Explain this Error when using an int as base class

Question

I know that it is wrong to do this, but I try to understand what is going on behind the scenes (and if you could potentially duck-type a class for extending it). The code is as follows:

>>> a = 2
>>> class C(a):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() takes at most 2 arguments (3 given)

I'm trying to understand why int() would be called in this example. It is the type of a but I don't get why that is important here.

You're not using `int` as a base class, you're trying to use *an integer* as the base class. — jonrsharpe, Sep 14 '15 at 17:55
@jonrsharpe Nono, I know it is "wrong", I'm interested in what and why this is happening — WorldSEnder, Sep 14 '15 at 17:56
It's trying to call `type(2)` (`int`) instead of `type(int)` (`type`), and `int` doesn't have the same signature as `type`. — jonrsharpe, Sep 14 '15 at 17:57

score 5 · Accepted Answer · edited May 23 '17 at 12:08

5

You are trying to inherit from an instance object, an integer object. Python treats type(a) as the metatype. The metatype is passed the class name, a tuple of the baseclasses and the body of the class as a dictionary; it is expected to produce the new class object.

In this case, that means type(2)('C', (2,), {}) is called:

>>> type(2)('C', (2,), {})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() takes at most 2 arguments (3 given)

Normally, that'd be the type() object itself:

>>> type('C', (object,), {})
<class '__main__.C'>

where I used object as a base class instead.

See What is a metaclass in Python? for everything you didn't want to know about how metaclasses work in Python.

edited May 23 '17 at 12:08

Community

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answered Sep 14 '15 at 17:56

Martijn Pieters

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so if I get this right, I would be able to "inherit from an instance" if the type of that object behaves like a metaclass, correct? – WorldSEnder Sep 14 '15 at 18:00
@WorldSEnder well, yes - `int` is an instance of `type`, for example! – jonrsharpe Sep 14 '15 at 18:01
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@WorldSEnder: yes, and all classes are *normally* instances of a metaclass, directly from `type` or a subclass of `type`. – Martijn Pieters Sep 14 '15 at 18:04

taesu · Answer 2 · 2015-09-14T17:59:21.420

1

You are basically doing this:

class C(2):
    pass

As far as I am aware, you cannot inherit from an instance of object in this fashion.

You want to do:

class C(int):
    pass

OR

class C(type(a)):
    pass

edited Sep 14 '15 at 17:59

answered Sep 14 '15 at 17:55

taesu

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Although this is correct, it doesn't actually answer the question the OP is asking – jonrsharpe Sep 14 '15 at 17:59

Explain this Error when using an int as base class

2 Answers2