Issue 9110. Adding ContextDecorator to contextlib. This enables the creation of…

Issue 9110. Adding ContextDecorator to contextlib. This enables the creation of APIs that act as decorators as well as context managers. contextlib.contextmanager changed to use ContextDecorator.

Doc/library/contextlib.rst

@@ -51,6 +51,11 @@ Functions provided:
    the exception has been handled, and execution will resume with the statement
    immediately following the :keyword:`with` statement.
 
+   contextmanager uses :class:`ContextDecorator` so the context managers it
+   creates can be used as decorators as well as in :keyword:`with` statements.
+
+   .. versionchanged:: 3.2
+      Use of :class:`ContextDecorator`.
 
 .. function:: closing(thing)
 
@@ -79,6 +84,58 @@ Functions provided:
    ``page.close()`` will be called when the :keyword:`with` block is exited.
 
 
+.. class:: ContextDecorator()
+
+   A base class that enables a context manager to also be used as a decorator.
+
+   Context managers inheriting from ``ContextDecorator`` have to implement
+   ``__enter__`` and ``__exit__`` as normal. ``__exit__`` retains its optional
+   exception handling even when used as a decorator.
+
+   Example::
+
+      from contextlib import ContextDecorator
+
+      class mycontext(ContextDecorator):
+         def __enter__(self):
+            print('Starting')
+            return self
+
+         def __exit__(self, *exc):
+            print('Finishing')
+            return False
+
+      >>> @mycontext()
+      ... def function():
+      ...    print('The bit in the middle')
+      ...
+      >>> function()
+      Starting
+      The bit in the middle
+      Finishing
+
+      >>> with mycontext():
+      ...    print('The bit in the middle')
+      ...
+      Starting
+      The bit in the middle
+      Finishing
+
+   Existing context managers that already have a base class can be extended by
+   using ``ContextDecorator`` as a mixin class::
+
+      from contextlib import ContextDecorator
+
+      class mycontext(ContextBaseClass, ContextDecorator):
+         def __enter__(self):
+            return self
+
+         def __exit__(self, *exc):
+            return False
+
+   .. versionadded:: 3.2
+
+
 .. seealso::
 
    :pep:`0343` - The "with" statement

Lib/contextlib.py

@@ -4,9 +4,20 @@ import sys
 from functools import wraps
 from warnings import warn
 
-__all__ = ["contextmanager", "closing"]
+__all__ = ["contextmanager", "closing", "ContextDecorator"]
 
-class GeneratorContextManager(object):
+
+class ContextDecorator(object):
+    "A base class or mixin that enables context managers to work as decorators."
+    def __call__(self, func):
+        @wraps(func)
+        def inner(*args, **kwds):
+            with self:
+                return func(*args, **kwds)
+        return inner
+
+
+class GeneratorContextManager(ContextDecorator):
     """Helper for @contextmanager decorator."""
 
     def __init__(self, gen):

Lib/test/test_contextlib.py

@@ -202,6 +202,169 @@ class LockContextTestCase(unittest.TestCase):
                 return True
         self.boilerPlate(lock, locked)
 
+
+class mycontext(ContextDecorator):
+    started = False
+    exc = None
+    catch = False
+
+    def __enter__(self):
+        self.started = True
+        return self
+
+    def __exit__(self, *exc):
+        self.exc = exc
+        return self.catch
+
+
+class TestContextDecorator(unittest.TestCase):
+
+    def test_contextdecorator(self):
+        context = mycontext()
+        with context as result:
+            self.assertIs(result, context)
+            self.assertTrue(context.started)
+
+        self.assertEqual(context.exc, (None, None, None))
+
+
+    def test_contextdecorator_with_exception(self):
+        context = mycontext()
+
+        with self.assertRaisesRegexp(NameError, 'foo'):
+            with context:
+                raise NameError('foo')
+        self.assertIsNotNone(context.exc)
+        self.assertIs(context.exc[0], NameError)
+
+        context = mycontext()
+        context.catch = True
+        with context:
+            raise NameError('foo')
+        self.assertIsNotNone(context.exc)
+        self.assertIs(context.exc[0], NameError)
+
+
+    def test_decorator(self):
+        context = mycontext()
+
+        @context
+        def test():
+            self.assertIsNone(context.exc)
+            self.assertTrue(context.started)
+        test()
+        self.assertEqual(context.exc, (None, None, None))
+
+
+    def test_decorator_with_exception(self):
+        context = mycontext()
+
+        @context
+        def test():
+            self.assertIsNone(context.exc)
+            self.assertTrue(context.started)
+            raise NameError('foo')
+
+        with self.assertRaisesRegexp(NameError, 'foo'):
+            test()
+        self.assertIsNotNone(context.exc)
+        self.assertIs(context.exc[0], NameError)
+
+
+    def test_decorating_method(self):
+        context = mycontext()
+
+        class Test(object):
+
+            @context
+            def method(self, a, b, c=None):
+                self.a = a
+                self.b = b
+                self.c = c
+
+        # these tests are for argument passing when used as a decorator
+        test = Test()
+        test.method(1, 2)
+        self.assertEqual(test.a, 1)
+        self.assertEqual(test.b, 2)
+        self.assertEqual(test.c, None)
+
+        test = Test()
+        test.method('a', 'b', 'c')
+        self.assertEqual(test.a, 'a')
+        self.assertEqual(test.b, 'b')
+        self.assertEqual(test.c, 'c')
+
+        test = Test()
+        test.method(a=1, b=2)
+        self.assertEqual(test.a, 1)
+        self.assertEqual(test.b, 2)
+
+
+    def test_typo_enter(self):
+        class mycontext(ContextDecorator):
+            def __unter__(self):
+                pass
+            def __exit__(self, *exc):
+                pass
+
+        with self.assertRaises(AttributeError):
+            with mycontext():
+                pass
+
+
+    def test_typo_exit(self):
+        class mycontext(ContextDecorator):
+            def __enter__(self):
+                pass
+            def __uxit__(self, *exc):
+                pass
+
+        with self.assertRaises(AttributeError):
+            with mycontext():
+                pass
+
+
+    def test_contextdecorator_as_mixin(self):
+        class somecontext(object):
+            started = False
+            exc = None
+
+            def __enter__(self):
+                self.started = True
+                return self
+
+            def __exit__(self, *exc):
+                self.exc = exc
+
+        class mycontext(somecontext, ContextDecorator):
+            pass
+
+        context = mycontext()
+        @context
+        def test():
+            self.assertIsNone(context.exc)
+            self.assertTrue(context.started)
+        test()
+        self.assertEqual(context.exc, (None, None, None))
+
+
+    def test_contextmanager_as_decorator(self):
+        state = []
+        @contextmanager
+        def woohoo(y):
+            state.append(y)
+            yield
+            state.append(999)
+
+        @woohoo(1)
+        def test(x):
+            self.assertEqual(state, [1])
+            state.append(x)
+        test('something')
+        self.assertEqual(state, [1, 'something', 999])
+
+
 # This is needed to make the test actually run under regrtest.py!
 def test_main():
     support.run_unittest(__name__)

Misc/NEWS

@@ -460,6 +460,10 @@ C-API
 Library
 -------
 
+- Issue #9110: Addition of ContextDecorator to contextlib, for creating APIs
+  that act as both context managers and decorators. contextmanager changes
+  to use ContextDecorator.
+
 - Implement importlib.abc.SourceLoader and deprecate PyLoader and PyPycLoader
   for removal in Python 3.4.