Why doesn't calling a Python string method do anything unless you assign its output?

python string replace
Katherina picture Katherina · Feb 8, 2012 · Viewed 101.2k times · Source

I try to do a simple string replacement, but I don't know why it doesn't seem to work:

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

I want to change the word hello to goodbye, thus it should change the string "hello world" to "goodbye world". But X just remains "hello world". Why is my code not working?

Answer

Tadeck picture Tadeck · Feb 8, 2012

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 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 "in-place", 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.

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