Check if global is set for import (pylint-dev#2090)

The following code should trigger a global-variable-unassigned, as it is assigned
by the import statement:

def func():
    global sys
    import sys


Closes pylint-dev#1453

ChangeLog

@@ -4,6 +4,10 @@ Pylint's ChangeLog
 What's New in Pylint 2.0?
 =========================
 
+    * Don't warn that a global variable is unused if it is defined by an import
+
+      Close #1453
+
     * Skip wildcard import check for `__init__.py`.
 
       Close #2026

doc/whatsnew/2.0.rst

@@ -161,3 +161,11 @@ Other Changes
   They also no longer need to document raising a NotImplementedError.
 
 * Skip wildcard import check for `__init__.py`.
+
+* Don't warn that a global variable is unused if it is defined by an import
+
+  .. code-block:: python
+
+    def func():
+        global sys
+        import sys

pylint/checkers/variables.py

@@ -844,14 +844,18 @@ def visit_global(self, node):
 
         module = frame.root()
         default_message = True
+        locals_ = node.scope().locals
         for name in node.names:
             try:
                 assign_nodes = module.getattr(name)
             except astroid.NotFoundError:
                 # unassigned global, skip
                 assign_nodes = []
 
-            if not assign_nodes:
+            not_defined_locally_by_import = not any(
+                isinstance(local, astroid.node_classes.Import)
+                for local in locals_.get(name, ()))
+            if not assign_nodes and not_defined_locally_by_import:
                 self.add_message('global-variable-not-assigned',
                                  args=name, node=node)
                 default_message = False
@@ -866,9 +870,10 @@ def visit_global(self, node):
                     # module level assignment
                     break
             else:
-                # global undefined at the module scope
-                self.add_message('global-variable-undefined', args=name, node=node)
-                default_message = False
+                if not_defined_locally_by_import:
+                    # global undefined at the module scope
+                    self.add_message('global-variable-undefined', args=name, node=node)
+                    default_message = False
 
         if default_message:
             self.add_message('global-statement', node=node)

pylint/test/functional/globals.py

@@ -23,3 +23,10 @@ def define_constant():
     """ok but somevar is not defined at the module scope"""
     global SOMEVAR  # [global-variable-undefined]
     SOMEVAR = 2
+
+
+# pylint: disable=invalid-name
+def global_with_import():
+    """should only warn for global-statement"""
+    global sys  # [global-statement]
+    import sys

pylint/test/functional/globals.txt

@@ -4,3 +4,4 @@ global-statement:11:fix_contant:Using the global statement
 global-variable-not-assigned:18:other:Using global for 'HOP' but no assignment is done
 undefined-variable:19:other:Undefined variable 'HOP'
 global-variable-undefined:24:define_constant:Global variable 'SOMEVAR' undefined at the module level
+global-statement:31:global_with_import:Using the global statement

pylint/test/unittest_checker_variables.py

@@ -17,6 +17,7 @@
 
 from pylint.checkers import variables
 from pylint.testutils import CheckerTestCase, linter, set_config, Message
+from pylint.interfaces import UNDEFINED
 
 class TestVariablesChecker(CheckerTestCase):
 
@@ -89,6 +90,16 @@ def test():
             self.checker.visit_module(node.root())
             self.checker.visit_functiondef(node)
 
+    def test_unassigned_global(self):
+        node = astroid.extract_node('''
+            def func():
+                global sys  #@
+                import sys, lala
+        ''')
+        msg = Message('global-statement', node=node, confidence=UNDEFINED)
+        with self.assertAddsMessages(msg):
+            self.checker.visit_global(node)
+
 
 class TestVariablesCheckerWithTearDown(CheckerTestCase):