Special Methods for Classes


  • 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

Bases: sage.misc.nested_class.NestedClassMetaclass

A metaclass providing support for special methods for classes.

From the Section 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 __call__() and of __get__() and __contains__() for the description of the respective protocols.


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.


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().

ClasscallMetaclass is an extension of the base type.


find a good name for this metaclass.


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)
sage: isinstance(Foo, type)
sage.misc.classcall_metaclass.typecall(cls, *args, **kwds)

Object construction

This is a faster equivalent to type.__call__(cls, <some arguments>).


  • cls – the class used for constructing the instance. It must be a builtin type or a new style class (inheriting from object).


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)
sage.misc.classcall_metaclass.timeCall(T, n, *args)

We illustrate some timing when using the classcall mechanism.


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 ClasscallMetaclass if 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

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