From bcf81947a9c3c06ddbe1873e7d01b42d36d41617 Mon Sep 17 00:00:00 2001
From: Danny Sepler <dsepler@flatiron.com>
Date: Fri, 24 Dec 2021 12:44:34 -0500
Subject: [PATCH] Global variables must be assigned to be used

---
 pyflakes/checker.py                   | 26 +++++++++++++++++++-----
 pyflakes/test/test_undefined_names.py | 29 +++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index 56fc3ca4..d956319b 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -262,10 +262,11 @@ class Binding:
                 the node that this binding was last used.
     """
 
-    def __init__(self, name, source):
+    def __init__(self, name, source, *, assigned=True):
         self.name = name
         self.source = source
         self.used = False
+        self.assigned = assigned
 
     def __str__(self):
         return self.name
@@ -1073,6 +1074,12 @@ def addBinding(self, node, value):
                 break
         existing = scope.get(value.name)
 
+        global_scope = self.scopeStack[-1]
+        if (existing and global_scope.get(value.name) == existing and
+                not existing.assigned):
+            # make sure the variable is in the global scope before setting as assigned
+            existing.assigned = True
+
         if (existing and not isinstance(existing, Builtin) and
                 not self.differentForks(node, existing.source)):
 
@@ -1155,6 +1162,10 @@ def handleNodeLoad(self, node):
                     continue
 
             binding = scope.get(name, None)
+
+            if getattr(binding, 'assigned', None) is False:
+                self.report(messages.UndefinedName, node, name)
+
             if isinstance(binding, Annotation) and not self._in_postponed_annotation:
                 continue
 
@@ -1224,12 +1235,19 @@ def handleNodeStore(self, node):
                     continue
                 # if the name was defined in that scope, and the name has
                 # been accessed already in the current scope, and hasn't
-                # been declared global
+                # been assigned globally
                 used = name in scope and scope[name].used
                 if used and used[0] is self.scope and name not in self.scope.globals:
                     # then it's probably a mistake
                     self.report(messages.UndefinedLocal,
                                 scope[name].used[1], name, scope[name].source)
+
+                    # and remove UndefinedName messages already reported for this name
+                    self.messages = [
+                        m for m in self.messages if not
+                        isinstance(m, messages.UndefinedName) or
+                        m.message_args[0] != name]
+
                     break
 
         parent_stmt = self.getParent(node)
@@ -1933,11 +1951,9 @@ def GLOBAL(self, node):
 
             # One 'global' statement can bind multiple (comma-delimited) names.
             for node_name in node.names:
-                node_value = Assignment(node_name, node)
+                node_value = Assignment(node_name, node, assigned=False)
 
                 # Remove UndefinedName messages already reported for this name.
-                # TODO: if the global is not used in this scope, it does not
-                # become a globally defined name.  See test_unused_global.
                 self.messages = [
                     m for m in self.messages if not
                     isinstance(m, messages.UndefinedName) or
diff --git a/pyflakes/test/test_undefined_names.py b/pyflakes/test/test_undefined_names.py
index f3b89eaa..b997e5cb 100644
--- a/pyflakes/test/test_undefined_names.py
+++ b/pyflakes/test/test_undefined_names.py
@@ -298,6 +298,35 @@ def c(): bar
         def b(): global bar; bar = 1
         ''')
 
+    def test_unassigned_global_is_undefined(self):
+        """
+        If a "global" is never given a value, it is undefined
+        """
+        self.flakes('''
+        def a():
+            global fu
+            fu
+        ''', m.UndefinedName)
+
+        self.flakes('''
+        global fu
+        fu
+        ''', m.UndefinedName)
+
+    def test_scope_defined_global(self):
+        """
+        If a "global" is defined inside of a function only,
+        outside of the function it is undefined
+        """
+        self.flakes('''
+        global fu
+        def a():
+            fu = 1
+            fu
+        a()
+        fu
+        ''', m.UndefinedName)
+
     def test_definedByGlobalMultipleNames(self):
         """
         "global" can accept multiple names.