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.
Note test_undefined_global for example of this.

pyflakes/checker.py

@@ -435,6 +435,7 @@ 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
 
@@ -574,6 +575,9 @@ def _in_doctest(self):
         return (len(self.scopeStack) >= 2 and
                 isinstance(self.scopeStack[1], DoctestScope))
 
+    def _global_scope(self):
+        return self.scopeStack[1 if self._in_doctest() else 0]
+
     @property
     def futuresAllowed(self):
         if not all(isinstance(scope, ModuleScope)
@@ -735,6 +739,11 @@ def addBinding(self, node, value):
             elif isinstance(existing, Importation) and value.redefines(existing):
                 existing.redefined.append(node)
 
+        if (isinstance(self.scope, FunctionScope) and
+                isinstance(value, Importation) and
+                value.name in self.scope.global_names):
+            self.store_global_scope(value.name, value)
+
         if value.name in self.scope:
             # then assume the rebound name is used as a global or within a loop
             value.used = self.scope[value.name].used
@@ -757,6 +766,12 @@ def handleNodeLoad(self, node):
         in_generators = None
         importStarred = None
 
+        if (isinstance(self.scope, FunctionScope) and
+                name in self.scope.global_names and
+                name not in self._global_scope()):
+            # report name declared with global statement but undefined
+            self.report(messages.UndefinedName, node, name)
+
         # try enclosing function scopes and global scope
         for scope in self.scopeStack[-1::-1]:
             if isinstance(scope, ClassScope):
@@ -833,6 +848,10 @@ def handleNodeStore(self, node):
                                 scope[name].used[1], name, scope[name].source)
                     break
 
+        if (isinstance(self.scope, FunctionScope) and
+                name in self.scope.global_names):
+            self.store_global_scope(name, Assignment(name, node))
+
         parent_stmt = self.getParent(node)
         if isinstance(parent_stmt, (ast.For, ast.comprehension)) or (
                 parent_stmt != node.parent and
@@ -868,8 +887,15 @@ def on_conditional_branch():
             # be executed.
             return
 
-        if isinstance(self.scope, FunctionScope) and name in self.scope.globals:
-            self.scope.globals.remove(name)
+        if isinstance(self.scope, FunctionScope):
+            if name in self.scope.globals:
+                self.scope.globals.remove(name)
+            if name in self.scope.global_names:
+                self.scope.global_names.remove(name)
+                try:
+                    del self._global_scope()[name]
+                except KeyError:
+                    self.report(messages.UndefinedName, node, name)
         else:
             try:
                 del self.scope[name]
@@ -1016,6 +1042,10 @@ def handleForwardAnnotation():
     def ignore(self, node):
         pass
 
+    def store_global_scope(self, name, value):
+        """This store name in global scope"""
+        self._global_scope()[name] = value
+
     # "stmt" type nodes
     DELETE = PRINT = FOR = ASYNCFOR = WHILE = IF = WITH = WITHITEM = \
         ASYNCWITH = ASYNCWITHITEM = TRYFINALLY = EXEC = \
@@ -1122,7 +1152,8 @@ 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)

pyflakes/test/test_imports.py

@@ -686,6 +686,11 @@ def f(): global foo; import foo
             def g(): foo.is_used()
         ''')
 
+        self.flakes('''
+            def f(): global foo; import foo.bar
+            def g(): foo.is_used()
+        ''')
+
     @skipIf(version_info >= (3,), 'deprecated syntax')
     def test_usedInBackquote(self):
         self.flakes('import fu; `fu`')

pyflakes/test/test_undefined_names.py

@@ -353,6 +353,14 @@ 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)