Fix quoted type annotations in unusual contexts (#516)

* Extract a context manager for when we're in a annotation context

* Detect quoted type annotations within typing.cast calls

* Refactor typing name check

This will make it easier to detect an unspecified typing module
member as well as opening up other checks (such as testing for
one of a collection many members all in one go).

* Detect quoted annotations within subscripts of typing classes

* Add nested quoted test case (Callable)

* Use a lambda here for clarity

This is slightly more usual than accessing a .__eq__ method and is
more obviously similar to the other usage of this helper.

2 files changed, 102 insertions, 13 deletions

diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index f0b7c37..c239950 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -8,6 +8,7 @@ import __future__
 import ast
 import bisect
 import collections
+import contextlib
 import doctest
 import functools
 import os
@@ -663,17 +664,26 @@ def getNodeName(node):
         return node.name
 
 
-def _is_typing(node, typing_attr, scope_stack):
+TYPING_MODULES = frozenset(('typing', 'typing_extensions'))
+
+
+def _is_typing_helper(node, is_name_match_fn, scope_stack):
+    """
+    Internal helper to determine whether or not something is a member of a
+    typing module. This is used as part of working out whether we are within a
+    type annotation context.
+
+    Note: you probably don't want to use this function directly. Instead see the
+    utils below which wrap it (`_is_typing` and `_is_any_typing_member`).
+    """
+
     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 expected_typing_names
+                    scope[name].module in TYPING_MODULES and
+                    is_name_match_fn(scope[name].real_name)
                 )
 
         return False
@@ -685,12 +695,33 @@ def _is_typing(node, typing_attr, scope_stack):
         ) or (
             isinstance(node, ast.Attribute) and
             isinstance(node.value, ast.Name) and
-            node.value.id in {'typing', 'typing_extensions'} and
-            node.attr == typing_attr
+            node.value.id in TYPING_MODULES and
+            is_name_match_fn(node.attr)
         )
     )
 
 
+def _is_typing(node, typing_attr, scope_stack):
+    """
+    Determine whether `node` represents the member of a typing module specified
+    by `typing_attr`.
+
+    This is used as part of working out whether we are within a type annotation
+    context.
+    """
+    return _is_typing_helper(node, lambda x: x == typing_attr, scope_stack)
+
+
+def _is_any_typing_member(node, scope_stack):
+    """
+    Determine whether `node` represents any member of a typing module.
+
+    This is used as part of working out whether we are within a type annotation
+    context.
+    """
+    return _is_typing_helper(node, lambda x: True, scope_stack)
+
+
 def is_typing_overload(value, scope_stack):
     return (
         isinstance(value.source, FUNCTION_TYPES) and
@@ -704,11 +735,8 @@ def is_typing_overload(value, scope_stack):
 def in_annotation(func):
     @functools.wraps(func)
     def in_annotation_func(self, *args, **kwargs):
-        orig, self._in_annotation = self._in_annotation, True
-        try:
+        with self._enter_annotation():
             return func(self, *args, **kwargs)
-        finally:
-            self._in_annotation = orig
     return in_annotation_func
 
 
@@ -1236,6 +1264,14 @@ class Checker(object):
             except KeyError:
                 self.report(messages.UndefinedName, node, name)
 
+    @contextlib.contextmanager
+    def _enter_annotation(self):
+        orig, self._in_annotation = self._in_annotation, True
+        try:
+            yield
+        finally:
+            self._in_annotation = orig
+
     def _handle_type_comments(self, node):
         for (lineno, col_offset), comment in self._type_comments.get(node, ()):
             comment = comment.split(':', 1)[1].strip()
@@ -1428,7 +1464,11 @@ class Checker(object):
             finally:
                 self._in_typing_literal = orig
         else:
-            self.handleChildren(node)
+            if _is_any_typing_member(node.value, self.scopeStack):
+                with self._enter_annotation():
+                    self.handleChildren(node)
+            else:
+                self.handleChildren(node)
 
     def _handle_string_dot_format(self, node):
         try:
@@ -1557,6 +1597,15 @@ class Checker(object):
                 node.func.attr == 'format'
         ):
             self._handle_string_dot_format(node)
+
+        if (
+            _is_typing(node.func, 'cast', self.scopeStack) and
+            len(node.args) >= 1 and
+            isinstance(node.args[0], ast.Str)
+        ):
+            with self._enter_annotation():
+                self.handleNode(node.args[0], node)
+
         self.handleChildren(node)
 
     def _handle_percent_format(self, node):
diff --git a/pyflakes/test/test_type_annotations.py b/pyflakes/test/test_type_annotations.py
index 15c658b..4804dda 100644
--- a/pyflakes/test/test_type_annotations.py
+++ b/pyflakes/test/test_type_annotations.py
@@ -449,6 +449,46 @@ class TestTypeAnnotations(TestCase):
             return None
         """)
 
+    def test_partially_quoted_type_assignment(self):
+        self.flakes("""
+        from queue import Queue
+        from typing import Optional
+
+        MaybeQueue = Optional['Queue[str]']
+        """)
+
+    def test_nested_partially_quoted_type_assignment(self):
+        self.flakes("""
+        from queue import Queue
+        from typing import Callable
+
+        Func = Callable[['Queue[str]'], None]
+        """)
+
+    def test_quoted_type_cast(self):
+        self.flakes("""
+        from typing import cast, Optional
+
+        maybe_int = cast('Optional[int]', 42)
+        """)
+
+    def test_type_cast_literal_str_to_str(self):
+        # Checks that our handling of quoted type annotations in the first
+        # argument to `cast` doesn't cause issues when (only) the _second_
+        # argument is a literal str which looks a bit like a type annoation.
+        self.flakes("""
+        from typing import cast
+
+        a_string = cast(str, 'Optional[int]')
+        """)
+
+    def test_quoted_type_cast_renamed_import(self):
+        self.flakes("""
+        from typing import cast as tsac, Optional as Maybe
+
+        maybe_int = tsac('Maybe[int]', 42)
+        """)
+
     @skipIf(version_info < (3,), 'new in Python 3')
     def test_literal_type_typing(self):
         self.flakes("""