The missing argument

Question

I have a class like following

class A:
     def __init__(self:A,size:int):
        self.size=size

when I input like:

>>>a=A(5)    
>>>a.size

it will come

>>>5

However when I input like:

>>>A.__init__(5)

it will have an missing argument 'size' error. I just don't understand that, Why I entered an init it still missing argument?

`self:A` should raise an error, as `A` is not defined when `__init__` gets defined. — Ashwini Chaudhary, Feb 23 '14 at 15:07

aIKid · Accepted Answer · 2014-02-23T15:10:10.570

3

__init__ needs a two arguments, self and an integer. Isn't that so simple?

For instance methods, Python will automatically pass a reference of the instance to your function. This is why all instance methods needs at least one argument, self.

This is your case:

>>> class A:
    def __init__(self)::

SyntaxError: invalid syntax
>>> class A:
    def __init__(self):
        print("Created!")


>>> a = A()
Created! #Success
>>> b = A.__init__()
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    b = A.__init__()
TypeError: __init__() missing 1 required positional argument: 'self'

Well, no wonder. __init__ needs an argument, but you're not passing anything.

However, if you insist on calling the function, you can just use another instance:

>>> b = A.__init__(a)
Created!

However this is just calling the function, and no new instance will be produced. b is None.

edited Feb 23 '14 at 15:10

answered Feb 23 '14 at 15:04

aIKid

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@ editor: Thanks, but that would be a good comment for the OP. – aIKid Feb 23 '14 at 15:08
Ok I get this. but when I entered A.__init__(5) why its missing the size argument not self? – user1813564 Feb 23 '14 at 15:15
1

Because the arguments are positional; it assumes that the first is `self`, so `size` must be missing. – jonrsharpe Feb 23 '14 at 15:16

score 1 · Answer 2 · edited May 23 '17 at 12:28

__init__ is the constructor function in Python; read the answers to this question which explain in detail that concept.

In order to call a function (or "method") on a class in Python you need a classmethod; this answer explains the role of classmethod vs instance methods in Python.

The main distinction is that:

for class methods the class is passed as the first argument to the classmethod,
for instance methods, the instance itself is passed as the first argument to the method

The missing argument

2 Answers2