Repository: PyCQA/pyflakes

Issue:
python 3.8 SyntaxWarning: "is" with a literal. Did you mean "=="?
Hi there -

I searched a bit for this but couldn't find it.   This is a warning generated by Python 3.8 and I think it's new.  It's very hard to get pytest to raise for this, so it would be better if it were in pyflakes / flake8.

demo:

```
"""test a syntax warning."""

x = 5

if x is ():
    print("hi")

```

```
$ /opt/python3.8/bin/python3 -m py_compile test.py
test.py:5: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if x is ():

```



Comments:

Patch:
diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index eca2002..351ab12 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -78,6 +78,51 @@ else:
     LOOP_TYPES = (ast.While, ast.For)
     FUNCTION_TYPES = (ast.FunctionDef,)
 
+
+if PY38_PLUS:
+    def _is_singleton(node):  # type: (ast.AST) -> bool
+        return (
+            isinstance(node, ast.Constant) and
+            isinstance(node.value, (bool, type(Ellipsis), type(None)))
+        )
+elif not PY2:
+    def _is_singleton(node):  # type: (ast.AST) -> bool
+        return isinstance(node, (ast.NameConstant, ast.Ellipsis))
+else:
+    def _is_singleton(node):  # type: (ast.AST) -> bool
+        return (
+            isinstance(node, ast.Name) and
+            node.id in {'True', 'False', 'Ellipsis', 'None'}
+        )
+
+
+def _is_tuple_constant(node):  # type: (ast.AST) -> bool
+    return (
+        isinstance(node, ast.Tuple) and
+        all(_is_constant(elt) for elt in node.elts)
+    )
+
+
+if PY38_PLUS:
+    def _is_constant(node):
+        return isinstance(node, ast.Constant) or _is_tuple_constant(node)
+else:
+    _const_tps = (ast.Str, ast.Num)
+    if not PY2:
+        _const_tps += (ast.Bytes,)
+
+    def _is_constant(node):
+        return (
+            isinstance(node, _const_tps) or
+            _is_singleton(node) or
+            _is_tuple_constant(node)
+        )
+
+
+def _is_const_non_singleton(node):  # type: (ast.AST) -> bool
+    return _is_constant(node) and not _is_singleton(node)
+
+
 # https://github.com/python/typed_ast/blob/1.4.0/ast27/Parser/tokenizer.c#L102-L104
 TYPE_COMMENT_RE = re.compile(r'^#\s*type:\s*')
 # https://github.com/python/typed_ast/blob/1.4.0/ast27/Parser/tokenizer.c#L1408-L1413
@@ -618,40 +663,55 @@ def getNodeName(node):
         return node.name
 
 
-def is_typing_overload(value, scope_stack):
-    def name_is_typing_overload(name):  # type: (str) -> bool
+def _is_typing(node, typing_attr, scope_stack):
+    def _bare_name_is_attr(name):
+        expected_typing_names = {
+            'typing.{}'.format(typing_attr),
+            'typing_extensions.{}'.format(typing_attr),
+        }
         for scope in reversed(scope_stack):
             if name in scope:
                 return (
                     isinstance(scope[name], ImportationFrom) and
-                    scope[name].fullName in (
-                        'typing.overload', 'typing_extensions.overload',
-                    )
+                    scope[name].fullName in expected_typing_names
                 )
 
         return False
 
-    def is_typing_overload_decorator(node):
-        return (
-            (
-                isinstance(node, ast.Name) and name_is_typing_overload(node.id)
-            ) or (
-                isinstance(node, ast.Attribute) and
-                isinstance(node.value, ast.Name) and
-                node.value.id == 'typing' and
-                node.attr == 'overload'
-            )
+    return (
+        (
+            isinstance(node, ast.Name) and
+            _bare_name_is_attr(node.id)
+        ) or (
+            isinstance(node, ast.Attribute) and
+            isinstance(node.value, ast.Name) and
+            node.value.id in {'typing', 'typing_extensions'} and
+            node.attr == typing_attr
         )
+    )
+
 
+def is_typing_overload(value, scope_stack):
     return (
         isinstance(value.source, FUNCTION_TYPES) and
         any(
-            is_typing_overload_decorator(dec)
+            _is_typing(dec, 'overload', scope_stack)
             for dec in value.source.decorator_list
         )
     )
 
 
+def in_annotation(func):
+    @functools.wraps(func)
+    def in_annotation_func(self, *args, **kwargs):
+        orig, self._in_annotation = self._in_annotation, True
+        try:
+            return func(self, *args, **kwargs)
+        finally:
+            self._in_annotation = orig
+    return in_annotation_func
+
+
 def make_tokens(code):
     # PY3: tokenize.tokenize requires readline of bytes
     if not isinstance(code, bytes):
@@ -738,6 +798,9 @@ class Checker(object):
     nodeDepth = 0
     offset = None
     traceTree = False
+    _in_annotation = False
+    _in_typing_literal = False
+    _in_deferred = False
 
     builtIns = set(builtin_vars).union(_MAGIC_GLOBALS)
     _customBuiltIns = os.environ.get('PYFLAKES_BUILTINS')
@@ -769,6 +832,7 @@ class Checker(object):
         for builtin in self.builtIns:
             self.addBinding(None, Builtin(builtin))
         self.handleChildren(tree)
+        self._in_deferred = True
         self.runDeferred(self._deferredFunctions)
         # Set _deferredFunctions to None so that deferFunction will fail
         # noisily if called after we've run through the deferred functions.
@@ -1009,12 +1073,30 @@ class Checker(object):
 
         self.scope[value.name] = value
 
+    def _unknown_handler(self, node):
+        # this environment variable configures whether to error on unknown
+        # ast types.
+        #
+        # this is silent by default but the error is enabled for the pyflakes
+        # testsuite.
+        #
+        # this allows new syntax to be added to python without *requiring*
+        # changes from the pyflakes side.  but will still produce an error
+        # in the pyflakes testsuite (so more specific handling can be added if
+        # needed).
+        if os.environ.get('PYFLAKES_ERROR_UNKNOWN'):
+            raise NotImplementedError('Unexpected type: {}'.format(type(node)))
+        else:
+            self.handleChildren(node)
+
     def getNodeHandler(self, node_class):
         try:
             return self._nodeHandlers[node_class]
         except KeyError:
             nodeType = getNodeType(node_class)
-        self._nodeHandlers[node_class] = handler = getattr(self, nodeType)
+        self._nodeHandlers[node_class] = handler = getattr(
+            self, nodeType, self._unknown_handler,
+        )
         return handler
 
     def handleNodeLoad(self, node):
@@ -1281,6 +1363,7 @@ class Checker(object):
         self.popScope()
         self.scopeStack = saved_stack
 
+    @in_annotation
     def handleStringAnnotation(self, s, node, ref_lineno, ref_col_offset, err):
         try:
             tree = ast.parse(s)
@@ -1304,6 +1387,7 @@ class Checker(object):
 
         self.handleNode(parsed_annotation, node)
 
+    @in_annotation
     def handleAnnotation(self, annotation, node):
         if isinstance(annotation, ast.Str):
             # Defer handling forward annotation.
@@ -1316,7 +1400,8 @@ class Checker(object):
                 messages.ForwardAnnotationSyntaxError,
             ))
         elif self.annotationsFutureEnabled:
-            self.deferFunction(lambda: self.handleNode(annotation, node))
+            fn = in_annotation(Checker.handleNode)
+            self.deferFunction(lambda: fn(self, annotation, node))
         else:
             self.handleNode(annotation, node)
 
@@ -1332,9 +1417,19 @@ class Checker(object):
 
     # "expr" type nodes
     BOOLOP = UNARYOP = IFEXP = SET = \
-        REPR = ATTRIBUTE = SUBSCRIPT = \
+        REPR = ATTRIBUTE = \
         STARRED = NAMECONSTANT = NAMEDEXPR = handleChildren
 
+    def SUBSCRIPT(self, node):
+        if _is_typing(node.value, 'Literal', self.scopeStack):
+            orig, self._in_typing_literal = self._in_typing_literal, True
+            try:
+                self.handleChildren(node)
+            finally:
+                self._in_typing_literal = orig
+        else:
+            self.handleChildren(node)
+
     def _handle_string_dot_format(self, node):
         try:
             placeholders = tuple(parse_format_string(node.func.value.s))
@@ -1575,7 +1670,27 @@ class Checker(object):
             self._handle_percent_format(node)
         self.handleChildren(node)
 
-    NUM = STR = BYTES = ELLIPSIS = CONSTANT = ignore
+    def STR(self, node):
+        if self._in_annotation and not self._in_typing_literal:
+            fn = functools.partial(
+                self.handleStringAnnotation,
+                node.s,
+                node,
+                node.lineno,
+                node.col_offset,
+                messages.ForwardAnnotationSyntaxError,
+            )
+            if self._in_deferred:
+                fn()
+            else:
+                self.deferFunction(fn)
+
+    if PY38_PLUS:
+        def CONSTANT(self, node):
+            if isinstance(node.value, str):
+                return self.STR(node)
+    else:
+        NUM = BYTES = ELLIPSIS = CONSTANT = ignore
 
     # "slice" type nodes
     SLICE = EXTSLICE = INDEX = handleChildren
@@ -1738,7 +1853,7 @@ class Checker(object):
                 break
             # Handle Try/TryFinally difference in Python < and >= 3.3
             if hasattr(n, 'finalbody') and isinstance(node, ast.Continue):
-                if n_child in n.finalbody:
+                if n_child in n.finalbody and not PY38_PLUS:
                     self.report(messages.ContinueInFinally, node)
                     return
         if isinstance(node, ast.Continue):
@@ -1799,6 +1914,10 @@ class Checker(object):
             addArgs(node.args.args)
             defaults = node.args.defaults
         else:
+            if PY38_PLUS:
+                for arg in node.args.posonlyargs:
+                    args.append(arg.arg)
+                    annotations.append(arg.annotation)
             for arg in node.args.args + node.args.kwonlyargs:
                 args.append(arg.arg)
                 annotations.append(arg.annotation)
@@ -2050,14 +2169,14 @@ class Checker(object):
             self.handleNode(node.value, node)
 
     def COMPARE(self, node):
-        literals = (ast.Str, ast.Num)
-        if not PY2:
-            literals += (ast.Bytes,)
-
         left = node.left
         for op, right in zip(node.ops, node.comparators):
-            if (isinstance(op, (ast.Is, ast.IsNot)) and
-                    (isinstance(left, literals) or isinstance(right, literals))):
+            if (
+                    isinstance(op, (ast.Is, ast.IsNot)) and (
+                        _is_const_non_singleton(left) or
+                        _is_const_non_singleton(right)
+                    )
+            ):
                 self.report(messages.IsLiteral, node)
             left = right
 
diff --git a/pyflakes/messages.py b/pyflakes/messages.py
index cf67cf8..e93a493 100644
--- a/pyflakes/messages.py
+++ b/pyflakes/messages.py
@@ -265,7 +265,7 @@ class InvalidPrintSyntax(Message):
 
 
 class IsLiteral(Message):
-    message = 'use ==/!= to compare str, bytes, and int literals'
+    message = 'use ==/!= to compare constant literals (str, bytes, int, float, tuple)'
 
 
 class FStringMissingPlaceholders(Message):
diff --git a/tox.ini b/tox.ini
index 5821483..f2256c8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,6 +5,7 @@ envlist =
 
 [testenv]
 deps = flake8==3.6.0
+setenv = PYFLAKES_ERROR_UNKNOWN=1
 commands =
     python -m unittest discover pyflakes
     flake8 pyflakes setup.py


Test patch:
diff --git a/pyflakes/test/test_is_literal.py b/pyflakes/test/test_is_literal.py
index 9280cda..fbbb205 100644
--- a/pyflakes/test/test_is_literal.py
+++ b/pyflakes/test/test_is_literal.py
@@ -198,3 +198,25 @@ class Test(TestCase):
         if 4 < x is 'foo':
             pass
         """, IsLiteral)
+
+    def test_is_tuple_constant(self):
+        self.flakes('''\
+            x = 5
+            if x is ():
+                pass
+        ''', IsLiteral)
+
+    def test_is_tuple_constant_containing_constants(self):
+        self.flakes('''\
+            x = 5
+            if x is (1, '2', True, (1.5, ())):
+                pass
+        ''', IsLiteral)
+
+    def test_is_tuple_containing_variables_ok(self):
+        # a bit nonsensical, but does not trigger a SyntaxWarning
+        self.flakes('''\
+            x = 5
+            if x is (x,):
+                pass
+        ''')
diff --git a/pyflakes/test/test_other.py b/pyflakes/test/test_other.py
index df2f790..282accb 100644
--- a/pyflakes/test/test_other.py
+++ b/pyflakes/test/test_other.py
@@ -493,8 +493,10 @@ class Test(TestCase):
                     continue
         ''')
 
+    @skipIf(version_info > (3, 8), "Python <= 3.8 only")
     def test_continueInFinally(self):
         # 'continue' inside 'finally' is a special syntax error
+        # that is removed in 3.8
         self.flakes('''
         while True:
             try:
@@ -2003,6 +2005,7 @@ class TestAsyncStatements(TestCase):
         ''', m.BreakOutsideLoop)
 
     @skipIf(version_info < (3, 5), 'new in Python 3.5')
+    @skipIf(version_info > (3, 8), "Python <= 3.8 only")
     def test_continueInAsyncForFinally(self):
         self.flakes('''
         async def read_data(db):
diff --git a/pyflakes/test/test_type_annotations.py b/pyflakes/test/test_type_annotations.py
index 289535d..15c658b 100644
--- a/pyflakes/test/test_type_annotations.py
+++ b/pyflakes/test/test_type_annotations.py
@@ -42,6 +42,7 @@ class TestTypeAnnotations(TestCase):
     def test_typingExtensionsOverload(self):
         """Allow intentional redefinitions via @typing_extensions.overload"""
         self.flakes("""
+        import typing_extensions
         from typing_extensions import overload
 
         @overload
@@ -54,6 +55,17 @@ class TestTypeAnnotations(TestCase):
 
         def f(s):
             return s
+
+        @typing_extensions.overload
+        def g(s):  # type: (None) -> None
+            pass
+
+        @typing_extensions.overload
+        def g(s):  # type: (int) -> int
+            pass
+
+        def g(s):
+            return s
         """)
 
     @skipIf(version_info < (3, 5), 'new in Python 3.5')
@@ -418,3 +430,72 @@ class TestTypeAnnotations(TestCase):
                 Y = 2
                 return Y
         """, m.UndefinedName)
+
+    @skipIf(version_info < (3, 8), 'new in Python 3.8')
+    def test_positional_only_argument_annotations(self):
+        self.flakes("""
+        from x import C
+
+        def f(c: C, /): ...
+        """)
+
+    @skipIf(version_info < (3,), 'new in Python 3')
+    def test_partially_quoted_type_annotation(self):
+        self.flakes("""
+        from queue import Queue
+        from typing import Optional
+
+        def f() -> Optional['Queue[str]']:
+            return None
+        """)
+
+    @skipIf(version_info < (3,), 'new in Python 3')
+    def test_literal_type_typing(self):
+        self.flakes("""
+        from typing import Literal
+
+        def f(x: Literal['some string']) -> None:
+            return None
+        """)
+
+    @skipIf(version_info < (3,), 'new in Python 3')
+    def test_literal_type_typing_extensions(self):
+        self.flakes("""
+        from typing_extensions import Literal
+
+        def f(x: Literal['some string']) -> None:
+            return None
+        """)
+
+    @skipIf(version_info < (3,), 'new in Python 3')
+    def test_literal_union_type_typing(self):
+        self.flakes("""
+        from typing import Literal
+
+        def f(x: Literal['some string', 'foo bar']) -> None:
+            return None
+        """)
+
+    @skipIf(version_info < (3,), 'new in Python 3')
+    def test_deferred_twice_annotation(self):
+        self.flakes("""
+            from queue import Queue
+            from typing import Optional
+
+
+            def f() -> "Optional['Queue[str]']":
+                return None
+        """)
+
+    @skipIf(version_info < (3, 7), 'new in Python 3.7')
+    def test_partial_string_annotations_with_future_annotations(self):
+        self.flakes("""
+            from __future__ import annotations
+
+            from queue import Queue
+            from typing import Optional
+
+
+            def f() -> Optional['Queue[str]']:
+                return None
+        """)