Changing one character in a string


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

For example:

text = "abcdefg";
text[1] = "Z";
Asked By: kostia



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.

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

Answered By: Martin Beckett
new = text[:1] + 'Z' + text[2:]
Answered By: Jochen Ritzel

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.

Answered By: scvalex

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

However, if you are using CPython, the implementation at, 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.

Answered By: Unknown

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'
# change all Z(s) to "W"
while "Z" in mytext:
      # replace "Z" to "W"
      mytext = mytext.replace('Z', 'W')
Answered By: K.Vee.Shanker.

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

edit: Changed str to s

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

Answered By: Mahmoud

Fastest method?

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

Method 1

Given by this answer

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

Which is pretty slow compared to ‘Method 2’

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

Method 2 (FAST METHOD)

Given by this answer

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

Which is much faster:

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

Method 3:

Byte array:

timeit.timeit("text = 'abcdefg'; s = bytearray(text); s[1] = 'Z'; str(s)", number=1000000)
Answered By: Mehdi Nellen

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)     
        newStr += i


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

Answered By: user5587487

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

Answered By: Paul Nathan

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)

My solution

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

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

Answered By: Manoj Kumar S

A solution combining find and replace methods in a single line if statement could be:

my_var = "stackoverflaw"
my_new_var = my_var.replace('a', 'o', 1) if my_var.find('s') != -1 else my_var
print(f"my_var = {my_var}")           # my_var = stackoverflaw
print(f"my_new_var = {my_new_var}")   # my_new_var = stackoverflow
Answered By: eapetcho

try this :

old_string = "mba"
string_list = list(old_string)
string_list[2] = "e"
//Replace 3rd element

new_string = "".join(string_list)


Answered By: passionatedevops

I like f-strings:

text = f'{text[:1]}Z{text[2:]}'

In my machine this method is 10% faster than the "fast method" of using + to concatenate strings:

>>> timeit.timeit("text = 'abcdefg'; text = text[:1] + 'Z' + text[2:]", number=1000000)
>>> timeit.timeit("text = 'abcdefg'; text = f'{text[:1]}Z{text[2:]}'", number =1000000)
Answered By: OsorioSP

To replace a character in a string

You can use either of the method:

Method 1

In general,

string = f'{string[:index]}{replacing_character}{string[index+1:]}'


text = f'{text[:1]}Z{text[2:]}'

Method 2

In general,

string = string[:index] + replacing_character + string[index+1:]


text = text[:1] + 'Z' + text[2:]
Answered By: Highbrow Director
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