Add support for PEP 563 (#356) (#374)

* Test with py3.7; Travis should use Xenial for 3.7 and nightly.

* Add support for PEP 563 (#356)

If a module imports annotations from __future__, postpone the evaluation
of annotations like it is done for annotation strings.

2 files changed, 36 insertions, 0 deletions

diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index 5c12820..e05bdf4 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -476,6 +476,7 @@ class GeneratorScope(Scope):
 class ModuleScope(Scope):
     """Scope for a module."""
     _futures_allowed = True
+    _annotations_future_enabled = False
 
 
 class DoctestScope(ModuleScope):
@@ -629,6 +630,19 @@ class Checker(object):
             self.scope._futures_allowed = False
 
     @property
+    def annotationsFutureEnabled(self):
+        scope = self.scopeStack[0]
+        if not isinstance(scope, ModuleScope):
+            return False
+        return scope._annotations_future_enabled
+
+    @annotationsFutureEnabled.setter
+    def annotationsFutureEnabled(self, value):
+        assert value is True
+        assert isinstance(self.scope, ModuleScope)
+        self.scope._annotations_future_enabled = True
+
+    @property
     def scope(self):
         return self.scopeStack[-1]
 
@@ -1068,6 +1082,8 @@ class Checker(object):
                 self.handleNode(parsed_annotation, node)
 
             self.deferFunction(handleForwardAnnotation)
+        elif self.annotationsFutureEnabled:
+            self.deferFunction(lambda: self.handleNode(annotation, node))
         else:
             self.handleNode(annotation, node)
 
@@ -1448,6 +1464,8 @@ class Checker(object):
                 if alias.name not in __future__.all_feature_names:
                     self.report(messages.FutureFeatureNotDefined,
                                 node, alias.name)
+                if alias.name == 'annotations':
+                    self.annotationsFutureEnabled = True
             elif alias.name == '*':
                 # Only Python 2, local import * is a SyntaxWarning
                 if not PY2 and not isinstance(self.scope, ModuleScope):
diff --git a/pyflakes/test/test_other.py b/pyflakes/test/test_other.py
index b8301ea..d0fff0a 100644
--- a/pyflakes/test/test_other.py
+++ b/pyflakes/test/test_other.py
@@ -2055,6 +2055,24 @@ class TestAsyncStatements(TestCase):
         a: 'a: "A"'
         ''', m.ForwardAnnotationSyntaxError)
 
+    @skipIf(version_info < (3, 7), 'new in Python 3.7')
+    def test_postponed_annotations(self):
+        self.flakes('''
+        from __future__ import annotations
+        def f(a: A) -> A: pass
+        class A:
+            b: B
+        class B: pass
+        ''')
+
+        self.flakes('''
+        from __future__ import annotations
+        def f(a: A) -> A: pass
+        class A:
+            b: Undefined
+        class B: pass
+        ''', m.UndefinedName)
+
     def test_raise_notimplemented(self):
         self.flakes('''
         raise NotImplementedError("This is fine")