Why do you need “self.” in Python to refer to instance variables?

Question

I have been programming into a number of languages like Java, Ruby, Haskell and Python. I have to switch between many languages per day due to different projects I work on. Now, the issue is I often forget to write self as the first parameter in the function definitions in Python same is with calling methods on the same object.

That said, I am quite amazed by this approach of Python. Basically we have to type more to get the things done, in the languages like Java and Ruby things are made simple by automatically referencing the variables in the current object.

My question is why is this self necessary? Is it purely a style choice, or is there a reason why Python can't let you omit self the way Java and C++ let you omit this?

Answer 1

1) Why is self required as an explicit parameter in method signatures?

Because methods are functions and foo.bar(baz) is just syntactic sugar for bar(foo, baz). Classes are just dictionaries where some of the values are functions. (Constructors are also just functions, which is why Python doesn't need new) You can say that Python makes it explicit that objects are built from simpler components. This is in accordance with the "explicit is better than implicit"-philosophy.

In contrast, in Java objects really are magic and cannot be reduced to simpler components in the language. In Java (at least until Java 8) a function is always a method owned by an object, and this ownership cannot be changed due to the static nature of the language. Therefore there is no ambiguity about what this refers to, so it makes sense to have it implicitly defined.

JavaScript is an example of a language that has an implicit this like Java, but where functions can exist separately from objects like in Python. This leads to a lot of confusion about what this refers to when functions are passed around and called in different contexts. Many instinctively think this must refer to some intrinsic property of the function, while it is actually purely determined by the way the function is called. I believe having this as an explicit parameter like in Python would make this much less confusing.

Some other benefits of the explicit self-parameter:

• Decorators are just functions which wraps other functions. Since methods are just functions, decorators works just as fine on methods. If there were some kind of implicit self, decorators would not work transparently on methods.

• Classmethods and static methods does not take an instance parameter. Classmethods take a class as the first argument (typically called cls). The explicit self or cls parameters makes it much clearer what is going on, and what you have access to in the method.

2) Why must instances variables always be qualified with"self.?

In Java, you don't need to prefix member variables with "this.", but in Python "self." is always required. The reason is that Python does not have an explicit syntax for declaring variables, so there would be no way to tell if x = 7 is supposed to declare a new local variable or assign to a member variable. Specifying self. solves this ambiguity.

Answer 2

There is a rather simple reason that AFAIK hasn't really been touched upon in the cross-site duplicate, nor here: Python started out as a procedural language. It was based on ABC, also a procedural language.

Object-Orientation was added later, and when it was added, Guido van Rossum wanted to add the minimal amount of features possible, in order to keep the design of Python simple. Python already had dicts and functions, so why add something entirely new to the language, when an object can simply be a dict of slots and a class can simply be a dict of functions? A method can be interpreted as a partially-applied function that closes over a single distinguished argument. And that's precisely how methods are implemented in Python: they aren't. They are just functions which receive an extra distinguished argument.

Answer 3

Here are my conclusions based on above answers and reading the Guido's own rambling on this topic:

The big idea

Functions are the important building blocks in Python (or we should say the only one), in fact we are kind of emulating OOP by using functions.

Given that a class is nothing but a dictionary of functions hence, we can attach any function to any class at run time. Basically it is because of this need to toss the functions around at run time we can do stuffs like Monkey Patching. Here the self parameter supporting the parametric Polymorphism.