1. Are all of my class's methods supposed to take 'self' as their first arg?
If by "class's methods" you mean methods on which you called classmethod, then no, they shouldn't take a 'self' parameter, they should take a 'cls' parameter because the first argument to the function will be the class:
class C(object):
@classmethod
def f(cls, *args):
# do stuffUndecorated methods (e.g. those that are not wrapped with classmethod or staticmethod) should, on the other hand, take a 'self' parameter.
2. Am I then supposed to call them with MyClass.foo() or instead:
bar = MyClass() bar.foo()
Classmethods should be called from the class. Python allows you to call them from the instance, but this almost never does what you want, e.g.:
py> d = {}
py> d.fromkeys(range(4))
{0: None, 1: None, 2: None, 3: None}
py> d
{}Note that 'd' is not updated -- I called a classmethod, not an instancemethod. If I had called dict.fromkeys instead, this would have been clearer.
3. Is "bound method" a synonym for instance method?
4. Is "unbound method" a synonym for class method?
No. To simplify things a little[1], a "bound method" is an instance method that has been associated with a specific instance, and an "unbound method" is an instance method that has not been associated with a specific instance. Consider the difference between str.join and ''.join:
py> str.join
<method 'join' of 'str' objects>
py> ', '.join
<built-in method join of str object at 0x01233620>
py> str.join(['a', 'b', 'c'])
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
TypeError: descriptor 'join' requires a 'str' object but received a 'list'
py> ', '.join(['a', 'b', 'c'])
'a, b, c'
py> str.join(', ', ['a', 'b', 'c'])
'a, b, c'In the example above, you can see that str.join is an "unbound method" -- when I try to call it without giving it an instance, it complains. On the other hand, ', '.join is a "bound method" because it has been bound to a specific instance of str (in this case, the instance ', '). When I call it without an instance, it doesn't complain because it's already been bound to an instance.
Where do the so-called "static methods" fit into all this? By the name of them, it sounds like the same thing as class methods...
Staticmethods, like classmethods, are associated with the class object, not the instance objects. The main difference is that when a staticmethod is called, no additional arguments are supplied, while when a classmethod is called, a first argument, the class, is inserted in the argument list:
py> class C(object): ... @classmethod ... def f(*args): ... print args ... @staticmethod ... def g(*args): ... print args ... py> C.f(1, 2, 3) (<class '__main__.C'>, 1, 2, 3) py> C.g(1, 2, 3) (1, 2, 3)
STeVe
[1] Technically, I think classmethods could also considered to be "bound methods" because in this case, the method is associated with a specific instance of 'type' (the class in which it resides) -- you can see this in the first argument that is supplied to the argument list of a classmethod.
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