Maybe a bit of example code will help: Notice the difference in the call signatures of foo
, class_foo
and static_foo
:
class A(object):
def foo(self, x):
print(f"executing foo({self}, {x})")
@classmethod
def class_foo(cls, x):
print(f"executing class_foo({cls}, {x})")
@staticmethod
def static_foo(x):
print(f"executing static_foo({x})")
a = A()
Below is the usual way an object instance calls a method. The object instance, a
, is implicitly passed as the first argument.
a.foo(1)
# executing foo(<__main__.A object at 0xb7dbef0c>, 1)
With classmethods, the class of the object instance is implicitly passed as the first argument instead of self
.
a.class_foo(1)
# executing class_foo(<class '__main__.A'>, 1)
You can also call class_foo
using the class. In fact, if you define something to be
a classmethod, it is probably because you intend to call it from the class rather than from a class instance. A.foo(1)
would have raised a TypeError, but A.class_foo(1)
works just fine:
A.class_foo(1)
# executing class_foo(<class '__main__.A'>, 1)
One use people have found for class methods is to create inheritable alternative constructors.
With staticmethods, neither self
(the object instance) nor cls
(the class) is implicitly passed as the first argument. They behave like plain functions except that you can call them from an instance or the class:
a.static_foo(1)
# executing static_foo(1)
A.static_foo('hi')
# executing static_foo(hi)
Staticmethods are used to group functions which have some logical connection with a class to the class.
foo
is just a function, but when you call a.foo
you don't just get the function,
you get a "partially applied" version of the function with the object instance a
bound as the first argument to the function. foo
expects 2 arguments, while a.foo
only expects 1 argument.
a
is bound to foo
. That is what is meant by the term "bound" below:
print(a.foo)
# <bound method A.foo of <__main__.A object at 0xb7d52f0c>>
With a.class_foo
, a
is not bound to class_foo
, rather the class A
is bound to class_foo
.
print(a.class_foo)
# <bound method type.class_foo of <class '__main__.A'>>
Here, with a staticmethod, even though it is a method, a.static_foo
just returns
a good 'ole function with no arguments bound. static_foo
expects 1 argument, and
a.static_foo
expects 1 argument too.
print(a.static_foo)
# <function static_foo at 0xb7d479cc>
And of course the same thing happens when you call static_foo
with the class A
instead.
print(A.static_foo)
# <function static_foo at 0xb7d479cc>
Best Answer
Some basics first.
With objects, you need to deal with their attributes. Ordinarily, we do
instance.attribute
. Sometimes we need more control (when we do not know the name of the attribute in advance).For example,
instance.attribute
would becomegetattr(instance, attribute_name)
. Using this model, we can get the attribute by supplying the attribute_name as a string.Use of
__getattr__
You can also tell a class how to deal with attributes which it doesn't explicitly manage and do that via
__getattr__
method.Python will call this method whenever you request an attribute that hasn't already been defined, so you can define what to do with it.
A classic use case:
Caveats and use of
__getattribute__
If you need to catch every attribute regardless whether it exists or not, use
__getattribute__
instead. The difference is that__getattr__
only gets called for attributes that don't actually exist. If you set an attribute directly, referencing that attribute will retrieve it without calling__getattr__
.__getattribute__
is called all the times.