Catch usage of undefined name with global statement

When we use undefined name declared by global statement,
pyflakes should catch the error but it doesn't.
This will fix pyflakes to catch this error.
Check test_undefined_globa

Fixes #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
@@ -1292,13 +1317,21 @@ def TUPLE(self, node):
 
     def IMPORT(self, node):
         for alias in node.names:
+            name = alias.name
             if '.' in alias.name and not alias.asname:
-                importation = SubmoduleImportation(alias.name, node)
+                importation = SubmoduleImportation(name, node)
             else:
                 name = alias.asname or alias.name
-                importation = Importation(name, node, alias.name)
+                importation = Importation(name, node, name)
             self.addBinding(node, importation)
 
+            if isinstance(self.scope, FunctionScope):
+                global_scope_index = 1 if self._in_doctest() else 0
+                global_scope = self.scopeStack[global_scope_index]
+                if name in self.scope.global_names:
+                    global_scope.setdefault(name,
+                                            Assignment(name, alias))
+
     def IMPORTFROM(self, node):
         if node.module == '__future__':
             if not self.futuresAllowed:
@@ -1331,6 +1364,13 @@ def IMPORTFROM(self, node):
                                               module, alias.name)
             self.addBinding(node, importation)
 
+            if isinstance(self.scope, FunctionScope):
+                global_scope_index = 1 if self._in_doctest() else 0
+                global_scope = self.scopeStack[global_scope_index]
+                if name in self.scope.global_names:
+                    global_scope.setdefault(name,
+                                            Assignment(name, alias))
+
     def TRY(self, node):
         handler_names = []
         # List the exception handlers

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)