Allow some builtins to be deleted

__builtins__, __file__ and __debug__ can be deleted.

Adds a test runner that ensures defined builtins can be
loaded, and can be deleted when the python runtime allows it.

pyflakes/checker.py

@@ -185,6 +185,16 @@ class Builtin(Definition):
 
     def __init__(self, name):
         super(Builtin, self).__init__(name, None)
+        self.can_delete = False
+        # __file__ can only be deleted on Python 3
+        if not PY2 and name == '__file__':
+            self.can_delete = True
+        # __builtins__ can always be deleted.
+        # __debug__ can be deleted sometimes and not deleted other times.
+        # Safest course of action is to assume it can be deleted, in
+        # order that no error is reported by pyflakes
+        elif name in ('__builtins__'):
+            self.can_delete = True
 
     def __repr__(self):
         return '<%s object %r at 0x%x>' % (self.__class__.__name__,
@@ -865,7 +875,8 @@ def on_conditional_branch():
             self.scope.globals.remove(name)
         else:
             binding = self.scope.get(name, None)
-            if not binding or isinstance(binding, Builtin):
+            if not binding or (
+                    isinstance(binding, Builtin) and not binding.can_delete):
                 self.report(messages.UndefinedName, node, name)
                 return
 

pyflakes/test/harness.py

@@ -15,6 +15,16 @@ class TestCase(unittest.TestCase):
 
     withDoctest = False
 
+    def pythonException(self, input, *expectedOutputs, **kw):
+        try:
+            compile(textwrap.dedent(input), '<test>', 'exec', PyCF_ONLY_AST)
+        except BaseException as e:
+            return e
+        try:
+            exec(textwrap.dedent(input), {})
+        except BaseException as e:
+            return e
+
     def flakes(self, input, *expectedOutputs, **kw):
         tree = compile(textwrap.dedent(input), "<test>", "exec", PyCF_ONLY_AST)
         if kw.get('is_segment'):

pyflakes/test/test_builtin.py

@@ -4,8 +4,15 @@
 from sys import version_info
 
 from pyflakes import messages as m
+from pyflakes.checker import Checker
 from pyflakes.test.harness import TestCase, skipIf
 
+try:
+    WindowsError
+    WIN = True
+except NameError:
+    WIN = False
+
 
 class TestBuiltins(TestCase):
 
@@ -39,3 +46,51 @@ def f():
 
         f()
         ''', m.UndefinedLocal)
+
+
+class TestLiveBuiltins(TestCase):
+
+    def test_exists(self):
+        for name in sorted(Checker.builtIns):
+            # __file__ does exist in this test harness
+            if name == '__file__':
+                continue
+
+            if name == 'WindowsError' and not WIN:
+                continue
+
+            source = '''
+            %s
+            ''' % name
+            e = self.pythonException(source)
+            self.assertIsNone(e)
+
+    def test_del(self):
+        for name in sorted(Checker.builtIns):
+            # __file__ does exist in this test harness
+            if name == '__file__':
+                continue
+
+            # __debug__ can be deleted sometimes and not deleted other times.
+            # Safest course of action is to assume it can be deleted, in
+            # order that no error is reported by pyflakes
+            if name == '__debug__':
+                continue
+
+            source = '''
+            del %s
+            ''' % name
+
+            e = self.pythonException(source)
+
+            if isinstance(e, SyntaxError):
+                if version_info < (3,):
+                    # SyntaxError: invalid syntax
+                    self.assertIn(name, ('print'))
+                else:
+                    # SyntaxError: can't delete keyword
+                    self.assertIn(name, ('None', 'True', 'False'))
+            elif isinstance(e, NameError):
+                self.flakes(source, m.UndefinedName)
+            else:
+                self.flakes(source)