Don't warn about unused _ magic var

Resolves PyCQA#202

Author: krader1961

pyflakes/checker.py

@@ -425,7 +425,9 @@ def unusedAssignments(self):
         Return a generator for the assignments which have not been used.
         """
         for name, binding in self.items():
-            if (not binding.used and name not in self.globals
+            if (not binding.used
+                    and name != '_'  # see issue #202
+                    and name not in self.globals
                     and not self.usesLocals
                     and isinstance(binding, Assignment)):
                 yield name, binding

pyflakes/test/test_imports.py

@@ -94,6 +94,13 @@ def test_importfrom_future(self):
         assert binding.source_statement == 'from __future__ import print_function'
         assert str(binding) == '__future__.print_function'
 
+    def test_unusedImport_underscore(self):
+        """
+        The magic underscore var should be reported as unused when used as an
+        import alias.
+        """
+        self.flakes('import fu as _', m.UnusedImport)
+
 
 class Test(TestCase):
 

pyflakes/test/test_other.py

@@ -1175,6 +1175,16 @@ def a():
             b = 1
         ''', m.UnusedVariable)
 
+    def test_unusedUnderscoreVariable(self):
+        """
+        Don't warn when the magic "_" (underscore) variable is unused.
+        See issue #202.
+        """
+        self.flakes('''
+        def a(unused_param):
+            _ = unused_param
+        ''')
+
     def test_unusedVariableAsLocals(self):
         """
         Using locals() it is perfectly valid to have unused variables