Python 'self' for function

Question

I have read the SO post on 'self' explained, and I have read the Python documentation on classes. I think I understand the use of self in Python classes and the convention therein.

However, being relatively new to Python and its idioms, I cannot understand why some use self in a procedural type function definition. For example, in the Python documentation on integer types, the example function is:

def bit_length(self):
    s = bin(self)       # binary representation:  bin(-37) --> '-0b100101'
    s = s.lstrip('-0b') # remove leading zeros and minus sign
    return len(s)       # len('100101') --> 6

Replacing self with num is the same functional result; ie:

def bit_length(num):
    s = bin(num)       # binary representation:  bin(-37) --> '-0b100101'
    s = s.lstrip('-0b') # remove leading zeros and minus sign
    return len(s)       # len('100101') --> 6

There is no idiom like __init__ etc that I can see here why self is being used in the first case. I have seen this use of self elsewhere in procedural functions as well, and find it confusing.

So my question: If there is no class or method, why use self in a function definition rather than a descriptive parameter name?

You can find an answer here - stackoverflow.com/questions/5067604/… — Vasin Yuriy
– Vasin Yuriy, Commented Oct 15, 2019 at 9:28

Lucas Moeskops · Accepted Answer · 2010-11-22 21:09:00Z

6

In the example bit_length is defined as a function for the int class, so there is actually a 'class or method'. The idea is that you 'ask' an integer to give its bit_length, hence it is defined to take self as an argument.

answered Nov 22, 2010 at 21:09

Lucas Moeskops

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1 Comment

Steven Rumbalski Over a year ago

In other words, the docs show an equivalent (but inefficient) implementation of one of int's methods, that's why it uses self.

Ignacio Vazquez-Abrams · Accepted Answer · 2010-11-22 21:06:41Z

4

No real reason. But if you're going to monkeypatch it onto an existing class then it acts as a bit of notification for anyone that may be reading the code.

answered Nov 22, 2010 at 21:06

Ignacio Vazquez-Abrams

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2 Comments

user395760 Over a year ago

This. Since methods are just functions (well, mostly) def do_stuff(self): ..., Clazz.do_stuff = do_stuff is pretty much the same as defining a method do_stuff as usual, only more ugly/non-obvious/hacky and therefore not useful except when you're monkey-patching.

Steven Rumbalski Over a year ago

-1. Incorrect. The docs show an equivalent implementation of one of int's methods, that's why it uses self.

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