'in-place' string modifications in Python

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 a

Answer 1

If I ever need to do something like that I just convert it to a mutable list

For example... (though it would be easier to use sort (see second example) )

>>> s = "abcdfe"
>>> s = list(s)
>>> s[4] = "e"
>>> s[5] = "f"
>>> s = ''.join(s)
>>> print s
abcdef
>>>
# second example
>>> s.sort()
>>> s = ''.join(s)

Answer 2

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)

Answer 3

string.translate is probably the closest function to what you're after.

Answer 4

>>> 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.

Answer 5

I did that like this:

import tempfile
import shutil

...

f_old = open(input_file, 'r')
with tempfile.NamedTemporaryFile() as tmp:
    for line in f_old:
        tmp.write(line.replace(old_string, new_string))
    f_old.close()
    tmp.flush()
    os.fsync(tmp)
    shutil.copy2(tmp.name, input_file)
    tmp.close()

Answer 6

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)