+    ambiguity: 'Literal["auto", "resolve", "explicit", "forest"]'
+    postlex: Optional[PostLex]
+    priority: 'Optional[Literal["auto", "normal", "invert"]]'
+    lexer_callbacks: Dict[str, Callable[[Token], Token]]
+    use_bytes: bool
+    edit_terminals: Optional[Callable[[TerminalDef], TerminalDef]]
+    import_paths: 'List[Union[str, Callable[[Union[None, str, PackageResource], str], Tuple[str, str]]]]'
+    source_path: Optional[str]
+
     OPTIONS_DOC = """
     **===  General Options  ===**
 
@@ -106,12 +146,10 @@ class LarkOptions(Serialize):
     # Adding a new option needs to be done in multiple places:
     # - In the dictionary below. This is the primary truth of which options `Lark.__init__` accepts
     # - In the docstring above. It is used both for the docstring of `LarkOptions` and `Lark`, and in readthedocs
-    # - In `lark-stubs/lark.pyi`:
-    #   - As attribute to `LarkOptions`
-    #   - As parameter to `Lark.__init__`
+    # - As an attribute of `LarkOptions` above
     # - Potentially in `_LOAD_ALLOWED_OPTIONS` below this class, when the option doesn't change how the grammar is loaded
     # - Potentially in `lark.tools.__init__`, if it makes sense, and it can easily be passed as a cmd argument
-    _defaults = {
+    _defaults: Dict[str, Any] = {
         'debug': False,
         'keep_all_tokens': False,
         'tree_class': None,
@@ -189,13 +227,7 @@ _VALID_PRIORITY_OPTIONS = ('auto', 'normal', 'invert', None)
 _VALID_AMBIGUITY_OPTIONS = ('auto', 'resolve', 'explicit', 'forest')
 
 
-class PostLex(ABC):
-    @abstractmethod
-    def process(self, stream):
-        return stream
-
-    always_accept = ()
-
+_T = TypeVar('_T')
 
 class Lark(Serialize):
     """Main interface for the library.
@@ -210,7 +242,15 @@ class Lark(Serialize):
         >>> Lark(r'''start: "foo" ''')
         Lark(...)
     """
-    def __init__(self, grammar, **options):
+
+    source_path: str
+    source_grammar: str
+    grammar: 'Grammar'
+    options: LarkOptions
+    lexer: Lexer
+    terminals: List[TerminalDef]
+
+    def __init__(self, grammar: 'Union[Grammar, str, IO[str]]', **options) -> None:
         self.options = LarkOptions(options)
 
         # Set regex or re module
@@ -374,6 +414,7 @@ class Lark(Serialize):
         if cache_fn:
             logger.debug('Saving grammar to cache: %s', cache_fn)
             with FS.open(cache_fn, 'wb') as f:
+                assert cache_md5 is not None
                 f.write(cache_md5.encode('utf8') + b'\n')
                 pickle.dump(used_files, f)
                 self.save(f)
@@ -476,7 +517,7 @@ class Lark(Serialize):
         return inst._load({'data': data, 'memo': memo}, **kwargs)
 
     @classmethod
-    def open(cls, grammar_filename, rel_to=None, **options):
+    def open(cls: Type[_T], grammar_filename: str, rel_to: Optional[str]=None, **options) -> _T:
         """Create an instance of Lark with the grammar given by its filename
 
         If ``rel_to`` is provided, the function will find the grammar filename in relation to it.
@@ -494,7 +535,7 @@ class Lark(Serialize):
             return cls(f, **options)
 
     @classmethod
-    def open_from_package(cls, package, grammar_path, search_paths=("",), **options):
+    def open_from_package(cls: Type[_T], package: str, grammar_path: str, search_paths: Tuple[str, ...]=("",), **options) -> _T:
         """Create an instance of Lark with the grammar loaded from within the package `package`.
         This allows grammar loading from zipapps.
 
@@ -515,7 +556,7 @@ class Lark(Serialize):
         return 'Lark(open(%r), parser=%r, lexer=%r, ...)' % (self.source_path, self.options.parser, self.options.lexer)
 
 
-    def lex(self, text, dont_ignore=False):
+    def lex(self, text: str, dont_ignore: bool=False) -> Iterator[Token]:
         """Only lex (and postlex) the text, without parsing it. Only relevant when lexer='standard'
 
         When dont_ignore=True, the lexer will return all tokens, even those marked for %ignore.
@@ -530,11 +571,11 @@ class Lark(Serialize):
             return self.options.postlex.process(stream)
         return stream
 
-    def get_terminal(self, name):
+    def get_terminal(self, name: str) -> TerminalDef:
         """Get information about a terminal"""
         return self._terminals_dict[name]
     
-    def parse_interactive(self, text=None, start=None):
+    def parse_interactive(self, text: Optional[str]=None, start: Optional[str]=None) -> 'InteractiveParser':
         """Start an interactive parsing session.
 
         Parameters:
@@ -548,7 +589,7 @@ class Lark(Serialize):
         """
         return self.parser.parse_interactive(text, start=start)
 
-    def parse(self, text, start=None, on_error=None):
+    def parse(self, text: str, start: Optional[str]=None, on_error: 'Optional[Callable[[UnexpectedInput], bool]]'=None) -> Tree:
         """Parse the given text, according to the options provided.
 
         Parameters:
diff --git a/lark/lexer.py b/lark/lexer.py
index 1b5250e..af698cb 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -1,20 +1,32 @@
 # Lexer Implementation
 
+from abc import abstractmethod, ABC
 import re
 from contextlib import suppress
 
-from .utils import classify, get_regexp_width, Serialize
+from .utils import classify, get_regexp_width, Py36, Serialize
 from .exceptions import UnexpectedCharacters, LexError, UnexpectedToken
 
 ###{standalone
 from copy import copy
 
+from types import ModuleType
+from typing import (
+    TypeVar, Type, Tuple, List, Dict, Iterator, Collection, Callable, Optional, FrozenSet, Any,
+    Pattern as REPattern, ClassVar, TYPE_CHECKING
+)
 
-class Pattern(Serialize):
-    raw = None
-    type = None
+if TYPE_CHECKING:
+    from .common import LexerConf
 
-    def __init__(self, value, flags=(), raw=None):
+class Pattern(Serialize, ABC):
+
+    value: str
+    flags: Collection[str]
+    raw: Optional[str]
+    type: ClassVar[str]
+
+    def __init__(self, value: str, flags: Collection[str]=(), raw: Optional[str]=None) -> None:
         self.value = value
         self.flags = frozenset(flags)
         self.raw = raw
@@ -29,41 +41,58 @@ class Pattern(Serialize):
     def __eq__(self, other):
         return type(self) == type(other) and self.value == other.value and self.flags == other.flags
 
-    def to_regexp(self):
+    @abstractmethod
+    def to_regexp(self) -> str:
         raise NotImplementedError()
 
-    def min_width(self):
+    @property
+    @abstractmethod
+    def min_width(self) -> int:
         raise NotImplementedError()
 
-    def max_width(self):
+    @property
+    @abstractmethod
+    def max_width(self) -> int:
         raise NotImplementedError()
 
-    def _get_flags(self, value):
-        for f in self.flags:
-            value = ('(?%s:%s)' % (f, value))
-        return value
+    if Py36:
+        # Python 3.6 changed syntax for flags in regular expression
+        def _get_flags(self, value):
+            for f in self.flags:
+                value = ('(?%s:%s)' % (f, value))
+            return value
+
+    else:
+        def _get_flags(self, value):
+            for f in self.flags:
+                value = ('(?%s)' % f) + value
+            return value
+
 
 
 class PatternStr(Pattern):
     __serialize_fields__ = 'value', 'flags'
 
-    type = "str"
+    type: ClassVar[str] = "str"
 
-    def to_regexp(self):
+    def to_regexp(self) -> str:
         return self._get_flags(re.escape(self.value))
 
     @property
-    def min_width(self):
+    def min_width(self) -> int:
+        return len(self.value)
+
+    @property
+    def max_width(self) -> int:
         return len(self.value)
-    max_width = min_width
 
 
 class PatternRE(Pattern):
     __serialize_fields__ = 'value', 'flags', '_width'
 
-    type = "re"
+    type: ClassVar[str] = "re"
 
-    def to_regexp(self):
+    def to_regexp(self) -> str:
         return self._get_flags(self.value)
 
     _width = None
@@ -73,11 +102,11 @@ class PatternRE(Pattern):
         return self._width
 
     @property
-    def min_width(self):
+    def min_width(self) -> int:
         return self._get_width()[0]
 
     @property
-    def max_width(self):
+    def max_width(self) -> int:
         return self._get_width()[1]
 
 
@@ -85,7 +114,11 @@ class TerminalDef(Serialize):
     __serialize_fields__ = 'name', 'pattern', 'priority'
     __serialize_namespace__ = PatternStr, PatternRE
 
-    def __init__(self, name, pattern, priority=1):
+    name: str
+    pattern: Pattern
+    priority: int
+
+    def __init__(self, name: str, pattern: Pattern, priority: int=1) -> None:
         assert isinstance(pattern, Pattern), pattern
         self.name = name
         self.pattern = pattern
@@ -94,12 +127,13 @@ class TerminalDef(Serialize):
     def __repr__(self):
         return '%s(%r, %r)' % (type(self).__name__, self.name, self.pattern)
 
-    def user_repr(self):
+    def user_repr(self) -> str:
         if self.name.startswith('__'): # We represent a generated terminal
             return self.pattern.raw or self.name
         else:
             return self.name
 
+_T = TypeVar('_T')
 
 class Token(str):
     """A string with meta-information, that is produced by the lexer.
@@ -122,6 +156,15 @@ class Token(str):
     """
     __slots__ = ('type', 'start_pos', 'value', 'line', 'column', 'end_line', 'end_column', 'end_pos')
 
+    type: str
+    start_pos: int
+    value: Any
+    line: int
+    column: int
+    end_line: int
+    end_column: int
+    end_pos: int
+
     def __new__(cls, type_, value, start_pos=None, line=None, column=None, end_line=None, end_column=None, end_pos=None):
         try:
             self = super(Token, cls).__new__(cls, value)
@@ -139,7 +182,7 @@ class Token(str):
         self.end_pos = end_pos
         return self
 
-    def update(self, type_=None, value=None):
+    def update(self, type_: Optional[str]=None, value: Optional[Any]=None) -> 'Token':
         return Token.new_borrow_pos(
             type_ if type_ is not None else self.type,
             value if value is not None else self.value,
@@ -147,7 +190,7 @@ class Token(str):
         )
 
     @classmethod
-    def new_borrow_pos(cls, type_, value, borrow_t):
+    def new_borrow_pos(cls: Type[_T], type_: str, value: Any, borrow_t: 'Token') -> _T:
         return cls(type_, value, borrow_t.start_pos, borrow_t.line, borrow_t.column, borrow_t.end_line, borrow_t.end_column, borrow_t.end_pos)
 
     def __reduce__(self):
@@ -281,13 +324,35 @@ def _regexp_has_newline(r):
     return '\n' in r or '\\n' in r or '\\s' in r or '[^' in r or ('(?s' in r and '.' in r)
 
 
-class Lexer(object):
+class LexerState(object):
+    __slots__ = 'text', 'line_ctr', 'last_token'
+
+    def __init__(self, text, line_ctr, last_token=None):
+        self.text = text
+        self.line_ctr = line_ctr
+        self.last_token = last_token
+
+    def __eq__(self, other):
+        if not isinstance(other, LexerState):
+            return NotImplemented
+
+        return self.text is other.text and self.line_ctr == other.line_ctr and self.last_token == other.last_token
+
+    def __copy__(self):
+        return type(self)(self.text, copy(self.line_ctr), self.last_token)
+
+
+_Callback = Callable[[Token], Token]
+
+class Lexer(ABC):
     """Lexer interface
 
     Method Signatures:
-        lex(self, text) -> Iterator[Token]
+        lex(self, lexer_state, parser_state) -> Iterator[Token]
     """
-    lex = NotImplemented
+    @abstractmethod
+    def lex(self, lexer_state: LexerState, parser_state: Any) -> Iterator[Token]:
+        ...
 
     def make_lexer_state(self, text):
         line_ctr = LineCounter(b'\n' if isinstance(text, bytes) else '\n')
@@ -296,7 +361,14 @@ class Lexer(object):
 
 class TraditionalLexer(Lexer):
 
-    def __init__(self, conf):
+    terminals: Collection[TerminalDef]
+    ignore_types: FrozenSet[str]
+    newline_types: FrozenSet[str]
+    user_callbacks: Dict[str, _Callback]
+    callback: Dict[str, _Callback]
+    re: ModuleType
+
+    def __init__(self, conf: 'LexerConf') -> None:
         terminals = list(conf.terminals)
         assert all(isinstance(t, TerminalDef) for t in terminals), terminals
 
@@ -329,7 +401,7 @@ class TraditionalLexer(Lexer):
 
         self._mres = None
 
-    def _build(self):
+    def _build(self) -> None:
         terminals, self.callback = _create_unless(self.terminals, self.g_regex_flags, self.re, self.use_bytes)
         assert all(self.callback.values())
 
@@ -343,23 +415,24 @@ class TraditionalLexer(Lexer):
         self._mres = build_mres(terminals, self.g_regex_flags, self.re, self.use_bytes)
 
     @property
-    def mres(self):
+    def mres(self) -> List[Tuple[REPattern, Dict[int, str]]]:
         if self._mres is None:
             self._build()
+            assert self._mres is not None
         return self._mres
 
-    def match(self, text, pos):
+    def match(self, text: str, pos: int) -> Optional[Tuple[str, str]]:
         for mre, type_from_index in self.mres:
             m = mre.match(text, pos)
             if m:
                 return m.group(0), type_from_index[m.lastindex]
 
-    def lex(self, state, parser_state):
+    def lex(self, state: LexerState, parser_state: Any) -> Iterator[Token]:
         with suppress(EOFError):
             while True:
                 yield self.next_token(state, parser_state)
 
-    def next_token(self, lex_state, parser_state=None):
+    def next_token(self, lex_state: LexerState, parser_state: Any=None) -> Token:
         line_ctr = lex_state.line_ctr
         while line_ctr.char_pos < len(lex_state.text):
             res = self.match(lex_state.text, line_ctr.char_pos)
@@ -395,27 +468,12 @@ class TraditionalLexer(Lexer):
         raise EOFError(self)
 
 
-class LexerState(object):
-    __slots__ = 'text', 'line_ctr', 'last_token'
-
-    def __init__(self, text, line_ctr, last_token=None):
-        self.text = text
-        self.line_ctr = line_ctr
-        self.last_token = last_token
-
-    def __eq__(self, other):
-        if not isinstance(other, LexerState):
-            return NotImplemented
-
-        return self.text is other.text and self.line_ctr == other.line_ctr and self.last_token == other.last_token
-
-    def __copy__(self):
-        return type(self)(self.text, copy(self.line_ctr), self.last_token)
-
-
 class ContextualLexer(Lexer):
 
-    def __init__(self, conf, states, always_accept=()):
+    lexers: Dict[str, TraditionalLexer]
+    root_lexer: TraditionalLexer
+
+    def __init__(self, conf: 'LexerConf', states: Dict[str, Collection[str]], always_accept: Collection[str]=()) -> None:
         terminals = list(conf.terminals)
         terminals_by_name = conf.terminals_by_name
 
@@ -443,7 +501,7 @@ class ContextualLexer(Lexer):
     def make_lexer_state(self, text):
         return self.root_lexer.make_lexer_state(text)
 
-    def lex(self, lexer_state, parser_state):
+    def lex(self, lexer_state: LexerState, parser_state: Any) -> Iterator[Token]:
         try:
             while True:
                 lexer = self.lexers[parser_state.position]
diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index d3c310d..3e9b95a 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -8,8 +8,9 @@ import pkgutil
 from ast import literal_eval
 from numbers import Integral
 from contextlib import suppress
+from typing import List, Tuple, Union, Callable, Dict, Optional
 
-from .utils import bfs, logger, classify_bool, is_id_continue, is_id_start, bfs_all_unique
+from .utils import bfs, Py36, logger, classify_bool, is_id_continue, is_id_start, bfs_all_unique
 from .lexer import Token, TerminalDef, PatternStr, PatternRE
 
 from .parse_tree_builder import ParseTreeBuilder
@@ -17,7 +18,7 @@ from .parser_frontends import ParsingFrontend
 from .common import LexerConf, ParserConf
 from .grammar import RuleOptions, Rule, Terminal, NonTerminal, Symbol
 from .utils import classify, dedup_list
-from .exceptions import GrammarError, UnexpectedCharacters, UnexpectedToken, ParseError
+from .exceptions import GrammarError, UnexpectedCharacters, UnexpectedToken, ParseError, UnexpectedInput
 
 from .tree import Tree, SlottedTree as ST
 from .visitors import Transformer, Visitor, v_args, Transformer_InPlace, Transformer_NonRecursive
@@ -475,7 +476,18 @@ class PrepareLiterals(Transformer_InPlace):
 
 
 def _make_joined_pattern(regexp, flags_set):
-    return PatternRE(regexp, ())
+    # In Python 3.6, a new syntax for flags was introduced, that allows us to restrict the scope
+    # of flags to a specific regexp group. We are already using it in `lexer.Pattern._get_flags`
+    # However, for prior Python versions, we still need to use global flags, so we have to make sure
+    # that there are no flag collisions when we merge several terminals.
+    flags = ()
+    if not Py36:
+        if len(flags_set) > 1:
+            raise GrammarError("Lark doesn't support joining terminals with conflicting flags in python <3.6!")
+        elif len(flags_set) == 1:
+            flags ,= flags_set
+
+    return PatternRE(regexp, flags)
 
 
 class TerminalTreeToPattern(Transformer):
@@ -540,7 +552,12 @@ def nr_deepcopy_tree(t):
 
 
 class Grammar:
-    def __init__(self, rule_defs, term_defs, ignore):
+
+    term_defs: List[Tuple[str, Tuple[Tree, int]]]
+    rule_defs: List[Tuple[str, Tuple[str, ...], Tree, RuleOptions]]
+    ignore: List[str]
+
+    def __init__(self, rule_defs: List[Tuple[str, Tuple[str, ...], Tree, RuleOptions]], term_defs: List[Tuple[str, Tuple[Tree, int]]], ignore: List[str]) -> None:
         self.term_defs = term_defs
         self.rule_defs = rule_defs
         self.ignore = ignore
@@ -679,14 +696,18 @@ class FromPackageLoader(object):
     pkg_name: The name of the package. You can probably provide `__name__` most of the time
     search_paths: All the path that will be search on absolute imports.
     """
-    def __init__(self, pkg_name, search_paths=("", )):
+
+    pkg_name: str
+    search_paths: Tuple[str, ...]
+
+    def __init__(self, pkg_name: str, search_paths: Tuple[str, ...]=("", )) -> None:
         self.pkg_name = pkg_name
         self.search_paths = search_paths
 
     def __repr__(self):
         return "%s(%r, %r)" % (type(self).__name__, self.pkg_name, self.search_paths)
 
-    def __call__(self, base_path, grammar_path):
+    def __call__(self, base_path: Union[None, str, PackageResource], grammar_path: str) -> Tuple[PackageResource, str]:
         if base_path is None:
             to_try = self.search_paths
         else:
@@ -863,7 +884,7 @@ def _search_interactive_parser(interactive_parser, predicate):
         if predicate(p):
             return path, p
 
-def find_grammar_errors(text, start='start'):
+def find_grammar_errors(text: str, start: str='start') -> List[Tuple[UnexpectedInput, str]]:
     errors = []
     def on_error(e):
         errors.append((e, _error_repr(e)))
@@ -912,7 +933,12 @@ def _mangle_exp(exp, mangle):
 
 
 class GrammarBuilder:
-    def __init__(self, global_keep_all_tokens=False, import_paths=None, used_files=None):
+
+    global_keep_all_tokens: bool
+    import_paths: List[Union[str, Callable]]
+    used_files: Dict[str, str]
+
+    def __init__(self, global_keep_all_tokens: bool=False, import_paths: Optional[List[Union[str, Callable]]]=None, used_files: Optional[Dict[str, str]]=None) -> None:
         self.global_keep_all_tokens = global_keep_all_tokens
         self.import_paths = import_paths or []
         self.used_files = used_files or {}
@@ -1056,7 +1082,7 @@ class GrammarBuilder:
         return name, exp, params, opts
 
 
-    def load_grammar(self, grammar_text, grammar_name="<?>", mangle=None):
+    def load_grammar(self, grammar_text: str, grammar_name: str="<?>", mangle: Optional[Callable[[str], str]]=None) -> None:
         tree = _parse_grammar(grammar_text, grammar_name)
 
         imports = {}
@@ -1119,7 +1145,7 @@ class GrammarBuilder:
         self._definitions = {k: v for k, v in self._definitions.items() if k in _used}
 
 
-    def do_import(self, dotted_path, base_path, aliases, base_mangle=None):
+    def do_import(self, dotted_path: Tuple[str, ...], base_path: Optional[str], aliases: Dict[str, str], base_mangle: Optional[Callable[[str], str]]=None) -> None:
         assert dotted_path
         mangle = _get_mangle('__'.join(dotted_path), aliases, base_mangle)
         grammar_path = os.path.join(*dotted_path) + EXT
@@ -1155,7 +1181,7 @@ class GrammarBuilder:
             assert False, "Couldn't import grammar %s, but a corresponding file was found at a place where lark doesn't search for it" % (dotted_path,)
 
 
-    def validate(self):
+    def validate(self) -> None:
         for name, (params, exp, _options) in self._definitions.items():
             for i, p in enumerate(params):
                 if p in self._definitions:
@@ -1184,7 +1210,7 @@ class GrammarBuilder:
         if not set(self._definitions).issuperset(self._ignore_names):
             raise GrammarError("Terminals %s were marked to ignore but were not defined!" % (set(self._ignore_names) - set(self._definitions)))
 
-    def build(self):
+    def build(self) -> Grammar:
         self.validate()
         rule_defs = []
         term_defs = []
diff --git a/lark/parser_frontends.py b/lark/parser_frontends.py
index e066d9a..926997a 100644
--- a/lark/parser_frontends.py
+++ b/lark/parser_frontends.py
@@ -7,7 +7,7 @@ from .parsers.lalr_parser import LALR_Parser
 from .tree import Tree
 from .common import LexerConf, ParserConf
 try:
-    import regex
+    import regex  # type: ignore
 except ImportError:
     regex = None
 import re
diff --git a/lark/reconstruct.py b/lark/reconstruct.py
index ab2fb38..aa8c753 100644
--- a/lark/reconstruct.py
+++ b/lark/reconstruct.py
@@ -1,11 +1,13 @@
 """Reconstruct text from a tree, based on Lark grammar"""
 
+from typing import List, Dict, Union, Callable, Iterable, Optional
 import unicodedata
 
+from .lark import Lark
 from .tree import Tree
 from .visitors import Transformer_InPlace
-from .lexer import Token, PatternStr
-from .grammar import Terminal, NonTerminal
+from .lexer import Token, PatternStr, TerminalDef
+from .grammar import Terminal, NonTerminal, Symbol
 
 from .tree_matcher import TreeMatcher, is_discarded_terminal
 from .utils import is_id_continue
@@ -21,7 +23,10 @@ def is_iter_empty(i):
 class WriteTokensTransformer(Transformer_InPlace):
     "Inserts discarded tokens into their correct place, according to the rules of grammar"
 
-    def __init__(self, tokens, term_subs):
+    tokens: Dict[str, TerminalDef]
+    term_subs: Dict[str, Callable[[Symbol], str]]
+
+    def __init__(self, tokens: Dict[str, TerminalDef], term_subs: Dict[str, Callable[[Symbol], str]]) -> None:
         self.tokens = tokens
         self.term_subs = term_subs
 
@@ -70,7 +75,9 @@ class Reconstructor(TreeMatcher):
         term_subs: a dictionary of [Terminal name as str] to [output text as str]
     """
 
-    def __init__(self, parser, term_subs=None):
+    write_tokens: WriteTokensTransformer
+
+    def __init__(self, parser: Lark, term_subs: Optional[Dict[str, Callable[[Symbol], str]]]=None) -> None:
         TreeMatcher.__init__(self, parser)
 
         self.write_tokens = WriteTokensTransformer({t.name:t for t in self.tokens}, term_subs or {})
@@ -87,7 +94,7 @@ class Reconstructor(TreeMatcher):
             else:
                 yield item
 
-    def reconstruct(self, tree, postproc=None, insert_spaces=True):
+    def reconstruct(self, tree: Tree, postproc: Optional[Callable[[Iterable[str]], Iterable[str]]]=None, insert_spaces: bool=True) -> str:
         x = self._reconstruct(tree)
         if postproc:
             x = postproc(x)
diff --git a/lark/tree.py b/lark/tree.py
index 468894a..90ec0fe 100644
--- a/lark/tree.py
+++ b/lark/tree.py
@@ -1,16 +1,35 @@
 try:
-    from future_builtins import filter
+    from future_builtins import filter  # type: ignore
 except ImportError:
     pass
 
+import sys
 from copy import deepcopy
 
 
 ###{standalone
 from collections import OrderedDict
+from typing import List, Callable, Iterator, Union, Optional, Any, TYPE_CHECKING
 
+if TYPE_CHECKING:
+    from .lexer import TerminalDef
+    if sys.version_info >= (3, 8):
+        from typing import Literal
+    else:
+        from typing_extensions import Literal
 
 class Meta:
+
+    empty: bool
+    line: int
+    column: int
+    start_pos: int
+    end_line: int
+    end_column: int
+    end_pos: int
+    orig_expansion: 'List[TerminalDef]'
+    match_tree: bool
+
     def __init__(self):
         self.empty = True
 
@@ -27,13 +46,17 @@ class Tree(object):
         meta: Line & Column numbers (if ``propagate_positions`` is enabled).
             meta attributes: line, column, start_pos, end_line, end_column, end_pos
     """
-    def __init__(self, data, children, meta=None):
+
+    data: str
+    children: 'List[Union[str, Tree]]'
+
+    def __init__(self, data: str, children: 'List[Union[str, Tree]]', meta: Optional[Meta]=None) -> None:
         self.data = data
         self.children = children
         self._meta = meta
 
     @property
-    def meta(self):
+    def meta(self) -> Meta:
         if self._meta is None:
             self._meta = Meta()
         return self._meta
@@ -57,7 +80,7 @@ class Tree(object):
 
         return l
 
-    def pretty(self, indent_str='  '):
+    def pretty(self, indent_str: str='  ') -> str:
         """Returns an indented string representation of the tree.
 
         Great for debugging.
@@ -73,10 +96,10 @@ class Tree(object):
     def __ne__(self, other):
         return not (self == other)
 
-    def __hash__(self):
+    def __hash__(self) -> int:
         return hash((self.data, tuple(self.children)))
 
-    def iter_subtrees(self):
+    def iter_subtrees(self) -> 'Iterator[Tree]':
         """Depth-first iteration.
 
         Iterates over all the subtrees, never returning to the same node twice (Lark's parse-tree is actually a DAG).
@@ -91,23 +114,23 @@ class Tree(object):
         del queue
         return reversed(list(subtrees.values()))
 
-    def find_pred(self, pred):
+    def find_pred(self, pred: 'Callable[[Tree], bool]') -> 'Iterator[Tree]':
         """Returns all nodes of the tree that evaluate pred(node) as true."""
         return filter(pred, self.iter_subtrees())
 
-    def find_data(self, data):
+    def find_data(self, data: str) -> 'Iterator[Tree]':
         """Returns all nodes of the tree whose data equals the given data."""
         return self.find_pred(lambda t: t.data == data)
 
 ###}
 
-    def expand_kids_by_index(self, *indices):
+    def expand_kids_by_index(self, *indices: int) -> None:
         """Expand (inline) children at the given indices"""
         for i in sorted(indices, reverse=True):  # reverse so that changing tail won't affect indices
             kid = self.children[i]
             self.children[i:i+1] = kid.children
 
-    def scan_values(self, pred):
+    def scan_values(self, pred: 'Callable[[Union[str, Tree]], bool]') -> Iterator[str]:
         """Return all values in the tree that evaluate pred(value) as true.
 
         This can be used to find all the tokens in the tree.
@@ -140,10 +163,10 @@ class Tree(object):
     def __deepcopy__(self, memo):
         return type(self)(self.data, deepcopy(self.children, memo), meta=self._meta)
 
-    def copy(self):
+    def copy(self) -> 'Tree':
         return type(self)(self.data, self.children)
 
-    def set(self, data, children):
+    def set(self, data: str, children: 'List[Union[str, Tree]]') -> None:
         self.data = data
         self.children = children
 
@@ -153,7 +176,7 @@ class SlottedTree(Tree):
     __slots__ = 'data', 'children', 'rule', '_meta'
 
 
-def pydot__tree_to_png(tree, filename, rankdir="LR", **kwargs):
+def pydot__tree_to_png(tree: Tree, filename: str, rankdir: 'Literal["TB", "LR", "BT", "RL"]'="LR", **kwargs) -> None:
     graph = pydot__tree_to_graph(tree, rankdir, **kwargs)
     graph.write_png(filename)
 
@@ -174,7 +197,7 @@ def pydot__tree_to_graph(tree, rankdir="LR", **kwargs):
     possible attributes, see https://www.graphviz.org/doc/info/attrs.html.
     """
 
-    import pydot
+    import pydot  # type: ignore
     graph = pydot.Dot(graph_type='digraph', rankdir=rankdir, **kwargs)
 
     i = [0]
diff --git a/lark/utils.py b/lark/utils.py
index c210dca..1214e97 100644
--- a/lark/utils.py
+++ b/lark/utils.py
@@ -6,13 +6,15 @@ from collections import deque
 ###{standalone
 import sys, re
 import logging
-logger = logging.getLogger("lark")
+logger: logging.Logger = logging.getLogger("lark")
 logger.addHandler(logging.StreamHandler())
 # Set to highest level, since we have some warnings amongst the code
 # By default, we should not output any log messages
 logger.setLevel(logging.CRITICAL)
 
 
+Py36 = (sys.version_info[:2] >= (3, 6))
+
 NO_VALUE = object()
 
 
@@ -132,7 +134,7 @@ def smart_decorator(f, create_decorator):
 
 
 try:
-    import regex
+    import regex  # type: ignore
 except ImportError:
     regex = None
 
diff --git a/lark/visitors.py b/lark/visitors.py
index 22beb47..2c7309f 100644
--- a/lark/visitors.py
+++ b/lark/visitors.py
@@ -1,3 +1,4 @@
+from abc import ABC
 from functools import wraps
 
 from .utils import smart_decorator, combine_alternatives
@@ -7,7 +8,12 @@ from .lexer import Token
 
 ###{standalone
 from inspect import getmembers, getmro
+from typing import TypeVar, Tuple, List, Callable, Generic, Type, Union, Optional
 
+_T = TypeVar('_T')
+_R = TypeVar('_R')
+_FUNC = Callable[..., _T]
+_DECORATED = Union[_FUNC, type]
 
 class Discard(Exception):
     """When raising the Discard exception in a transformer callback,
@@ -46,7 +52,7 @@ class _Decoratable:
         return cls
 
 
-class Transformer(_Decoratable):
+class Transformer(_Decoratable, ABC, Generic[_T]):
     """Transformers visit each node of the tree, and run the appropriate method on it according to the node's data.
 
     Methods are provided by the user via inheritance, and called according to ``tree.data``.
@@ -74,7 +80,7 @@ class Transformer(_Decoratable):
     """
     __visit_tokens__ = True   # For backwards compatibility
 
-    def __init__(self,  visit_tokens=True):
+    def __init__(self,  visit_tokens: bool=True) -> None:
         self.__visit_tokens__ = visit_tokens
 
     def _call_userfunc(self, tree, new_children=None):
@@ -125,11 +131,11 @@ class Transformer(_Decoratable):
         children = list(self._transform_children(tree.children))
         return self._call_userfunc(tree, children)
 
-    def transform(self, tree):
+    def transform(self, tree: Tree) -> _T:
         "Transform the given tree, and return the final result"
         return self._transform_tree(tree)
 
-    def __mul__(self, other):
+    def __mul__(self, other: 'Transformer[_T]') -> 'TransformerChain[_T]':
         """Chain two transformers together, returning a new transformer.
         """
         return TransformerChain(self, other)
@@ -149,16 +155,19 @@ class Transformer(_Decoratable):
         return token
 
 
-class TransformerChain(object):
-    def __init__(self, *transformers):
+class TransformerChain(Generic[_T]):
+
+    transformers: Tuple[Transformer[_T], ...]
+
+    def __init__(self, *transformers: Transformer[_T]) -> None:
         self.transformers = transformers
 
-    def transform(self, tree):
+    def transform(self, tree: Tree) -> _T:
         for t in self.transformers:
             tree = t.transform(tree)
         return tree
 
-    def __mul__(self, other):
+    def __mul__(self, other: Transformer[_T]) -> 'TransformerChain[_T]':
         return TransformerChain(*self.transformers + (other,))
 
 
@@ -239,19 +248,19 @@ class VisitorBase:
         return cls
 
 
-class Visitor(VisitorBase):
+class Visitor(VisitorBase, ABC, Generic[_T]):
     """Tree visitor, non-recursive (can handle huge trees).
 
     Visiting a node calls its methods (provided by the user via inheritance) according to ``tree.data``
     """
 
-    def visit(self, tree):
+    def visit(self, tree: Tree) -> Tree:
         "Visits the tree, starting with the leaves and finally the root (bottom-up)"
         for subtree in tree.iter_subtrees():
             self._call_userfunc(subtree)
         return tree
 
-    def visit_topdown(self,tree):
+    def visit_topdown(self, tree: Tree) -> Tree:
         "Visit the tree, starting at the root, and ending at the leaves (top-down)"
         for subtree in tree.iter_subtrees_topdown():
             self._call_userfunc(subtree)
@@ -266,7 +275,7 @@ class Visitor_Recursive(VisitorBase):
     Slightly faster than the non-recursive version.
     """
 
-    def visit(self, tree):
+    def visit(self, tree: Tree) -> Tree:
         "Visits the tree, starting with the leaves and finally the root (bottom-up)"
         for child in tree.children:
             if isinstance(child, Tree):
@@ -275,7 +284,7 @@ class Visitor_Recursive(VisitorBase):
         self._call_userfunc(tree)
         return tree
 
-    def visit_topdown(self,tree):
+    def visit_topdown(self,tree: Tree) -> Tree:
         "Visit the tree, starting at the root, and ending at the leaves (top-down)"
         self._call_userfunc(tree)
 
@@ -286,16 +295,7 @@ class Visitor_Recursive(VisitorBase):
         return tree
 
 
-def visit_children_decor(func):
-    "See Interpreter"
-    @wraps(func)
-    def inner(cls, tree):
-        values = cls.visit_children(tree)
-        return func(cls, values)
-    return inner
-
-
-class Interpreter(_Decoratable):
+class Interpreter(_Decoratable, ABC, Generic[_T]):
     """Interpreter walks the tree starting at the root.
 
     Visits the tree, starting with the root and finally the leaves (top-down)
@@ -307,7 +307,7 @@ class Interpreter(_Decoratable):
     This allows the user to implement branching and loops.
     """
 
-    def visit(self, tree):
+    def visit(self, tree: Tree) -> _T:
         f = getattr(self, tree.data)
         wrapper = getattr(f, 'visit_wrapper', None)
         if wrapper is not None:
@@ -315,7 +315,7 @@ class Interpreter(_Decoratable):
         else:
             return f(tree)
 
-    def visit_children(self, tree):
+    def visit_children(self, tree: Tree) -> List[_T]:
         return [self.visit(child) if isinstance(child, Tree) else child
                 for child in tree.children]
 
@@ -326,6 +326,16 @@ class Interpreter(_Decoratable):
         return self.visit_children(tree)
 
 
+_InterMethod = Callable[[Type[Interpreter], _T], _R]
+
+def visit_children_decor(func: _InterMethod) -> _InterMethod:
+    "See Interpreter"
+    @wraps(func)
+    def inner(cls, tree):
+        values = cls.visit_children(tree)
+        return func(cls, values)
+    return inner
+
 # Decorators
 
 def _apply_decorator(obj, decorator, **kwargs):
@@ -380,7 +390,7 @@ def _vargs_tree(f, data, children, meta):
     return f(Tree(data, children, meta))
 
 
-def v_args(inline=False, meta=False, tree=False, wrapper=None):
+def v_args(inline: bool=False, meta: bool=False, tree: bool=False, wrapper: Optional[Callable]=None) -> Callable[[_DECORATED], _DECORATED]:
     """A convenience decorator factory for modifying the behavior of user-supplied visitor methods.
 
     By default, callback methods of transformers/visitors accept one argument - a list of the node's children.
diff --git a/tox.ini b/tox.ini
index 33f9f64..ef19e2c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,12 +1,16 @@
 [tox]
-envlist = py36, py37, py38, py39, pypy, pypy3
+envlist = py27, py34, py35, py36, py37, py38, py39, pypy, pypy3
 skip_missing_interpreters=true
 
 [travis]
+2.7 = py27
+3.4 = py34
+3.5 = py35
 3.6 = py36
 3.7 = py37
 3.8 = py38
 3.9 = py39
+pypy = pypy
 pypy3 = pypy3
 
 [testenv]


Test patch:
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index a635e58..c106bc6 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9.0-rc - 3.9, pypy3]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9.0-rc - 3.9, pypy3]
 
     steps:
       - uses: actions/checkout@v2
diff --git a/tests/test_parser.py b/tests/test_parser.py
index ac409b3..ff4e064 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -8,7 +8,7 @@ import os
 import sys
 from copy import copy, deepcopy
 
-from lark.utils import isascii
+from lark.utils import Py36, isascii
 
 from lark import Token, Transformer_NonRecursive, LexError
 
@@ -1545,6 +1545,7 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual( g.parse('"hello"').children, ['"hello"'])
             self.assertEqual( g.parse("'hello'").children, ["'hello'"])
 
+        @unittest.skipIf(not Py36, "Required re syntax only exists in python3.6+")
         def test_join_regex_flags(self):
             g = r"""
                 start: A