'str' object does not support item assignment in Python

The other answers are correct, but you can, of course, do something like:

>>> str1 = "mystring"
>>> list1 = list(str1)
>>> list1[5] = 'u'
>>> str1 = ''.join(list1)
>>> print(str1)
mystrung
>>> type(str1)
<type 'str'>

if you really want to.

In Python, strings are immutable, so you can't change their characters in-place.

You can, however, do the following:

for i in str:
    srr += i

The reasons this works is that it's a shortcut for:

for i in str:
    srr = srr + i

The above creates a new string with each iteration, and stores the reference to that new string in srr.

Python strings are immutable so what you are trying to do in C will be simply impossible in python. You will have to create a new string.

I would like to read some characters from a string and put it into other string.

Then use a string slice:

>>> s1 = 'Hello world!!'
>>> s2 = s1[6:12]
>>> print s2
world!