Class instantiation adding extra arguments to function

Question

I have a python3 class with the following code

class Cow:

    def __init__(self, *bovines):
        for i in bovines:
            self.i = i
            print(i)

    def moo(a,b):
        print(a)

animal = Cow
animal.moo('a','b')

It prints, correctly, a.

However, if I run the following (the only difference being that animal = Cow('Annie') instead of animal = Cow)

class Cow:

    def __init__(self, *bovines):
        for i in bovines:
            self.i = i
            print(i)

    def moo(a,b):
        print(a)

animal = Cow('Annie')
animal.moo('a','b')

Then moo returns the error

TypeError: moo() takes 2 positional arguments but 3 were given

I imagine this has something to do with the function accepting whatever is __init__'d as an argument, but I'm not sure how to work around this.

Thanks for any help!

"I imagine this has something to do with the function accepting whatever is `__init__`'d as an argument" Notice how the code you posted does **not** contain a method named `__init__`? It has been named `__init` instead. This is, presumably, the cause of the problem - although the usage example you posted doesn't make any sense: "instantiate `animal` with `cow`" already doesn't make sense, and you show the same code block again without indicating what is done differently to cause the problem. — Karl Knechtel, Jul 07 '21 at 01:02
In the future, please don't rely on vague descriptions such as "with a structure something like this" - instead, create a [minimal, reproducible example](https://stackoverflow.com/help/minimal-reproducible-example). — Karl Knechtel, Jul 07 '21 at 01:03
Thank you. This was an error in the formatting. The code blocks aren't identical now. I think this is minimal and reproducible now. I also fixed capitalization. — Michael, Jul 07 '21 at 01:04
Ah, now I understand. Please see https://stackoverflow.com/questions/23944657/typeerror-method-takes-1-positional-argument-but-2-were-given . — Karl Knechtel, Jul 07 '21 at 01:46
Your "only" difference is a *huge* difference. `animal = Cow` doesn't create an instance of `Cow` at all; it just makes `animal` another reference to the class. — chepner, Jul 07 '21 at 02:11

score 0 · Accepted Answer · edited Jul 07 '21 at 01:32

0

When defining a method for a class in python, you need to have self as the first parameter passed into the definition. See below:

def moo(self, a, b):
    print(a)

The type error is because you are passing in self and then the parameters you'd like to pass in (a & b) to the function moo by calling it. Hence "TypeError: moo() takes 2 positional arguments but 3 were given".

If you were defining a function but not passing in any parameters, you'd still define it as:

def foo(self):
  # do stuff

edited Jul 07 '21 at 01:32

Michael

5
3

answered Jul 07 '21 at 01:15

zahlawi

97
8

Thank you! This was the issue and now this is solved. – Michael Jul 07 '21 at 01:28

Class instantiation adding extra arguments to function

1 Answers1