A classmethod
object is a descriptor. You need to understand how descriptors work.
In a nutshell, a descriptor is an object which has a method __get__
, which takes three arguments: self
, an instance
, and an instance type
.
During normal attribute lookup, if a looked-up object A
has a method __get__
, that method gets called and what it returns is substituted in place for the object A
. This is how functions (which are also descriptors) become bound methods when you call a method on an object.
class Foo(object):
def bar(self, arg1, arg2):
print arg1, arg2
foo = Foo()
# this:
foo.bar(1,2) # prints '1 2'
# does about the same thing as this:
Foo.__dict__['bar'].__get__(foo, type(foo))(1,2) # prints '1 2'
A classmethod
object works the same way. When it gets looked up, its __get__
method gets called. The __get__
of a classmethod discards the argument corresponding to the instance
(if there was one) and only passes along the instance_type
when it calls __get__
on the wrapped function.
A illustrative doodle:
In [14]: def foo(cls):
....: print cls
....:
In [15]: classmethod(foo)
Out[15]: <classmethod object at 0x756e50>
In [16]: cm = classmethod(foo)
In [17]: cm.__get__(None, dict)
Out[17]: <bound method type.foo of <type 'dict'>>
In [18]: cm.__get__(None, dict)()
<type 'dict'>
In [19]: cm.__get__({}, dict)
Out[19]: <bound method type.foo of <type 'dict'>>
In [20]: cm.__get__({}, dict)()
<type 'dict'>
In [21]: cm.__get__("Some bogus unused string", dict)()
<type 'dict'>
More info on descriptors can be found here (among other places):
http://users.rcn.com/python/download/Descriptor.htm
For the specific task of getting the name of the function wrapped by a classmethod
:
In [29]: cm.__get__(None, dict).im_func.__name__
Out[29]: 'foo'