Make global-variable-not-assigned check local scope

This checker now checks whether the names after the global keyword
are reassigned in the local scope.
This closes pylint-dev#1375

ChangeLog

@@ -77,6 +77,11 @@ Release date: TBA
 
   Closes #4900
 
+* The ``global-variable-not-assigned`` checker now catches global variables that are never reassigned in a
+  local scope
+
+  Closes #1375
+
 
 What's New in Pylint 2.10.3?
 ============================

doc/whatsnew/2.11.rst

@@ -79,3 +79,8 @@ Other Changes
 * Fix a bug where pylint complained if the cache's parent directory does not exist
 
   Closes #4900
+
+* The ``global-variable-not-assigned`` checker now catches global variables that are never reassigned in a
+  local scope
+
+  Closes #1375

pylint/checkers/utils.py

@@ -1558,3 +1558,11 @@ def is_node_in_guarded_import_block(node: nodes.NodeNG) -> bool:
     return isinstance(node.parent, nodes.If) and (
         node.parent.is_sys_guard() or node.parent.is_typing_guard()
     )
+
+
+def is_reassigned_after_current(node: nodes.NodeNG, varname: str) -> bool:
+    """Check if the given variable name is reassigned in the same scope after the current node"""
+    for assign_node in node.scope().nodes_of_class(nodes.AssignName):
+        if assign_node.name == varname and node.lineno < assign_node.lineno:
+            return True
+    return False

pylint/checkers/variables.py

@@ -931,7 +931,10 @@ def visit_global(self, node: nodes.Global) -> None:
             not_defined_locally_by_import = not any(
                 isinstance(local, nodes.Import) for local in locals_.get(name, ())
             )
-            if not assign_nodes and not_defined_locally_by_import:
+            if (
+                not utils.is_reassigned_after_current(node, name)
+                and not_defined_locally_by_import
+            ):
                 self.add_message("global-variable-not-assigned", args=name, node=node)
                 default_message = False
                 continue

tests/functional/g/globals.py

@@ -30,3 +30,16 @@ def global_with_import():
     """should only warn for global-statement"""
     global sys  # [global-statement]
     import sys  # pylint: disable=import-outside-toplevel
+
+
+def global_no_assign():
+    """Not assigning anything to the global makes 'global' superfluous"""
+    global CONSTANT  # [global-variable-not-assigned]
+    print(CONSTANT)
+
+
+def global_operator_assign():
+    """Operator assigns should only throw a global statement error"""
+    global CONSTANT  # [global-statement]
+    print(CONSTANT)
+    CONSTANT += 1

tests/functional/g/globals.txt

@@ -5,3 +5,5 @@ global-variable-not-assigned:18:4:other:Using global for 'HOP' but no assignment
 undefined-variable:19:10:other:Undefined variable 'HOP'
 global-variable-undefined:24:4:define_constant:Global variable 'SOMEVAR' undefined at the module level
 global-statement:31:4:global_with_import:Using the global statement
+global-variable-not-assigned:37:4:global_no_assign:Using global for 'CONSTANT' but no assignment is done:HIGH
+global-statement:43:4:global_operator_assign:Using the global statement:HIGH