Question

I tried this code to do simple string replacement:

X = "hello world"
X.replace("hello", "goodbye")

Why doesn’t X change, from "hello world" to "goodbye world"?

Answer 1

This is because strings are immutable in Python.

Which means that X.replace("hello","goodbye") returns a copy of X with replacements made. Because of that you need to replace this line:

X.replace("hello", "goodbye")

with this line:

X = X.replace("hello", "goodbye")

More broadly, this is true for all Python string methods that change a string’s content, e.g. replace,strip,translate,lower/upper,join,…

You must assign their output to something if you want to use it and not throw it away, e.g.

X  = X.strip(' \t')
X2 = X.translate(...)
Y  = X.lower()
Z  = X.upper()
A  = X.join(':')
B  = X.capitalize()
C  = X.casefold()

and so on.