Ignore underscore redefinition (#73)

* Ignore underscore redefinition

* Add test: ignore underscore redefinition

* Redefining an importation should always raise a warning

pyflakes/checker.py

@@ -679,8 +679,9 @@ def addBinding(self, node, value):
                     self.report(messages.RedefinedInListComp,
                                 node, value.name, existing.source)
                 elif not existing.used and value.redefines(existing):
-                    self.report(messages.RedefinedWhileUnused,
-                                node, value.name, existing.source)
+                    if value.name != '_' or isinstance(existing, Importation):
+                        self.report(messages.RedefinedWhileUnused,
+                                    node, value.name, existing.source)
 
             elif isinstance(existing, Importation) and value.redefines(existing):
                 existing.redefined.append(node)

pyflakes/test/test_other.py

@@ -151,6 +151,25 @@ def a(): pass
         def a(): pass
         ''', m.RedefinedWhileUnused)
 
+    def test_redefinedUnderscoreFunction(self):
+        """
+        Test that shadowing a function definition named with underscore doesn't
+        raise anything.
+        """
+        self.flakes('''
+        def _(): pass
+        def _(): pass
+        ''')
+
+    def test_redefinedUnderscoreImportation(self):
+        """
+        Test that shadowing an underscore importation raises a warning.
+        """
+        self.flakes('''
+        from .i18n import _
+        def _(): pass
+        ''', m.RedefinedWhileUnused)
+
     def test_redefinedClassFunction(self):
         """
         Test that shadowing a function definition in a class suite with another