Changing a string in a subroutine in Python

Question

def modify(text):
    text = text[:1] + 'Z' + text[2:]
    return

text = 'abcdefg'
modify(text)
print text

This does not change the string. What is the best way to make such changes?

Welcome to stack over flow. Post what you are expecting also. — WoodChopper, Dec 08 '15 at 16:52

score 2 · Answer 1 · answered Dec 08 '15 at 16:41

2

Strings are immutable, you can't change them in a function and see the change outside the function.

The proper way to do this is to return the modified string and reassign it where it was called.

def modify(text):
    text = text[:1] + 'Z' + text[2:]
    return text

text = 'abcdefg'
text = modify(text)
print text

answered Dec 08 '15 at 16:41

Mark Ransom

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I dont' think this is about immutable strings. I think the issue is that the OP thinks that assignment to a name that has been passed as an argument affects the calling scope. You offer the correct solution, but the incorrect diagnosis. – Steven Rumbalski Dec 08 '15 at 16:44
@StevenRumbalski in that case they should check out http://stackoverflow.com/questions/986006/how-do-i-pass-a-variable-by-reference – Mark Ransom Dec 08 '15 at 16:46
Yes, that addresses it perfectly. Matter of fact, I'm closing this question in favor of that one. – Steven Rumbalski Dec 08 '15 at 16:47

score 1 · Answer 2 · answered Dec 08 '15 at 16:41

You must return the modified text from your modify function, otherwise, your modification won't take any effect:

def modify(text):
    text = text[:1] + 'Z' + text[2:]
    return text

And since text inside modify function is a local variable, the modification won't affect the global text variable (which you passed to modify function) until you assign it:

text = modify(text)

Changing a string in a subroutine in Python

2 Answers2