'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 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.

Answer 1

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'

Answer 2

>>> 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 3

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

Answer 4

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.

Answer 5

def modifyIdx(s, idx, newchar):
    return s[:idx] + newchar + s[idx+1:]

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)

Answer 7

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 8

You can use StringIO or cStringIO classes to receive file-like mutable interface of string.

Answer 9

you can use the UserString module:

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

Answer 10

Here is an example using translate to switch "-" with "." and uppercase "a"s

>>> from string import maketrans
>>> trans_table = maketrans(".-a","-.A")
>>> "foo-bar.".translate(trans_table)
'foo.bAr-'

This is much more efficient that flipping to byte array and back if you just need to do single char replacements

Answer 11

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.

Answer 12

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)