add some useful utilities for skipping tests with unittest's new skipping ability

most significantly apply a modified portion of the patch from #4242 with
patches for skipping implementation details

Lib/test/test_support.py

@@ -8,6 +8,7 @@ import errno
 import socket
 import sys
 import os
+import platform
 import shutil
 import warnings
 import unittest
@@ -23,7 +24,8 @@ __all__ = ["Error", "TestFailed", "TestSkipped", "ResourceDenied", "import_modul
            "captured_stdout", "TransientResource", "transient_internet",
            "run_with_locale", "set_memlimit", "bigmemtest", "bigaddrspacetest",
            "BasicTestRunner", "run_unittest", "run_doctest", "threading_setup",
-           "threading_cleanup", "reap_children"]
+           "threading_cleanup", "reap_children", "cpython_only",
+           "check_impl_detail"]
 
 class Error(Exception):
     """Base class for regression test exceptions."""
@@ -531,6 +533,21 @@ def captured_output(stream_name):
 def captured_stdout():
     return captured_output("stdout")
 
+def gc_collect():
+    """Force as many objects as possible to be collected.
+
+    In non-CPython implementations of Python, this is needed because timely
+    deallocation is not guaranteed by the garbage collector.  (Even in CPython
+    this can be the case in case of reference cycles.)  This means that __del__
+    methods may be called later than expected and weakrefs may remain alive for
+    longer than expected.  This function tries its best to force all garbage
+    objects to disappear.
+    """
+    import gc
+    gc.collect()
+    gc.collect()
+    gc.collect()
+
 
 #=======================================================================
 # Decorator for running a function in a different locale, correctly resetting
@@ -682,6 +699,55 @@ class BasicTestRunner:
         test(result)
         return result
 
+def _id(obj):
+    return obj
+
+def requires_resource(resource):
+    if resource_is_enabled(resource):
+        return _id
+    else:
+        return unittest.skip("resource {0!r} is not enabled".format(resource))
+
+def cpython_only(test):
+    """
+    Decorator for tests only applicable on CPython.
+    """
+    return impl_detail(cpython=True)(test)
+
+def impl_detail(msg=None, **guards):
+    if check_impl_detail():
+        return _id
+    if msg is None:
+        guardnames, default = _parse_guards(guards)
+        if default:
+            msg = "implementation detail not available on {0}"
+        else:
+            msg = "implementation detail specific to {0}"
+        guardnames = sorted(guardnames.keys())
+        msg = msg.format(' or '.join(guardnames))
+    return unittest.skip(msg)
+
+def _parse_guards(guards):
+    # Returns a tuple ({platform_name: run_me}, default_value)
+    if not guards:
+        return ({'cpython': True}, False)
+    is_true = guards.values()[0]
+    assert guards.values() == [is_true] * len(guards)   # all True or all False
+    return (guards, not is_true)
+
+# Use the following check to guard CPython's implementation-specific tests --
+# or to run them only on the implementation(s) guarded by the arguments.
+def check_impl_detail(**guards):
+    """This function returns True or False depending on the host platform.
+       Examples:
+          if check_impl_detail():               # only on CPython (default)
+          if check_impl_detail(jython=True):    # only on Jython
+          if check_impl_detail(cpython=False):  # everywhere except on CPython
+    """
+    guards, default = _parse_guards(guards)
+    return guards.get(platform.python_implementation().lower(), default)
+
+
 
 def _run_suite(suite):
     """Run tests from a unittest.TestSuite-derived class."""