How do I modify a string in place? | Python

You can't, because strings are immutable. If you need an object with this ability, try converting the string to a list or use the array module:
>>> s = "Hello, world" >>> a = list(s)
>>>print a
['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd'] >>> a[7:] = list("there!")
>>>''.join(a)
'Hello, there!'
>>> import array
>>> a = array.array('c', s) >>> print a
array('c', 'Hello, world')
>>> a[0] = 'y' ; print a
array('c', 'yello world')
>>> a.tostring()
'yello, world'