python – Changing one character in a string

The Question :

427 people think this question is useful

What is the easiest way in Python to replace a character in a string?

For example:

text = "abcdefg";
text[1] = "Z";
^



598 people think this answer is useful

Don’t modify strings.

Work with them as lists; turn them into strings only when needed.

>>> s = list("Hello zorld")
>>> s
['H', 'e', 'l', 'l', 'o', ' ', 'z', 'o', 'r', 'l', 'd']
>>> s[6] = 'W'
>>> s
['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd']
>>> "".join(s)
'Hello World'



Python strings are immutable (i.e. they can’t be modified). There are a lot of reasons for this. Use lists until you have no choice, only then turn them into strings.

221 people think this answer is useful

Fastest method?

There are three ways. For the speed seekers I recommend ‘Method 2’

Method 1

text = 'abcdefg'
new = list(text)
new[6] = 'W'
''.join(new)



Which is pretty slow compared to ‘Method 2’

timeit.timeit("text = 'abcdefg'; s = list(text); s[6] = 'W'; ''.join(s)", number=1000000)
1.0411581993103027



Method 2 (FAST METHOD)

text = 'abcdefg'
text = text[:1] + 'Z' + text[2:]



Which is much faster:

timeit.timeit("text = 'abcdefg'; text = text[:1] + 'Z' + text[2:]", number=1000000)
0.34651994705200195



Method 3:

Byte array:

timeit.timeit("text = 'abcdefg'; s = bytearray(text); s[1] = 'Z'; str(s)", number=1000000)
1.0387420654296875



131 people think this answer is useful
new = text[:1] + 'Z' + text[2:]



40 people think this answer is useful

Python strings are immutable, you change them by making a copy.
The easiest way to do what you want is probably:

text = "Z" + text[1:]



The text[1:] returns the string in text from position 1 to the end, positions count from 0 so ‘1’ is the second character.

edit: You can use the same string slicing technique for any part of the string

text = text[:1] + "Z" + text[2:]



Or if the letter only appears once you can use the search and replace technique suggested below

13 people think this answer is useful

Starting with python 2.6 and python 3 you can use bytearrays which are mutable (can be changed element-wise unlike strings):

s = "abcdefg"
b_s = bytearray(s)
b_s[1] = "Z"
s = str(b_s)
print s
aZcdefg



edit: Changed str to s

edit2: As Two-Bit Alchemist mentioned in the comments, this code does not work with unicode.

6 people think this answer is useful

Like other people have said, generally Python strings are supposed to be immutable.

However, if you are using CPython, the implementation at python.org, it is possible to use ctypes to modify the string structure in memory.

Here is an example where I use the technique to clear a string.

Mark data as sensitive in python

I mention this for the sake of completeness, and this should be your last resort as it is hackish.

5 people think this answer is useful

This code is not mine. I couldn’t recall the site form where, I took it. Interestingly, you can use this to replace one character or more with one or more charectors. Though this reply is very late, novices like me (anytime) might find it useful.

Change Text function.

mytext = 'Hello Zorld'
mytext = mytext.replace('Z', 'W')
print mytext,



3 people think this answer is useful

Strings are immutable in Python, which means you cannot change the existing string. But if you want to change any character in it, you could create a new string out it as follows,

def replace(s, position, character):
return s[:position] + character + s[position+1:]



replace(‘King’, 1, ‘o’)
// result: Kong

Note: If you give the position value greater than the length of the string, it will append the character at the end.

replace(‘Dog’, 10, ‘s’)
// result: Dogs

2 people think this answer is useful

Actually, with strings, you can do something like this:

oldStr = 'Hello World!'
newStr = ''

for i in oldStr:
if 'a' < i < 'z':
newStr += chr(ord(i)-32)
else:
newStr += i
print(newStr)

'HELLO WORLD!'



Basically, I’m “adding”+”strings” together into a new string :).

0 people think this answer is useful

if your world is 100% ascii/utf-8(a lot of use cases fit in that box):

b = bytearray(s, 'utf-8')
# process - e.g., lowercasing:
#    b[0] = b[i+1] - 32
s = str(b, 'utf-8')



python 3.7.3

0 people think this answer is useful

I would like to add another way of changing a character in a string.

>>> text = '~~~~~~~~~~~'
>>> text = text[:1] + (text[1:].replace(text[0], '+', 1))
'~+~~~~~~~~~'



How faster it is when compared to turning the string into list and replacing the ith value then joining again?.

List approach

>>> timeit.timeit("text = '~~~~~~~~~~~'; s = list(text); s[1] = '+'; ''.join(s)", number=1000000)
0.8268570480013295



My solution

>>> timeit.timeit("text = '~~~~~~~~~~~'; text=text[:1] + (text[1:].replace(text[0], '+', 1))", number=1000000)
0.588400217000526



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