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.
- class sage.misc.classcall_metaclass.ClasscallMetaclass[source]¶
- Bases: - NestedClassMetaclass- A metaclass providing support for special methods for classes. - From the Section Special method names of the Python Reference Manual: - `a class - clscan 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 - clsto 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 - __call__()and of- __get__()and- __contains__()for the description of the respective protocols.- Warning - For technical reasons, - __classcall__,- __classcall_private__,- __classcontains__, and- __classget__must be defined as- staticmethod()’s, even though they receive the class itself as their first argument.- Warning - For efficiency reasons, the resolution for the special methods is done once for all, upon creation of the class. Thus, later dynamic changes to those methods are ignored. But see also - _set_classcall().- ClasscallMetaclassis an extension of the base- type.- Todo - find a good name for this metaclass. - 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): pass sage: x = Foo(); x <__main__.Foo object at 0x...> sage: issubclass(Foo, object) True sage: isinstance(Foo, type) True - >>> from sage.all import * >>> from sage.misc.classcall_metaclass import ClasscallMetaclass >>> class Foo(metaclass=ClasscallMetaclass): pass >>> x = Foo(); x <__main__.Foo object at 0x...> >>> issubclass(Foo, object) True >>> isinstance(Foo, type) True 
- sage.misc.classcall_metaclass.timeCall(T, n, *args)[source]¶
- 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) - >>> from sage.all import * >>> from sage.misc.classcall_metaclass import ( ... ClasscallMetaclass, CRef, C2, C3, C2C, timeCall) >>> timeCall(object, Integer(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(): ....: def __init__(self, i): ....: self.i = i+i1 - >>> from sage.all import * >>> %timeit timeCall(object, Integer(1000)) # not tested 625 loops, best of 3: 41.4 µs per loop >>> i1 = int(Integer(1)); i3 = int(Integer(3)) # don't use Sage's Integer >>> class PRef(): ... def __init__(self, i): ... self.i = i+i1 - For a Python class, compared to the reference class there is a 10% overhead in using - ClasscallMetaclassif there is no classcall defined:- sage: class P(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 - >>> from sage.all import * >>> class P(metaclass=ClasscallMetaclass): ... def __init__(self, i): ... self.i = i+i1 >>> %timeit timeCall(PRef, Integer(1000), i3) # not tested 625 loops, best of 3: 420 µs per loop >>> %timeit timeCall(P, Integer(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 - >>> from sage.all import * >>> %timeit timeCall(CRef, Integer(1000), i3) # not tested 625 loops, best of 3: 266 µs per loop >>> %timeit timeCall(C2, Integer(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 - >>> from sage.all import * >>> class PC(object, metaclass=ClasscallMetaclass): ... @staticmethod ... def __classcall__(cls, i): ... return i+i1 >>> %timeit timeCall(C2C, Integer(1000), i3) # not tested 625 loops, best of 3: 148 µs per loop >>> %timeit timeCall(PC, Integer(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 - >>> from sage.all import * >>> %timeit timeCall(C2C.__classcall__, Integer(1000), C2C, i3) # not tested 625 loops, best of 3: 33 µs per loop >>> %timeit timeCall(PC.__classcall__, Integer(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 - >>> from sage.all import * >>> %timeit timeCall(C2, Integer(1000), i3) # not tested 625 loops, best of 3: 330 µs per loop >>> %timeit timeCall(C3, Integer(1000), i3) # not tested 625 loops, best of 3: 328 µs per loop 
- sage.misc.classcall_metaclass.typecall(cls, *args, **kwds)[source]¶
- 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- object)
 - EXAMPLES: - sage: from sage.misc.classcall_metaclass import typecall sage: class Foo(): pass sage: typecall(Foo) <__main__.Foo object at 0x...> sage: typecall(list) [] sage: typecall(Integer, 2) 2 - >>> from sage.all import * >>> from sage.misc.classcall_metaclass import typecall >>> class Foo(): pass >>> typecall(Foo) <__main__.Foo object at 0x...> >>> typecall(list) [] >>> typecall(Integer, Integer(2)) 2