Question

In Python, strings are immutable.

What is the standard idiom to walk through a string character-by-character and modify it?

The only methods I can think of are some genuinely stanky hacks related to joining against a result string.

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In C:

for(int i = 0; i < strlen(s); i++){   s[i] = F(s[i]);}

This is super expressive and says exactly what I am doing. That is what I am looking for.

I guess since they are immutable you can't "modify it"... do you mean constructing a new string char-by-char? — Aug 12 '10 at 0:02
This is really rare. Can you give a context or a use case for doing this? — Aug 12 '10 at 0:21
@Paul Nathan: "Adding noise to a message"? As in "replacing characters at random"? What's wrong with replace for this? — Aug 12 '10 at 2:43
@S.Lott: Replace selects against pre-known elements in the list ``mystring.replace("1", "i")`; whereas I want to iterate by index - I don't know what elements will be in the list per se. (Why does this have to be such a pain?) — Aug 12 '10 at 5:51

167k 26 316 464 · Answer 1 · 2010-08-12 00:25:54Z

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The Python analog of your C:

for(int i = 0; i < strlen(s); i++){   s[i] = F(s[i]);}

would be:

s = "".join(F(c) for c in s)

which is also very expressive. It says exactly what is happening, but in a functional style rather than a procedural style.

answered

Aug 12 '10 at 0:25

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Interesting; I sort of like it, except for the null string. Probably as good as I'm getting though. –

Aug 12 '10 at 0:35

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A significant part of the C to me is that it works in-place. –

Aug 12 '10 at 7:33

@Joe Koberg: A significant part of the Python is that it's shorter and more clear. "in place" is limiting because you can't make the string longer. –

Aug 12 '10 at 10:12

I understand that, but the OP seemed to be focusing on it. And rejected many standard python string transformation methods. That it didn't work in-place was the only thing left. –

Aug 12 '10 at 15:26

Isn't map() the same as the list comprehension and join? –

Aug 12 '10 at 20:53

score 11 · Answer 2 · 2010-08-12 00:02:39Z

Don't use a string, use something mutable like bytearray:

#!/usr/bin/pythons = bytearray("my dog has fleas")for n in xrange(len(s)):    s[n] = chr(s[n]).upper()print s

Results in:

MY DOG HAS FLEAS

Edit:

Since this is a bytearray, you aren't (necessarily) working with characters. You're working with bytes. So this works too:

s = bytearray("\x81\x82\x83")for n in xrange(len(s)):    s[n] = s[n] + 1print repr(s)

gives:

bytearray(b'\x82\x83\x84')

If you want to modify characters in a Unicode string, you'd maybe want to work with , though that doesn't support Unicode directly.

They aren't Characters, however, just bytes. Works only for ASCII, not Unicode. — Aug 12 '10 at 0:20

1,280 1 11 25 · Answer 3 · 2010-08-12 00:18:24Z

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you can use the UserString module:

>>> import UserString... s = UserString.MutableString('Python')... print sPython>>> s[0] = 'c'>>> print scython

answered

Aug 12 '10 at 0:18

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Note that MutableString was deprecated in Python 2.6 and removed in Python 3 - –

Dec 8 '14 at 15:28

score 6 · Answer 4 · 2010-08-12 02:00:24Z

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I'd say the most Pythonic way is to use :

s = map(func, s) # func has been applied to every character in s

This is the equivalent of writing:

s = "".join(func(c) for c in s)

answered

Aug 12 '10 at 2:00

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Minor correction/point: One still needs to re-join the returned list (/iter/map-obj in Py3+). So the above should be: s = "".join(map(func, s)) –

Apr 3 '13 at 20:54

19.1k 5 44 72 · Answer 5 · 2010-08-11 23:56:27Z

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Strings are iterable and can be walked through like lists. Strings also have a number of basic methods such as .replace() that might be what you're looking for. All string methods return a new string. So instead of modifying the string in place you can simply replace its existing value.

>>> mystring = 'robot drama'>>> mystring = mystring.replace('r', 'g')>>> mystring'gobot dgama'

answered

Aug 11 '10 at 23:56

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Strings are immutable and cannot be assigned to by member. –

Aug 11 '10 at 23:58

Correct. All string operations return a copy of the input string. The variable name is NOT immutable, however, so by re-assigning a string operation to the same variable name, you are effectively "mutating" the string. –

Aug 12 '10 at 0:00

"In the gage case of altegations to a stging, which do not incgease og decgease its length, it would still be useful if one could use the slice notation fog in-place adjustments: myStr[0:3][:] = 'new'".replace("g","r") –

Aug 20 '15 at 20:00

3,979 7 30 58 · Answer 6 · 2010-08-11 23:56:41Z

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>>> mystring = "Th1s 1s my str1ng">>> mystring.replace("1", "i")'This is my string'

If you want to store this new string you'll have to mystring = mystring.replace("1", "i"). This is because in Python strings are immutable.

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Aug 11 '10 at 23:56

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1: Don't use "string" as a variable name. 2: That doesn't modify the variable. –

Aug 11 '10 at 23:58

That doesn't select by index and modify it. –

Aug 12 '10 at 0:00

@bstpierre: There you go. :) –

Aug 12 '10 at 0:01

@Paul Nathan: Where was "by index" a requirement? –

Aug 12 '10 at 0:19

score 1 · Answer 7

Assigning a particular character to a particular index in a string is not a particularly common operation, so if you find yourself needing to do it, think about whether there may be a better way to accomplish the task. But if you do need to, probably the most standard way would be to convert the string to a list, make your modifications, and then convert it back to a string.

s = 'abcdefgh'l = list(s)l[3] = 'r's2 = ''.join(l)

EDIT: As posted in bstpierre's answer, bytearray is probably even better for this task than list, as long as you're not working with Unicode strings.

s = 'abcdefgh'b = bytearray(s)b[3] = 'r's2 = str(b)

12 Answers