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?

CodePudding user response：

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")]

CodePudding user response：

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.