Repository: asottile/pyupgrade

Issue:
constant fold isinstance(x, (str, str))


input
```python
import ham, value
import six


if isinstance(value, (six.text_type, str)):
    ham()
```

actual output (with pyupgrade --py38-plus)

```python
import ham, value
import six


if isinstance(value, (str, str)):
    ham()
```

expected output:

```python
import ham, value
import six


if isinstance(value, str):
    ham()
```

Comments:

Patch:
diff --git a/README.md b/README.md
index e1b4caf..9576436 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,22 @@ A fix for [python-modernize/python-modernize#178]
 
 [python-modernize/python-modernize#178]: https://github.com/python-modernize/python-modernize/issues/178
 
+### constant fold `isinstance` / `issubclass` / `except`
+
+```diff
+-isinstance(x, (int, int))
++isinstance(x, int)
+
+-issubclass(y, (str, str))
++issubclass(y, str)
+
+ try:
+     raises()
+-except (Error1, Error1, Error2):
++except (Error1, Error2):
+     pass
+```
+
 ### unittest deprecated aliases
 
 Rewrites [deprecated unittest method aliases](https://docs.python.org/3/library/unittest.html#deprecated-aliases) to their non-deprecated forms.
diff --git a/pyupgrade/_ast_helpers.py b/pyupgrade/_ast_helpers.py
index 9bcd981..8a79d3a 100644
--- a/pyupgrade/_ast_helpers.py
+++ b/pyupgrade/_ast_helpers.py
@@ -56,3 +56,13 @@ def is_async_listcomp(node: ast.ListComp) -> bool:
         any(gen.is_async for gen in node.generators) or
         contains_await(node)
     )
+
+
+def is_type_check(node: ast.AST) -> bool:
+    return (
+        isinstance(node, ast.Call) and
+        isinstance(node.func, ast.Name) and
+        node.func.id in {'isinstance', 'issubclass'} and
+        len(node.args) == 2 and
+        not has_starargs(node)
+    )
diff --git a/pyupgrade/_plugins/constant_fold.py b/pyupgrade/_plugins/constant_fold.py
new file mode 100644
index 0000000..07c3de5
--- /dev/null
+++ b/pyupgrade/_plugins/constant_fold.py
@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+import ast
+from typing import Iterable
+
+from tokenize_rt import Offset
+
+from pyupgrade._ast_helpers import ast_to_offset
+from pyupgrade._ast_helpers import is_type_check
+from pyupgrade._data import register
+from pyupgrade._data import State
+from pyupgrade._data import TokenFunc
+from pyupgrade._token_helpers import constant_fold_tuple
+
+
+def _to_name(node: ast.AST) -> str | None:
+    if isinstance(node, ast.Name):
+        return node.id
+    elif isinstance(node, ast.Attribute):
+        base = _to_name(node.value)
+        if base is None:
+            return None
+        else:
+            return f'{base}.{node.attr}'
+    else:
+        return None
+
+
+def _can_constant_fold(node: ast.Tuple) -> bool:
+    seen = set()
+    for el in node.elts:
+        name = _to_name(el)
+        if name is not None:
+            if name in seen:
+                return True
+            else:
+                seen.add(name)
+    else:
+        return False
+
+
+def _cbs(node: ast.AST | None) -> Iterable[tuple[Offset, TokenFunc]]:
+    if isinstance(node, ast.Tuple) and _can_constant_fold(node):
+        yield ast_to_offset(node), constant_fold_tuple
+
+
+@register(ast.Call)
+def visit_Call(
+        state: State,
+        node: ast.Call,
+        parent: ast.AST,
+) -> Iterable[tuple[Offset, TokenFunc]]:
+    if is_type_check(node):
+        yield from _cbs(node.args[1])
+
+
+@register(ast.Try)
+def visit_Try(
+        state: State,
+        node: ast.Try,
+        parent: ast.AST,
+) -> Iterable[tuple[Offset, TokenFunc]]:
+    for handler in node.handlers:
+        yield from _cbs(handler.type)
diff --git a/pyupgrade/_plugins/exceptions.py b/pyupgrade/_plugins/exceptions.py
index 64dabd6..37b39fa 100644
--- a/pyupgrade/_plugins/exceptions.py
+++ b/pyupgrade/_plugins/exceptions.py
@@ -13,7 +13,7 @@ from pyupgrade._data import register
 from pyupgrade._data import State
 from pyupgrade._data import TokenFunc
 from pyupgrade._data import Version
-from pyupgrade._token_helpers import arg_str
+from pyupgrade._token_helpers import constant_fold_tuple
 from pyupgrade._token_helpers import find_op
 from pyupgrade._token_helpers import parse_call_args
 from pyupgrade._token_helpers import replace_name
@@ -45,34 +45,13 @@ def _fix_except(
         *,
         at_idx: dict[int, _Target],
 ) -> None:
-    # find all the arg strs in the tuple
-    except_index = i
-    while tokens[except_index].src != 'except':
-        except_index -= 1
-    start = find_op(tokens, except_index, '(')
+    start = find_op(tokens, i, '(')
     func_args, end = parse_call_args(tokens, start)
 
-    arg_strs = [arg_str(tokens, *arg) for arg in func_args]
+    for i, target in reversed(at_idx.items()):
+        tokens[slice(*func_args[i])] = [Token('NAME', target.target)]
 
-    # rewrite the block without dupes
-    args = []
-    for i, arg in enumerate(arg_strs):
-        target = at_idx.get(i)
-        if target is not None:
-            args.append(target.target)
-        else:
-            args.append(arg)
-
-    unique_args = tuple(dict.fromkeys(args))
-
-    if len(unique_args) > 1:
-        joined = '({})'.format(', '.join(unique_args))
-    elif tokens[start - 1].name != 'UNIMPORTANT_WS':
-        joined = f' {unique_args[0]}'
-    else:
-        joined = unique_args[0]
-
-    tokens[start:end] = [Token('CODE', joined)]
+    constant_fold_tuple(start, tokens)
 
 
 def _get_rewrite(
diff --git a/pyupgrade/_plugins/six_simple.py b/pyupgrade/_plugins/six_simple.py
index e583a7b..d490d9e 100644
--- a/pyupgrade/_plugins/six_simple.py
+++ b/pyupgrade/_plugins/six_simple.py
@@ -7,6 +7,7 @@ from typing import Iterable
 from tokenize_rt import Offset
 
 from pyupgrade._ast_helpers import ast_to_offset
+from pyupgrade._ast_helpers import is_type_check
 from pyupgrade._data import register
 from pyupgrade._data import State
 from pyupgrade._data import TokenFunc
@@ -36,14 +37,6 @@ NAMES_TYPE_CTX = {
 }
 
 
-def _is_type_check(node: ast.AST | None) -> bool:
-    return (
-        isinstance(node, ast.Call) and
-        isinstance(node.func, ast.Name) and
-        node.func.id in {'isinstance', 'issubclass'}
-    )
-
-
 @register(ast.Attribute)
 def visit_Attribute(
         state: State,
@@ -62,7 +55,7 @@ def visit_Attribute(
         ):
             return
 
-        if node.attr in NAMES_TYPE_CTX and _is_type_check(parent):
+        if node.attr in NAMES_TYPE_CTX and is_type_check(parent):
             new = NAMES_TYPE_CTX[node.attr]
         else:
             new = NAMES[node.attr]
@@ -106,7 +99,7 @@ def visit_Name(
         ):
             return
 
-        if node.id in NAMES_TYPE_CTX and _is_type_check(parent):
+        if node.id in NAMES_TYPE_CTX and is_type_check(parent):
             new = NAMES_TYPE_CTX[node.id]
         else:
             new = NAMES[node.id]
diff --git a/pyupgrade/_token_helpers.py b/pyupgrade/_token_helpers.py
index 3d28bc7..5936ab7 100644
--- a/pyupgrade/_token_helpers.py
+++ b/pyupgrade/_token_helpers.py
@@ -470,6 +470,23 @@ def replace_argument(
     tokens[start_idx:end_idx] = [Token('SRC', new)]
 
 
+def constant_fold_tuple(i: int, tokens: list[Token]) -> None:
+    start = find_op(tokens, i, '(')
+    func_args, end = parse_call_args(tokens, start)
+    arg_strs = [arg_str(tokens, *arg) for arg in func_args]
+
+    unique_args = tuple(dict.fromkeys(arg_strs))
+
+    if len(unique_args) > 1:
+        joined = '({})'.format(', '.join(unique_args))
+    elif tokens[start - 1].name != 'UNIMPORTANT_WS':
+        joined = f' {unique_args[0]}'
+    else:
+        joined = unique_args[0]
+
+    tokens[start:end] = [Token('CODE', joined)]
+
+
 def has_space_before(i: int, tokens: list[Token]) -> bool:
     return i >= 1 and tokens[i - 1].name in {UNIMPORTANT_WS, 'INDENT'}
 


Test patch:
diff --git a/tests/features/constant_fold_test.py b/tests/features/constant_fold_test.py
new file mode 100644
index 0000000..142ab02
--- /dev/null
+++ b/tests/features/constant_fold_test.py
@@ -0,0 +1,91 @@
+from __future__ import annotations
+
+import pytest
+
+from pyupgrade._data import Settings
+from pyupgrade._main import _fix_plugins
+
+
+@pytest.mark.parametrize(
+    's',
+    (
+        pytest.param(
+            'isinstance(x, str)',
+            id='isinstance nothing duplicated',
+        ),
+        pytest.param(
+            'issubclass(x, str)',
+            id='issubclass nothing duplicated',
+        ),
+        pytest.param(
+            'try: ...\n'
+            'except Exception: ...\n',
+            id='try-except nothing duplicated',
+        ),
+        pytest.param(
+            'isinstance(x, (str, (str,)))',
+            id='only consider flat tuples',
+        ),
+        pytest.param(
+            'isinstance(x, (f(), a().g))',
+            id='only consider names and dotted names',
+        ),
+    ),
+)
+def test_constant_fold_noop(s):
+    assert _fix_plugins(s, settings=Settings()) == s
+
+
+@pytest.mark.parametrize(
+    ('s', 'expected'),
+    (
+        pytest.param(
+            'isinstance(x, (str, str, int))',
+
+            'isinstance(x, (str, int))',
+
+            id='isinstance',
+        ),
+        pytest.param(
+            'issubclass(x, (str, str, int))',
+
+            'issubclass(x, (str, int))',
+
+            id='issubclass',
+        ),
+        pytest.param(
+            'try: ...\n'
+            'except (Exception, Exception, TypeError): ...\n',
+
+            'try: ...\n'
+            'except (Exception, TypeError): ...\n',
+
+            id='except',
+        ),
+
+        pytest.param(
+            'isinstance(x, (str, str))',
+
+            'isinstance(x, str)',
+
+            id='folds to 1',
+        ),
+
+        pytest.param(
+            'isinstance(x, (a.b, a.b, a.c))',
+            'isinstance(x, (a.b, a.c))',
+            id='folds dotted names',
+        ),
+        pytest.param(
+            'try: ...\n'
+            'except(a, a): ...\n',
+
+            'try: ...\n'
+            'except a: ...\n',
+
+            id='deduplication to 1 does not cause syntax error with except',
+        ),
+    ),
+)
+def test_constant_fold(s, expected):
+    assert _fix_plugins(s, settings=Settings()) == expected