Implementing a custom string method

Question

How do you add a custom method to a built-in python datatype? For example, I'd like to implement one of the solutions from this question but be able to call it as follows:

>>> s = "A string with extra whitespace"
>>> print s.strip_inner()
>>> A string with extra whitespace

So how would I define a custom .strip_inner() string method?

• user395760: possible duplicate of Extending builtin classes in python

Answer 1

You can't. And you don't need to.

See Extending builtin classes in python for an alternative solution. Subclassing is the way to go here.

• mwolfe02: You're right. I didn't want to. I wanted to subclass I just couldn't remember what it was called so I couldn't google the solution myself. Thanks.
• mwolfe02: On closer inspection that is not exactly what I was trying to do (as you say, I can't do exactly what I want), but it is certainly a workable solution.

Answer 2

The built-in classes such as str are implemented in C, so you can't manipulate them. What you can do, instead, is extend the str class:

>>> class my_str(str):
... def strip_inner(self):
... return re.sub(r'\s{2,}', ' ', s)
... 
>>> s = my_str("A string with extra whitespace")
>>> print s.strip_inner()
A string with extra whitespace

Answer 3

You can't add methods to built-in classes. But what's wrong with using functions? strip_inner(s) is just fine and pythonic.

If you need polymorphism, then just use if isinstance(obj, type_) to determine what to do, or for something more extensible, use a generic function package, like PEAK-Rules.

• Piotr Dobrogost: +1 for the first paragraph and -1 for the second one. As to why function may be better than method see methods vs. functions