Difference between calling classmethod or regular method from another classmethod

Question

I have been lurking SO a long time and it happens that now it's my first time to ask a question since I couldn't find answer anywhere. I really hope that the title is not that confusing, I don't know how else to name it.

I am working on a Matrix class which I need for my college project (LUP decomposition, matrix inverse, solving linear equations...). Basic matrix operations, operator overloading, some helper methods, nothing too fancy.

As I was writing the code I came up with something that bothers me. I have a private method _makeMatrix. That method creates intended Matrix object. Then, I have classmethods like createFromList and createFromFile and similar, basically methods that allow a different way to create Matrix object. All of these methods call _makeMatrix to actually create Matrix object.

So, my question is, what is the difference between these two, except that in second case I can call _makeMatrix without creating object (which I obviously don't want to because _makeMatrix is intended to be private):

def _makeMatrix(r):
    # some code that creates Matrix object m
    return m

@classmethod
def createFromList(cls, matxList)
    # code that makes matxList suitable for passing to _makeMatrix()
    r = matxList
    return Matrix._makeMatrix(r)

and

@classmethod
def _makeMatrix(cls, r):
    # some code that creates Matrix object m
    return m

@classmethod
def createFromList(cls, matxList)
    # code that makes matxList suitable for passing to _makeMatrix()
    r = matxList
    return cls._makeMatrix(r)

What are the exact differences between these two and are there any benefits/drawbacks of using one or another approach?

On both codes you may write c._makeMatrix(r). BTW, the correct name for this object per PEP-8 is cls — JBernardo
– JBernardo, Commented Oct 14, 2015 at 0:53
What is _makeMatrix in the first case? Instance or Static method? Instance Method would need self as 1st argument. Static method would need a decorator. — sureshvv
– sureshvv, Commented Oct 14, 2015 at 4:46
@JBernardo Thx, I didn't know PEP-8 defines that too.; @sureshvv Yes, you are right, I missed that, self should be there. I've edited my code to address that. — dosvarog
– dosvarog, Commented Oct 14, 2015 at 8:25
@dosvarog Now your code doesn't work... You will be missing an argument when you call Matrix._makeMatrix(r) — JBernardo
– JBernardo, Commented Oct 14, 2015 at 14:07
@JBernardo Yes, you are right. I was on mobile phone so I didn't have my code in front of me. My original code was without self, like your third example. I haven't realized that before, so what is going on here? Does Python implicitly treat that method as a static one, even though I didn't decorate it with @staticmethod? If I put self in _makeMatrix method, I have to call it with cls._makeMatrix(cls, r), so I'm guessing there is also no difference between all these calls like in the previous examples? All this is a bit confusing, I have to admit. — dosvarog
– dosvarog, Commented Oct 14, 2015 at 18:54

JBernardo · Accepted Answer · 2015-10-14 00:58:24Z

2

The proper decorator for this example should be staticmethod. Here's why:

class MyClass:
    @classmethod
    def foo(cls, arg):
        # Here I can reference the class to work on things related to it
        # even if it is being accessed by an object of this class 
        pass


    @staticmethod
    def bar(arg):
        # This one behaves as a pure function unaware of the class it is
        # located on; even if it is accessed by an object of this class
        pass


    def baz(arg):
        # This one is tricky. Because it has no "self", it should crash
        # when accessed by an object of the class but not on the class
        # itself!
        pass

Testing:

x = MyClass()
x.foo(10) # OK, but `cls` is unused
x.bar(10) # OK!
x.baz(10) # Crash!!

MyClass.foo(10) # Ok, but `cls` is unused
MyClass.bar(10) # OK!
MyClass.baz(10) # OK!

So, whenever you need a "pure" function inside a class, i.e. a function which doesn't need any information from the class it is inserted this should be a staticmethod. If you don't want to instantiate an object but require some information from the class, you want classmethod.

edited Oct 14, 2015 at 0:58

answered Oct 14, 2015 at 0:51

JBernardo

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2 Comments

dosvarog Over a year ago

Generally, I agree with you, but this doesn't quite answer my question. What I really want to know is, what is the difference between defining _makeMatrix as classmethod and calling it from createFromList like cls._makeMatrix or leaving it as instance (regular) method and calling it from createFromList like Matrix._makeMatrix? What is difference between these two calls? Is it just semantics or is Python internally doing something different?

JBernardo Over a year ago

@dosvarog There is no difference whatsoever because in this context cls is Matrix. But it is always better to write cls (so your subclasses can work too)

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