Closes #249

pyflakes/checker.py

@@ -417,6 +417,7 @@ def __init__(self):
         super(FunctionScope, self).__init__()
         # Simplify: manage the special locals as globals
         self.globals = self.alwaysUsed.copy()
+        self.global_names = []
         self.returnValue = None     # First non-empty return
         self.isGenerator = False    # Detect a generator
 
@@ -430,6 +431,13 @@ def unusedAssignments(self):
                     and isinstance(binding, Assignment)):
                 yield name, binding
 
+    def usedAssignments(self):
+        for name, binding in self.items():
+            if (name not in self.globals and
+                    not self.usesLocals and
+                    isinstance(binding, Assignment)):
+                yield name, binding
+
 
 class GeneratorScope(Scope):
     pass
@@ -1052,7 +1060,8 @@ def GLOBAL(self, node):
                     m.message_args[0] != node_name]
 
                 # Bind name to global scope if it doesn't exist already.
-                global_scope.setdefault(node_name, node_value)
+                if isinstance(self.scope, FunctionScope):
+                    self.scope.global_names.append(node_name)
 
                 # Bind name to non-global scopes, but as already "used".
                 node_value.used = (global_scope, node)
@@ -1074,17 +1083,33 @@ def NAME(self, node):
         """
         Handle occurrence of Name (which can be a load/store/delete access.)
         """
+        global_scope_index = 1 if self._in_doctest() else 0
+        global_scope = self.scopeStack[global_scope_index]
         # Locate the name in locals / function / globals scopes.
         if isinstance(node.ctx, (ast.Load, ast.AugLoad)):
             self.handleNodeLoad(node)
             if (node.id == 'locals' and isinstance(self.scope, FunctionScope)
                     and isinstance(node.parent, ast.Call)):
                 # we are doing locals() call in current scope
                 self.scope.usesLocals = True
+            if (isinstance(self.scope, FunctionScope) and
+                    node.id in self.scope.global_names):
+                if node.id not in global_scope:
+                    self.report(messages.UndefinedName, node, node.id)
         elif isinstance(node.ctx, (ast.Store, ast.AugStore)):
             self.handleNodeStore(node)
+            if (isinstance(self.scope, FunctionScope) and
+                    node.id in self.scope.global_names):
+                global_scope.setdefault(node.id, Assignment(node.id, node))
         elif isinstance(node.ctx, ast.Del):
             self.handleNodeDelete(node)
+            if (isinstance(self.scope, FunctionScope) and
+                    node.id in self.scope.global_names):
+                if not node.id in global_scope:
+                    self.report(messages.UndefinedName, node, node.id)
+                else:
+                    global_scope.pop(node.id, None)
+                    self.scope.global_names.remove(node.id)
         else:
             # must be a Param context -- this only happens for names in function
             # arguments, but these aren't dispatched through here

pyflakes/test/test_undefined_names.py

@@ -337,6 +337,14 @@ def f2():
             global m
         ''', m.UndefinedName)
 
+    def test_undefined_global(self):
+        """Use an undefined name with global statement"""
+        self.flakes('''
+        def f():
+            global m
+            print(m)
+        ''', m.UndefinedName)
+
     def test_del(self):
         """Del deletes bindings."""
         self.flakes('a = 1; del a; a', m.UndefinedName)