Question

What is the best / correct way to use item assignment for python string ?

i.e s = "ABCDEFGH" s[1] = 'a' s[-1]='b' ?

Normal way will throw : 'str' object does not support item assignment

Answer 1

Strings are immutable. That means you can’t assign to them at all. You could use formatting:

>>> s = 'abc{0}efg'.format('d')
>>> s
'abcdefg'

Or concatenation:

>>> s = 'abc' + 'd' + 'efg'
>>> s
'abcdefg'

Or replacement (thanks Odomontois for reminding me):

>>> s = 'abc0efg'
>>> s.replace('0', 'd')
'abcdefg'

But keep in mind that all of these methods create copies of the string, rather than modifying it in-place. If you want in-place modification, you could use a bytearray — though that will only work for plain ascii strings, as alexis points out.

>>> b = bytearray('abc0efg')
>>> b[3] = 'd'
>>> b
bytearray(b'abcdefg')

Or you could create a list of characters and manipulate that. This is probably the most efficient and correct way to do frequent, large-scale string manipulation:

>>> l = list('abc0efg')
>>> l[3] = 'd'
>>> l
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> ''.join(l)
'abcdefg'

And consider the re module for more complex operations.

String formatting and list manipulation are the two methods that are most likely to be correct and efficient IMO — string formatting when only a few insertions are required, and list manipulation when you need to frequently update your string.