TypeError: method() takes 1 positional argument but 2 were given

If I have a class…

class MyClass:

    def method(arg):
        print(arg)

…which I use to create an object…

my_object = MyClass()

…on which I call method("foo") like so…

>>> my_object.method("foo")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: method() takes exactly 1 positional argument (2 given)

…why does Python tell me I gave it two arguments, when I only gave one?