Close #19330 by using public classes in contextlib

- added test cases to ensure docstrings are reasonable
- also updates various comments in contextlib for accuracy
- identifed #19404 as an issue making it difficult to provide
  good help output on generator based context manager instances

Lib/contextlib.py

@@ -37,6 +37,16 @@ class _GeneratorContextManager(ContextDecorator):
     def __init__(self, func, *args, **kwds):
         self.gen = func(*args, **kwds)
         self.func, self.args, self.kwds = func, args, kwds
+        # Issue 19330: ensure context manager instances have good docstrings
+        doc = getattr(func, "__doc__", None)
+        if doc is None:
+            doc = type(self).__doc__
+        self.__doc__ = doc
+        # Unfortunately, this still doesn't provide good help output when
+        # inspecting the created context manager instances, since pydoc
+        # currently bypasses the instance docstring and shows the docstring
+        # for the class instead.
+        # See http://bugs.python.org/issue19404 for more details.
 
     def _recreate_cm(self):
         # _GCM instances are one-shot context managers, so the
@@ -117,9 +127,6 @@ def contextmanager(func):
     return helper
 
 
-# Unfortunately, this was originally published as a class, so
-# backwards compatibility prevents the use of the wrapper function
-# approach used for the other classes
 class closing(object):
     """Context to automatically close something at the end of a block.
 
@@ -144,8 +151,18 @@ class closing(object):
     def __exit__(self, *exc_info):
         self.thing.close()
 
-class _RedirectStdout:
-    """Helper for redirect_stdout."""
+class redirect_stdout:
+    """Context manager for temporarily redirecting stdout to another file
+
+        # How to send help() to stderr
+        with redirect_stdout(sys.stderr):
+            help(dir)
+
+        # How to write help() to a file
+        with open('help.txt', 'w') as f:
+            with redirect_stdout(f):
+                help(pow)
+    """
 
     def __init__(self, new_target):
         self._new_target = new_target
@@ -163,25 +180,19 @@ class _RedirectStdout:
         self._old_target = self._sentinel
         sys.stdout = restore_stdout
 
-# Use a wrapper function since we don't care about supporting inheritance
-# and a function gives much cleaner output in help()
-def redirect_stdout(target):
-    """Context manager for temporarily redirecting stdout to another file
 
-        # How to send help() to stderr
-        with redirect_stdout(sys.stderr):
-            help(dir)
 
-        # How to write help() to a file
-        with open('help.txt', 'w') as f:
-            with redirect_stdout(f):
-                help(pow)
-    """
-    return _RedirectStdout(target)
+class suppress:
+    """Context manager to suppress specified exceptions
+
+    After the exception is suppressed, execution proceeds with the next
+    statement following the with statement.
 
+         with suppress(FileNotFoundError):
+             os.remove(somefile)
+         # Execution still resumes here if the file was already removed
+    """
 
-class _SuppressExceptions:
-    """Helper for suppress."""
     def __init__(self, *exceptions):
         self._exceptions = exceptions
 
@@ -189,30 +200,17 @@ class _SuppressExceptions:
         pass
 
     def __exit__(self, exctype, excinst, exctb):
-        # Unlike isinstance and issubclass, exception handling only
-        # looks at the concrete type heirarchy (ignoring the instance
-        # and subclass checking hooks). However, all exceptions are
-        # also required to be concrete subclasses of BaseException, so
-        # if there's a discrepancy in behaviour, we currently consider it
-        # the fault of the strange way the exception has been defined rather
-        # than the fact that issubclass can be customised while the
-        # exception checks can't.
+        # Unlike isinstance and issubclass, CPython exception handling
+        # currently only looks at the concrete type hierarchy (ignoring
+        # the instance and subclass checking hooks). While Guido considers
+        # that a bug rather than a feature, it's a fairly hard one to fix
+        # due to various internal implementation details. suppress provides
+        # the simpler issubclass based semantics, rather than trying to
+        # exactly reproduce the limitations of the CPython interpreter.
+        #
         # See http://bugs.python.org/issue12029 for more details
         return exctype is not None and issubclass(exctype, self._exceptions)
 
-# Use a wrapper function since we don't care about supporting inheritance
-# and a function gives much cleaner output in help()
-def suppress(*exceptions):
-    """Context manager to suppress specified exceptions
-
-    After the exception is suppressed, execution proceeds with the next
-    statement following the with statement.
-
-         with suppress(FileNotFoundError):
-             os.remove(somefile)
-         # Execution still resumes here if the file was already removed
-    """
-    return _SuppressExceptions(*exceptions)
 
 # Inspired by discussions on http://bugs.python.org/issue13585
 class ExitStack(object):

Lib/test/test_contextlib.py

@@ -14,6 +14,20 @@ except ImportError:
 
 class ContextManagerTestCase(unittest.TestCase):
 
+    def test_instance_docstring_given_function_docstring(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        # See http://bugs.python.org/issue19404 for why this doesn't currently
+        # affect help() output :(
+        def gen_with_docstring():
+            """This has a docstring"""
+            yield
+        gen_docstring = gen_with_docstring.__doc__
+        cm_with_docstring = contextmanager(gen_with_docstring)
+        self.assertEqual(cm_with_docstring.__doc__, gen_docstring)
+        obj = cm_with_docstring()
+        self.assertEqual(obj.__doc__, gen_docstring)
+        self.assertNotEqual(obj.__doc__, type(obj).__doc__)
+
     def test_contextmanager_plain(self):
         state = []
         @contextmanager
@@ -109,7 +123,11 @@ class ContextManagerTestCase(unittest.TestCase):
 
 class ClosingTestCase(unittest.TestCase):
 
-    # XXX This needs more work
+    def test_instance_docs(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        cm_docstring = closing.__doc__
+        obj = closing(None)
+        self.assertEqual(obj.__doc__, cm_docstring)
 
     def test_closing(self):
         state = []
@@ -205,6 +223,7 @@ class LockContextTestCase(unittest.TestCase):
 
 
 class mycontext(ContextDecorator):
+    """Example decoration-compatible context manager for testing"""
     started = False
     exc = None
     catch = False
@@ -220,6 +239,12 @@ class mycontext(ContextDecorator):
 
 class TestContextDecorator(unittest.TestCase):
 
+    def test_instance_docs(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        cm_docstring = mycontext.__doc__
+        obj = mycontext()
+        self.assertEqual(obj.__doc__, cm_docstring)
+
     def test_contextdecorator(self):
         context = mycontext()
         with context as result:
@@ -373,6 +398,12 @@ class TestContextDecorator(unittest.TestCase):
 
 class TestExitStack(unittest.TestCase):
 
+    def test_instance_docs(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        cm_docstring = ExitStack.__doc__
+        obj = ExitStack()
+        self.assertEqual(obj.__doc__, cm_docstring)
+
     def test_no_resources(self):
         with ExitStack():
             pass
@@ -634,6 +665,12 @@ class TestExitStack(unittest.TestCase):
 
 class TestRedirectStdout(unittest.TestCase):
 
+    def test_instance_docs(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        cm_docstring = redirect_stdout.__doc__
+        obj = redirect_stdout(None)
+        self.assertEqual(obj.__doc__, cm_docstring)
+
     def test_redirect_to_string_io(self):
         f = io.StringIO()
         msg = "Consider an API like help(), which prints directly to stdout"
@@ -671,6 +708,12 @@ class TestRedirectStdout(unittest.TestCase):
 
 class TestSuppress(unittest.TestCase):
 
+    def test_instance_docs(self):
+        # Issue 19330: ensure context manager instances have good docstrings
+        cm_docstring = suppress.__doc__
+        obj = suppress()
+        self.assertEqual(obj.__doc__, cm_docstring)
+
     def test_no_result_from_enter(self):
         with suppress(ValueError) as enter_result:
             self.assertIsNone(enter_result)
@@ -683,16 +726,26 @@ class TestSuppress(unittest.TestCase):
         with suppress(TypeError):
             len(5)
 
+    def test_exception_hierarchy(self):
+        with suppress(LookupError):
+            'Hello'[50]
+
+    def test_other_exception(self):
+        with self.assertRaises(ZeroDivisionError):
+            with suppress(TypeError):
+                1/0
+
+    def test_no_args(self):
+        with self.assertRaises(ZeroDivisionError):
+            with suppress():
+                1/0
+
     def test_multiple_exception_args(self):
         with suppress(ZeroDivisionError, TypeError):
             1/0
         with suppress(ZeroDivisionError, TypeError):
             len(5)
 
-    def test_exception_hierarchy(self):
-        with suppress(LookupError):
-            'Hello'[50]
-
     def test_cm_is_reentrant(self):
         ignore_exceptions = suppress(Exception)
         with ignore_exceptions:

Misc/NEWS

@@ -21,6 +21,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #19330: the unnecessary wrapper functions have been removed from the
+  implementations of the new contextlib.redirect_stdout and
+  contextlib.suppress context managers, which also ensures they provide
+  reasonable help() output on instances
+
 - Issue #18685: Restore re performance to pre-PEP 393 levels.
 
 - Issue #19339: telnetlib module is now using time.monotonic() when available