CODEWITHSUNDEEP
pythonstringreplace
qiaop
qiaop

Reputation: 169

How does str.replace work?

I have a conceptual question. I was trying to get rid of the punctuations in a string, so I wrote the following function:

def nopunc(str):
    for x in string.punctuation:
        str = str.replace(x, " ")
    return str

This worked. However, when I changed the function a little bit:

def nopunc(str):
    for x in string.punctuation:
        str1 = str.replace(x, " ")
    return str1

it didn't work. Instead of having str = str.replace(x, ""), I gave the string a new name called str1. Why would that cause a problem? Does it have something to do with .replace?

Upvotes: 0

Views: 3102

Answers (2)

user2555451
user2555451

Reputation:

Because strings are immutable, str.replace(x, " ") does not modify the string object referenced by str in any way. Instead, it returns a copy of this object were x is replaced with " ".

In your first for-loop, every iteration assigns this newly created object to the existing name str. So, your first function is basically equivalent to:

def nopunc(str):
    str = str.replace(string.punctuation[0], " ")
    str = str.replace(string.punctuation[1], " ")
    str = str.replace(string.punctuation[2], " ")
    ...
    str = str.replace(string.punctuation[31], " ")
    return str

Notice how the value of the name str is continually updated, thereby saving any changes.

The second for-loop however just repeatedly reassigns the name str1 to str.replace(x, " "). This means that your second function is no different than doing:

def nopunc(str):
    str1 = str.replace(string.punctuation[31], " ")
    return str1

Upvotes: 4

Mathias Dolidon
Mathias Dolidon

Reputation: 3913

Replace does not change the value held by str. It is your = assignment that does.

In the first case, all replacements add up, because each call with a different punctuation mark replaces stuff in the string resulting from the previous replacement.

In the second case, all replacements are lost because each call with a new punctuation mark replaces stuff in the (unchanging) original string, and only returns the last replacement (which is replacing tildes with spaces).

Upvotes: 2

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