How to use a method that takes an argument as the function in a map function, using a class.method notation?

Question

I'm running the following code and cannot understand why python thinks that I'm not passing an argument to the endswith method:

filtered_list = list(filter(str.endswith("mp4"), my_files))

This is the error that I'm getting:

TypeError: endswith() takes at least 1 argument (0 given)

I know I can do the same thing with a lambda function instead of using the method with the class name, or to use the methodcaller, but I think this way it is shorter and cooler and I don't understand why it doesn't work. Maybe when using a class.method notation I can't pass any arguments and have to use it without paranthases?

Answer 1

It doesn't work because you're immediately calling str.endswith, but with one argument too few. (Try replacing str.endswith("mp4") with input("Hello?"), so you see it's evaluated immediately.)

>>> str.endswith("foo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: endswith() takes at least 1 argument (0 given)
>>> str.endswith("foo", "oo")
True

This has to do with how "foo".endswith is a bound method, i.e. the "generic" str.endswith, but with the first "self" argument already bound to the string instance "foo".

(The error message is a bit cryptic, or "off-by-one", even, because you are indeed supposed to pass one argument in addition to "self").

This is how methods work for user-created classes too; for an instance inst of class MyClass, inst.somefunc(x) is equivalent to MyClass.somefunc(inst, x).

Anyway, the Pythonic, i.e. way cool, way to say what you want is

filtered_list = [f for f in my_files if f.endswith(".mp4")]

Answer 2

filter requires a callable that returns a Boolean value; str.endswith("mp4") does not evaluate to such a callable. You have to use a lambda expression or methodcaller because those do evaluate to callables.