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"?
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Karl Knechtel
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Katherina
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2 Answers
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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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1Pedantic point: strings can be mutated for `x += 'a'` special case, e.g. https://stackoverflow.com/a/40996908/6260170 – Chris_Rands Oct 08 '18 at 15:24
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@Chris_Rands: Looks like you are right, but from outside it does not have a real mutability effect - some implementations check if this is really used, and if not (so mutability would not be observed), it would actually mutate. Is that true? – Tadeck Oct 09 '18 at 16:35
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@Tadeck Well I think it is *real* but it's merely a CPython optimisation and not directly relevant here (that's why I said I was being pedantic) – Chris_Rands Oct 11 '18 at 20:29
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All string functions as lower, upper, strip are returning a string without modifying the original. If you try to modify a string, as you might think well it is an iterable, it will fail.
x = 'hello'
x[0] = 'i' #'str' object does not support item assignment
There is a good reading about the importance of strings being immutable: Why are Python strings immutable? Best practices for using them
user1767754
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