Another solution.

Author: PetterS

pyflakes/checker.py

@@ -325,7 +325,6 @@ def __init__(self, name, source):
         self.name = name
         self.source = source
         self.used = False
-        self.during_type_checking = False
 
     def __str__(self):
         return self.name
@@ -613,6 +612,17 @@ 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__
@@ -1119,8 +1129,6 @@ def addBinding(self, node, value):
                 break
         existing = scope.get(value.name)
 
-        value.during_type_checking = self._in_type_checking
-
         if (existing and not isinstance(existing, Builtin) and
                 not self.differentForks(node, existing.source)):
 
@@ -1152,6 +1160,8 @@ def addBinding(self, node, value):
         # in scope
         if value.name not in self.scope or not isinstance(value, Annotation):
             self.scope[value.name] = value
+        self.scope[Scope.TYPE_CHECKING_ONLY][value.name] = self._in_type_checking
+
 
     def _unknown_handler(self, node):
         # this environment variable configures whether to error on unknown
@@ -1220,7 +1230,8 @@ def handleNodeLoad(self, node):
                         scope[n.fullName].used = (self.scope, node)
                     except KeyError:
                         pass
-                if n.during_type_checking and not self._in_annotation:
+                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