Issue 9732: addition of getattr_static to the inspect module
This commit is contained in:
Michael Foord
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89197fe93c
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@ -435,6 +435,14 @@ Glossary
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its first :term:`argument` (which is usually called ``self``).
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See :term:`function` and :term:`nested scope`.
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method resolution order
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Method Resolution Order is the order in which base classes are searched
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for a member during lookup. See `The Python 2.3 Method Resolution Order
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<http://www.python.org/download/releases/2.3/mro/>`_.
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MRO
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See :term:`method resolution order`.
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mutable
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Mutable objects can change their value but keep their :func:`id`. See
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also :term:`immutable`.
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@ -563,3 +563,70 @@ line.
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entry in the list represents the caller; the last entry represents where the
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exception was raised.
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Fetching attributes statically
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------------------------------
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Both :func:`getattr` and :func:`hasattr` can trigger code execution when
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fetching or checking for the existence of attributes. Descriptors, like
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properties, will be invoked and :meth:`__getattr__` and :meth:`__getattribute__`
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may be called.
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For cases where you want passive introspection, like documentation tools, this
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can be inconvenient. `getattr_static` has the same signature as :func:`getattr`
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but avoids executing code when it fetches attributes.
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.. function:: getattr_static(obj, attr, default=None)
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Retrieve attributes without triggering dynamic lookup via the
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descriptor protocol, `__getattr__` or `__getattribute__`.
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Note: this function may not be able to retrieve all attributes
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that getattr can fetch (like dynamically created attributes)
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and may find attributes that getattr can't (like descriptors
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that raise AttributeError). It can also return descriptors objects
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instead of instance members.
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There are several cases that will break `getattr_static` or be handled
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incorrectly. These are pathological enough not to worry about (i.e. if you do
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any of these then you deserve to have everything break anyway):
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* :data:`~object.__dict__` existing (e.g. as a property) but returning the
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wrong dictionary or even returning something other than a
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dictionary
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* classes created with :data:`~object.__slots__` that have the `__slots__`
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member deleted from the class, or a fake `__slots__` attribute
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attached to the instance, or any other monkeying with
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`__slots__`
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* objects that lie about their type by having `__class__` as a
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descriptor (`getattr_static` traverses the :term:`MRO` of whatever type
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`obj.__class__` returns instead of the real type)
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* type objects that lie about their :term:`MRO`
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Descriptors are not resolved (for example slot descriptors or
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getset descriptors on objects implemented in C). The descriptor
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is returned instead of the underlying attribute.
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You can handle these with code like the following. Note that
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for arbitrary getset descriptors invoking these may trigger
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code execution::
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# example code for resolving the builtin descriptor types
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class _foo(object):
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__slots__ = ['foo']
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slot_descriptor = type(_foo.foo)
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getset_descriptor = type(type(open(__file__)).name)
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wrapper_descriptor = type(str.__dict__['__add__'])
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descriptor_types = (slot_descriptor, getset_descriptor, wrapper_descriptor)
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result = getattr_static(some_object, 'foo')
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if type(result) in descriptor_types:
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try:
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result = result.__get__()
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except AttributeError:
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# descriptors can raise AttributeError to
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# indicate there is no underlying value
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# in which case the descriptor itself will
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# have to do
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pass
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@ -1054,3 +1054,67 @@ def stack(context=1):
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def trace(context=1):
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"""Return a list of records for the stack below the current exception."""
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return getinnerframes(sys.exc_info()[2], context)
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# ------------------------------------------------ static version of getattr
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_sentinel = object()
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def _check_instance(obj, attr):
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instance_dict = {}
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try:
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instance_dict = object.__getattribute__(obj, "__dict__")
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except AttributeError:
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pass
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return instance_dict.get(attr, _sentinel)
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def _check_class(klass, attr):
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for entry in getmro(klass):
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try:
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return entry.__dict__[attr]
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except KeyError:
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pass
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return _sentinel
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def getattr_static(obj, attr, default=_sentinel):
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"""Retrieve attributes without triggering dynamic lookup via the
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descriptor protocol, __getattr__ or __getattribute__.
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Note: this function may not be able to retrieve all attributes
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that getattr can fetch (like dynamically created attributes)
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and may find attributes that getattr can't (like descriptors
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that raise AttributeError). It can also return descriptor objects
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instead of instance members in some cases. See the
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documentation for details.
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"""
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instance_result = _sentinel
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if not isinstance(obj, type):
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instance_result = _check_instance(obj, attr)
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klass = obj.__class__
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else:
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klass = obj
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klass_result = _check_class(klass, attr)
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if instance_result is not _sentinel and klass_result is not _sentinel:
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if (_check_class(type(klass_result), '__get__') is not _sentinel and
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_check_class(type(klass_result), '__set__') is not _sentinel):
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return klass_result
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if instance_result is not _sentinel:
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return instance_result
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if klass_result is not _sentinel:
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return klass_result
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if obj is klass:
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# for types we check the metaclass too
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for entry in getmro(type(klass)):
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try:
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return entry.__dict__[attr]
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except KeyError:
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pass
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if default is not _sentinel:
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return default
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raise AttributeError(attr)
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@ -706,12 +706,162 @@ class TestGetcallargsUnboundMethods(TestGetcallargsMethods):
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locs = dict(locs or {}, inst=self.inst)
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return (func, 'inst,' + call_params_string, locs)
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class TestGetattrStatic(unittest.TestCase):
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def test_basic(self):
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class Thing(object):
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x = object()
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thing = Thing()
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self.assertEqual(inspect.getattr_static(thing, 'x'), Thing.x)
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self.assertEqual(inspect.getattr_static(thing, 'x', None), Thing.x)
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with self.assertRaises(AttributeError):
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inspect.getattr_static(thing, 'y')
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self.assertEqual(inspect.getattr_static(thing, 'y', 3), 3)
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def test_inherited(self):
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class Thing(object):
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x = object()
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class OtherThing(Thing):
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pass
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something = OtherThing()
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self.assertEqual(inspect.getattr_static(something, 'x'), Thing.x)
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def test_instance_attr(self):
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class Thing(object):
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x = 2
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def __init__(self, x):
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self.x = x
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thing = Thing(3)
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self.assertEqual(inspect.getattr_static(thing, 'x'), 3)
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del thing.x
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self.assertEqual(inspect.getattr_static(thing, 'x'), 2)
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def test_property(self):
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class Thing(object):
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@property
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def x(self):
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raise AttributeError("I'm pretending not to exist")
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thing = Thing()
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self.assertEqual(inspect.getattr_static(thing, 'x'), Thing.x)
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def test_descriptor(self):
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class descriptor(object):
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def __get__(*_):
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raise AttributeError("I'm pretending not to exist")
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desc = descriptor()
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class Thing(object):
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x = desc
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thing = Thing()
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self.assertEqual(inspect.getattr_static(thing, 'x'), desc)
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def test_classAttribute(self):
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class Thing(object):
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x = object()
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self.assertEqual(inspect.getattr_static(Thing, 'x'), Thing.x)
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def test_inherited_classattribute(self):
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class Thing(object):
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x = object()
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class OtherThing(Thing):
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pass
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self.assertEqual(inspect.getattr_static(OtherThing, 'x'), Thing.x)
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def test_slots(self):
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class Thing(object):
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y = 'bar'
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__slots__ = ['x']
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def __init__(self):
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self.x = 'foo'
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thing = Thing()
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self.assertEqual(inspect.getattr_static(thing, 'x'), Thing.x)
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self.assertEqual(inspect.getattr_static(thing, 'y'), 'bar')
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del thing.x
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self.assertEqual(inspect.getattr_static(thing, 'x'), Thing.x)
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def test_metaclass(self):
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class meta(type):
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attr = 'foo'
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class Thing(object, metaclass=meta):
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pass
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self.assertEqual(inspect.getattr_static(Thing, 'attr'), 'foo')
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class sub(meta):
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pass
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class OtherThing(object, metaclass=sub):
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x = 3
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self.assertEqual(inspect.getattr_static(OtherThing, 'attr'), 'foo')
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class OtherOtherThing(OtherThing):
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pass
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# this test is odd, but it was added as it exposed a bug
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self.assertEqual(inspect.getattr_static(OtherOtherThing, 'x'), 3)
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def test_no_dict_no_slots(self):
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self.assertEqual(inspect.getattr_static(1, 'foo', None), None)
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self.assertNotEqual(inspect.getattr_static('foo', 'lower'), None)
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def test_no_dict_no_slots_instance_member(self):
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# returns descriptor
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with open(__file__) as handle:
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self.assertEqual(inspect.getattr_static(handle, 'name'), type(handle).name)
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def test_inherited_slots(self):
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# returns descriptor
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class Thing(object):
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__slots__ = ['x']
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def __init__(self):
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self.x = 'foo'
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class OtherThing(Thing):
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pass
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# it would be nice if this worked...
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# we get the descriptor instead of the instance attribute
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self.assertEqual(inspect.getattr_static(OtherThing(), 'x'), Thing.x)
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def test_descriptor(self):
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class descriptor(object):
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def __get__(self, instance, owner):
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return 3
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class Foo(object):
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d = descriptor()
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foo = Foo()
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# for a non data descriptor we return the instance attribute
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foo.__dict__['d'] = 1
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self.assertEqual(inspect.getattr_static(foo, 'd'), 1)
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# if the descriptor is a data-desciptor we should return the
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# descriptor
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descriptor.__set__ = lambda s, i, v: None
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self.assertEqual(inspect.getattr_static(foo, 'd'), Foo.__dict__['d'])
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def test_metaclass_with_descriptor(self):
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class descriptor(object):
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def __get__(self, instance, owner):
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return 3
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class meta(type):
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d = descriptor()
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class Thing(object, metaclass=meta):
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pass
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self.assertEqual(inspect.getattr_static(Thing, 'd'), meta.__dict__['d'])
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def test_main():
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run_unittest(
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TestDecorators, TestRetrievingSourceCode, TestOneliners, TestBuggyCases,
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TestInterpreterStack, TestClassesAndFunctions, TestPredicates,
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TestGetcallargsFunctions, TestGetcallargsMethods,
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TestGetcallargsUnboundMethods)
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TestGetcallargsUnboundMethods, TestGetattrStatic
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)
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if __name__ == "__main__":
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test_main()
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@ -25,6 +25,8 @@ Library
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complex zeros on systems where the log1p function fails to respect
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the sign of zero. This fixes a test failure on AIX.
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- Issue #9732: Addition of getattr_static to the inspect module.
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- Issue #10446: Module documentation generated by pydoc now links to a
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version-specific online reference manual.
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@ -5,7 +5,9 @@
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##################################################################
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[project attributes]
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proj.directory-list = [{'dirloc': loc('..'),
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'excludes': [u'Lib/__pycache__'],
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'excludes': [u'Lib/__pycache__',
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u'Doc/build',
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u'build'],
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'filter': '*',
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'include_hidden': False,
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'recursive': True,
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