r"""
Special Methods for Classes
AUTHORS:
- Nicolas M. Thiery (2009-2011) implementation of
``__classcall__``, ``__classget__``, ``__classcontains__``;
- Florent Hivert (2010-2012): implementation of ``__classcall_private__``,
documentation, Cythonization and optimization.
"""
include '../ext/python.pxi'
cdef extern from "Python.h":
ctypedef PyObject *(*callfunc)(type, object, object) except NULL
ctypedef struct PyTypeObject_call "PyTypeObject":
callfunc tp_call
cdef PyTypeObject_call PyType_Type
__all__ = ['ClasscallMetaclass', 'typecall', 'timeCall']
cdef class ClasscallMetaclass(NestedClassMetaclass):
"""
A metaclass providing support for special methods for classes.
From the Section :python:`Special method names
<reference/datamodel.html#special-method-names>` of the Python Reference
Manual:
\`a class ``cls`` can implement certain operations on its instances
that are invoked by special syntax (such as arithmetic operations or
subscripting and slicing) by defining methods with special
names\'.
The purpose of this metaclass is to allow for the class ``cls`` to
implement analogues of those special methods for the operations on the
class itself.
Currently, the following special methods are supported:
- ``.__classcall__`` (and ``.__classcall_private__``) for
customizing ``cls(...)`` (analogue of ``.__call__``).
- ``.__classcontains__`` for customizing membership testing
``x in cls`` (analogue of ``.__contains__``).
- ``.__classget__`` for customizing the binding behavior in
``foo.cls`` (analogue of ``.__get__``).
See the documentation of :meth:`.__call__` and of :meth:`.__get__`
and :meth:`.__contains__` for the description of the respective
protocols.
.. warning:: for technical reasons, ``__classcall__``,
``__classcall_private__``, ``__classcontains__``, and
``__classget__`` must be defined as :func:`staticmethod`'s, even
though they receive the class itself as their first argument.
``ClasscallMetaclass`` is an extension of the base :class:`type`.
TODO: find a good name for this metaclass.
TESTS::
sage: PerfectMatchings(2).list()
[PerfectMatching [(2, 1)]]
.. note::
If a class is put in this metaclass it automatically becomes a
new-style class::
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class Foo:
... __metaclass__ = ClasscallMetaclass
sage: x = Foo(); x
<__main__.Foo object at 0x...>
sage: issubclass(Foo, object)
True
sage: isinstance(Foo, type)
True
"""
_included_private_doc_ = ['__call__', '__contains__', '__get__']
def __cinit__(self, *args, **opts):
r"""
TESTS::
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class FOO(object):
... __metaclass__ = ClasscallMetaclass
sage: isinstance(FOO, ClasscallMetaclass) # indirect doctest
True
"""
if '__classcall_private__' in self.__dict__:
self.classcall = self.__classcall_private__
elif hasattr(self, "__classcall__"):
self.classcall = self.__classcall__
else:
self.classcall = None
self.classcontains = getattr(self, "__classcontains__", None)
self.classget = getattr(self, "__classget__", None)
def __call__(cls, *args, **opts):
r"""
This method implements ``cls(<some arguments>)``.
Let ``cls`` be a class in :class:`ClasscallMetaclass`, and
consider a call of the form::
cls(<some arguments>)
- If ``cls`` defines a method ``__classcall_private__``, then
this results in a call to::
cls.__classcall_private__(cls, <some arguments>)
- Otherwise, if ``cls`` has a method ``__classcall__``, then instead
the following is called::
cls.__classcall__(cls, <some arguments>)
- If neither of these two methods are implemented, then the standard
``type.__call__(cls, <some arguments>)`` is called, which in turn
uses :meth:`~object.__new__` and :meth:`~object.__init__` as usual
(see Section :python:`Basic Customization
<reference/datamodel.html#basic-customization>` in the Python
Reference Manual).
.. warning:: for technical reasons, ``__classcall__`` must be
defined as a :func:`staticmethod`, even though it receives
the class itself as its first argument.
EXAMPLES::
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class Foo(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcall__(cls):
... print "calling classcall"
... return type.__call__(cls)
... def __new__(cls):
... print "calling new"
... return super(Foo, cls).__new__(cls)
... def __init__(self):
... print "calling init"
sage: Foo()
calling classcall
calling new
calling init
<__main__.Foo object at ...>
This behavior is inherited::
sage: class Bar(Foo): pass
sage: Bar()
calling classcall
calling new
calling init
<__main__.Bar object at ...>
We now show the usage of ``__classcall_private__``::
sage: class FooNoInherits(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcall_private__(cls):
... print "calling private classcall"
... return type.__call__(cls)
...
sage: FooNoInherits()
calling private classcall
<__main__.FooNoInherits object at ...>
Here the behavior is not inherited::
sage: class BarNoInherits(FooNoInherits): pass
sage: BarNoInherits()
<__main__.BarNoInherits object at ...>
We now show the usage of both::
sage: class Foo2(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcall_private__(cls):
... print "calling private classcall"
... return type.__call__(cls)
... @staticmethod
... def __classcall__(cls):
... print "calling classcall with %s"%cls
... return type.__call__(cls)
...
sage: Foo2()
calling private classcall
<__main__.Foo2 object at ...>
sage: class Bar2(Foo2): pass
sage: Bar2()
calling classcall with <class '__main__.Bar2'>
<__main__.Bar2 object at ...>
.. rubric:: Discussion
Typical applications include the implementation of factories or of
unique representation (see :class:`UniqueRepresentation`). Such
features are traditionaly implemented by either using a wrapper
function, or fiddling with :meth:`~object.__new__`.
The benefit, compared with fiddling directly with
:meth:`~object.__new__` is a clear separation of the three distinct
roles:
- ``cls.__classcall__``: what ``cls(<...>)`` does
- ``cls.__new__``: memory allocation for a *new* instance
- ``cls.__init__``: initialization of a newly created instance
The benefit, compared with using a wrapper function, is that the
user interface has a single handle for the class::
sage: x = Partition([3,2,2])
sage: isinstance(x, Partition) # todo: not implemented
instead of::
sage: isinstance(x, sage.combinat.partition.Partition_class)
True
Another difference is that ``__classcall__`` is inherited by
subclasses, which may be desirable, or not. If not, one should
instead define the method ``__classcall_private__`` which will
not be called for subclasses. Specifically, if a class ``cls``
defines both methods ``__classcall__`` and
``__classcall_private__`` then, for any subclass ``sub`` of ``cls``:
- ``cls(<args>)`` will call ``cls.__classcall_private__(cls, <args>)``
- ``sub(<args>)`` will call ``cls.__classcall__(sub, <args>)``
TESTS:
We check that the idiom ``method_name in cls.__dict__`` works
for extension types::
sage: "_sage_" in SageObject.__dict__, "_sage_" in Parent.__dict__
(True, False)
We check for memory leaks::
sage: class NOCALL(object):
... __metaclass__ = ClasscallMetaclass
... pass
sage: sys.getrefcount(NOCALL())
1
We check that exception are correctly handled::
sage: class Exc(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcall__(cls):
... raise ValueError, "Calling classcall"
sage: Exc()
Traceback (most recent call last):
...
ValueError: Calling classcall
"""
if cls.classcall is not None:
return cls.classcall(cls, *args, **opts)
else:
res = <object> PyType_Type.tp_call(cls, args, opts)
Py_XDECREF(res)
return res
def __get__(cls, instance, owner):
r"""
This method implements instance binding behavior for nested classes.
Suppose that a class ``Outer`` contains a nested class ``cls`` which
is an instance of this metaclass. For any object ``obj`` of ``cls``,
this method implements a instance binding behavior for ``obj.cls`` by
delegating it to ``cls.__classget__(Outer, obj, owner)`` if available.
Otherwise, ``obj.cls`` results in ``cls``, as usual.
Similarily, a class binding as in ``Outer.cls`` is delegated
to ``cls.__classget__(Outer, None, owner)`` if available and
to ``cls`` if not.
.. warning:: for technical reasons, ``__classget__`` must be
defined as a :func:`staticmethod`, even though it receives
the class itself as its first argument.
For technical details, and in particular the description of the
``owner`` argument, see the Section :python:`Implementing Descriptor
<reference/datamodel.html#implementing-descriptors>` in the Python
reference manual.
EXAMPLES:
We show how to implement a nested class ``Outer.Inner`` with a
binding behavior, as if it was a method of ``Outer``: namely,
for ``obj`` an instance of ``Outer``, calling
``obj.Inner(...)`` is equivalent to ``Outer.Inner(obj, ...)``::
sage: import functools
sage: from sage.misc.nested_class import NestedClassMetaclass
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class Outer:
... __metaclass__ = NestedClassMetaclass # workaround for python pickling bug
...
... class Inner(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classget__(cls, instance, owner):
... print "calling __classget__(%s, %s, %s)"%(
... cls, instance, owner)
... if instance is None:
... return cls
... return functools.partial(cls, instance)
... def __init__(self, instance):
... self.instance = instance
sage: obj = Outer()
sage: bar = obj.Inner()
calling __classget__(<class '__main__.Outer.Inner'>, <__main__.Outer object at 0x...>, <class '__main__.Outer'>)
sage: bar.instance == obj
True
Calling ``Outer.Inner`` returns the (unbinded) class as usual::
sage: Inner = Outer.Inner
calling __classget__(<class '__main__.Outer.Inner'>, None, <class '__main__.Outer'>)
sage: Inner
<class '__main__.Outer.Inner'>
sage: type(bar) is Inner
True
.. warning:: Inner has to be a new style class (i.e. a subclass of object).
.. warning::
calling ``obj.Inner`` does no longer return a class::
sage: bind = obj.Inner
calling __classget__(<class '__main__.Outer.Inner'>, <__main__.Outer object at 0x...>, <class '__main__.Outer'>)
sage: bind
<functools.partial object at 0x...>
"""
if cls.classget:
return cls.classget(cls, instance, owner)
else:
return cls
def __contains__(cls, x):
r"""
This method implements membership testing for a class
Let ``cls`` be a class in :class:`ClasscallMetaclass`, and consider
a call of the form::
x in cls
If ``cls`` defines a method ``__classcontains__``, then this
results in a call to::
cls.__classcontains__(cls, x)
.. warning:: for technical reasons, ``__classcontains__`` must
be defined as a :func:`staticmethod`, even though it
receives the class itself as its first argument.
EXAMPLES:
We construct a class which implements membership testing, and
which contains ``1`` and no other x::
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class Foo(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcontains__(cls, x):
... return x == 1
sage: 1 in Foo
True
sage: 2 in Foo
False
We now check that for a class without ``__classcontains__``
method, we emulate the usual error message::
sage: from sage.misc.classcall_metaclass import ClasscallMetaclass
sage: class Bar(object):
... __metaclass__ = ClasscallMetaclass
sage: 1 in Bar
Traceback (most recent call last):
...
TypeError: argument of type 'type' is not iterable
"""
if cls.classcontains:
return cls.classcontains(cls, x)
else:
return x in object
def typecall(type cls, *args, **opts):
r"""
Object construction
This is a faster equivalent to ``type.__call__(cls, <some arguments>)``.
INPUT:
- ``cls`` -- the class used for constructing the instance. It must be
a builtin type or a new style class (inheriting from :class:`object`).
EXAMPLES::
sage: from sage.misc.classcall_metaclass import typecall
sage: class Foo(object): pass
sage: typecall(Foo)
<__main__.Foo object at 0x...>
sage: typecall(list)
[]
sage: typecall(Integer, 2)
2
.. warning::
:func:`typecall` doesn't work for old style class (not inheriting from
:class:`object`)::
sage: class Bar: pass
sage: typecall(Bar)
Traceback (most recent call last):
...
TypeError: Argument 'cls' has incorrect type (expected type, got classobj)
"""
res = <object> PyType_Type.tp_call(cls, args, opts)
Py_XDECREF(res)
return res
class CRef(object):
def __init__(self, i):
"""
TESTS::
sage: from sage.misc.classcall_metaclass import CRef
sage: P = CRef(2); P.i
3
"""
self.i = i+1
class C2(object):
__metaclass__ = ClasscallMetaclass
def __init__(self, i):
"""
TESTS::
sage: from sage.misc.classcall_metaclass import C2
sage: P = C2(2); P.i
3
"""
self.i = i+1
class C3(object, metaclass = ClasscallMetaclass):
def __init__(self, i):
"""
TESTS::
sage: from sage.misc.classcall_metaclass import C3
sage: P = C3(2); P.i
3
"""
self.i = i+1
class C2C(object):
__metaclass__ = ClasscallMetaclass
@staticmethod
def __classcall__(cls, i):
"""
TESTS::
sage: from sage.misc.classcall_metaclass import C2C
sage: C2C(2)
3
"""
return i+1
def timeCall(T, int n, *args):
r"""
We illustrate some timing when using the classcall mechanism.
EXAMPLES::
sage: from sage.misc.classcall_metaclass import (
... ClasscallMetaclass, CRef, C2, C3, C2C, timeCall)
sage: timeCall(object, 1000)
For reference let construct basic objects and a basic Python class::
sage: %timeit timeCall(object, 1000) # not tested
625 loops, best of 3: 41.4 µs per loop
sage: i1 = int(1); i3 = int(3) # don't use Sage's Integer
sage: class PRef(object):
... def __init__(self, i):
... self.i = i+i1
For a Python class, compared to the reference class there is a 10%
overhead in using :class:`ClasscallMetaclass` if there is no classcall
defined::
sage: class P(object):
... __metaclass__ = ClasscallMetaclass
... def __init__(self, i):
... self.i = i+i1
sage: %timeit timeCall(PRef, 1000, i3) # not tested
625 loops, best of 3: 420 µs per loop
sage: %timeit timeCall(P, 1000, i3) # not tested
625 loops, best of 3: 458 µs per loop
For a Cython class (not cdef since they doesn't allows metaclasses), the
overhead is a little larger::
sage: %timeit timeCall(CRef, 1000, i3) # not tested
625 loops, best of 3: 266 µs per loop
sage: %timeit timeCall(C2, 1000, i3) # not tested
625 loops, best of 3: 298 µs per loop
Let's now compare when there is a classcall defined::
sage: class PC(object):
... __metaclass__ = ClasscallMetaclass
... @staticmethod
... def __classcall__(cls, i):
... return i+i1
sage: %timeit timeCall(C2C, 1000, i3) # not tested
625 loops, best of 3: 148 µs per loop
sage: %timeit timeCall(PC, 1000, i3) # not tested
625 loops, best of 3: 289 µs per loop
The overhead of the indirection ( ``C(...) ->
ClasscallMetaclass.__call__(...) -> C.__classcall__(...)``) is
unfortunately quite large in this case (two method calls instead of
one). In reasonable usecases, the overhead should be mostly hidden by the
computations inside the classcall::
sage: %timeit timeCall(C2C.__classcall__, 1000, C2C, i3) # not tested
625 loops, best of 3: 33 µs per loop
sage: %timeit timeCall(PC.__classcall__, 1000, PC, i3) # not tested
625 loops, best of 3: 131 µs per loop
Finally, there is no significant difference between Cython's V2 and V3
syntax for metaclass::
sage: %timeit timeCall(C2, 1000, i3) # not tested
625 loops, best of 3: 330 µs per loop
sage: %timeit timeCall(C3, 1000, i3) # not tested
625 loops, best of 3: 328 µs per loop
"""
cdef int i
for 0<=i<n:
T(*args)
