Handle code guarded by 'if TYPE_CHECKING' correctly

pyflakes/checker.py

@@ -587,6 +587,17 @@ def _add_to_names(container):
 
 class Scope(dict):
     importStarred = False       # set to True when import * is found
+    # Special key for checking whether a binding is defined only for type checking.
+    TYPE_CHECKING_ONLY = object()
+
+    def __init__(self):
+        super(Scope, self).__init__(self)
+        self[self.TYPE_CHECKING_ONLY] = collections.defaultdict(bool)
+
+    def items(self):
+        for key, val in super(Scope, self).items():
+            if key != self.TYPE_CHECKING_ONLY:
+                yield key, val
 
     def __repr__(self):
         scope_cls = self.__class__.__name__
@@ -829,6 +840,7 @@ class Checker(object):
     _in_annotation = False
     _in_typing_literal = False
     _in_deferred = False
+    _in_type_checking = False
 
     builtIns = set(builtin_vars).union(_MAGIC_GLOBALS)
     _customBuiltIns = os.environ.get('PYFLAKES_BUILTINS')
@@ -1099,6 +1111,7 @@ def addBinding(self, node, value):
             # then assume the rebound name is used as a global or within a loop
             value.used = self.scope[value.name].used
 
+        self.scope[Scope.TYPE_CHECKING_ONLY][value.name] = self._in_type_checking
         self.scope[value.name] = value
 
     def _unknown_handler(self, node):
@@ -1165,6 +1178,11 @@ def handleNodeLoad(self, node):
                         scope[n.fullName].used = (self.scope, node)
                     except KeyError:
                         pass
+                if (self.scope[Scope.TYPE_CHECKING_ONLY][name]
+                        and not self._in_annotation):
+                    # Only defined during type-checking; this does not count. Real code
+                    # (not an annotation) using this binding will not work.
+                    continue
             except KeyError:
                 pass
             else:
@@ -1833,7 +1851,12 @@ def DICT(self, node):
     def IF(self, node):
         if isinstance(node.test, ast.Tuple) and node.test.elts != []:
             self.report(messages.IfTuple, node)
+
+        prev = self._in_type_checking
+        if _is_typing(node.test, 'TYPE_CHECKING', self.scopeStack):
+            self._in_type_checking = True
         self.handleChildren(node)
+        self._in_type_checking = prev
 
     IFEXP = IF
 

pyflakes/test/test_imports.py

@@ -1031,6 +1031,35 @@ def test_futureImportStar(self):
         from __future__ import *
         ''', m.FutureFeatureNotDefined)
 
+    def test_ignoresTypingImports(self):
+        """Ignores imports within 'if TYPE_CHECKING' checking normal code."""
+        self.flakes('''
+        from typing import TYPE_CHECKING
+        if TYPE_CHECKING:
+            from a import b
+        b()
+        ''', m.UndefinedName)
+
+    def test_ignoresTypingClassDefinitino(self):
+        """Ignores definitions within 'if TYPE_CHECKING' checking normal code."""
+        self.flakes('''
+        from typing import TYPE_CHECKING
+        if TYPE_CHECKING:
+            class T:
+                ...
+        t = T()
+        ''', m.UndefinedName)
+
+    def test_usesTypingImportsForAnnotations(self):
+        """Uses imports within 'if TYPE_CHECKING' checking annotations."""
+        self.flakes('''
+        from typing import TYPE_CHECKING
+        if TYPE_CHECKING:
+            from a import b
+        def f() -> "b":
+            pass
+        ''')
+
 
 class TestSpecialAll(TestCase):
     """