Python modify string in place

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. -- 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.  python  string share improve this question edited Aug 12 '10 at 0:31 asked  Aug 11 '10 at 23:52 Paul Nathan 25.4k 12 84 164

I guess since they are immutable you can't "modify it"... do you mean constructing a new string char-by-char?–  Joe Koberg  Aug 12 '10 at 0:02 This is really rare. Can you give a context or a use case for doing this? –  S.Lott  Aug 12 '10 at 0:21 Adding noise to a message for testing. –  Paul Nathan  Aug 12 '10 at 0:22 @Paul Nathan: "Adding noise to a message"? As in "replacing characters at random"? What's wrong with replace for this? –  S.Lott  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?)–  Paul Nathan  Aug 12 '10 at 5:51 show 1 more comment

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up vote 12 down vote

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. share improve this answer answered  Aug 12 '10 at 0:25 Ned Batchelder 167k 26 316 464

Interesting; I sort of like it, except for the null string. Probably as good as I'm getting though. –  Paul Nathan Aug 12 '10 at 0:35 1   A significant part of the C to me is that it works in-place. –  Joe Koberg  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. –  S.Lott  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. –  Joe Koberg  Aug 12 '10 at 15:26 Isn't map() the same as the list comprehension and join? –  Javier  Aug 12 '10 at 20:53 show 2 more comments

up vote 11 down vote

Don't use a string, use something mutable like bytearray: #!/usr/bin/python s = 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] + 1 print 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 memoryview, though that doesn't support Unicode directly. share improve this answer edited Aug 12 '10 at 0:30 answered  Aug 12 '10 at 0:02 bstpierre 15.2k 9 42 80

2   They aren't Characters, however, just bytes. Works only for ASCII, not Unicode. –  S.Lott  Aug 12 '10 at 0:20 1   Works for valid bytes, not just ASCII. –  bstpierre  Aug 12 '10 at 0:30 add a comment

up vote 9 down vote	you can use the UserString module: >>> import UserString ... s = UserString.MutableString('Python') ... print s Python >>> s[0] = 'c' >>> print s cython share improve this answer answered  Aug 12 '10 at 0:18 killown 1,280 1 11 25
	1   Note that MutableString was deprecated in Python 2.6 and removed in Python 3 - docs.python.org/2/library/…–  msanders  Dec 8 '14 at 15:28 add a comment

up vote 6 down vote	I'd say the most Pythonic way is to use map(): 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) share improve this answer edited Aug 12 '10 at 2:09 answered  Aug 12 '10 at 2:00 Javier 2,292 3 19 40
	1   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)) –  pythonlarry  Apr 3 '13 at 20:54  add a comment

up vote 1 down vote

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' share improve this answer answered  Aug 11 '10 at 23:56 jathanism 19.1k 5 44 72

Strings are immutable and cannot be assigned to by member. –  Paul Nathan  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. –  jathanism  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") –  CoDEmanX  Aug 20 '15 at 20:00  add a comment

up vote 1 down vote	>>> 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. share improve this answer answered  Aug 11 '10 at 23:56 Jungle Hunter 3,979 7 30 58
	1: Don't use "string" as a variable name. 2: That doesn't modify the variable. –  bstpierre  Aug 11 '10 at 23:58 That doesn't select by index and modify it. –  Paul Nathan  Aug 12 '10 at 0:00 @bstpierre: There you go. :) –  Jungle Hunter  Aug 12 '10 at 0:01  @Paul Nathan: Where was "by index" a requirement? –  S.Lott  Aug 12 '10 at 0:19 add a comment

up vote 1 down vote

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)