Add string in a certain position in Python

No. Python Strings are immutable.

>>> s='355879ACB6'
>>> s[4:4] = '-'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

It is, however, possible to create a new string that has the inserted character:

>>> s[:4] + '-' + s[4:]
'3558-79ACB6'

This seems very easy:

>>> hash = "355879ACB6"
>>> hash = hash[:4] + '-' + hash[4:]
>>> print hash
3558-79ACB6

However if you like something like a function do as this:

def insert_dash(string, index):
    return string[:index] + '-' + string[index:]

print insert_dash("355879ACB6", 5)

As strings are immutable another way to do this would be to turn the string into a list, which can then be indexed and modified without any slicing trickery. However, to get the list back to a string you'd have to use .join() using an empty string.

>>> hash = '355879ACB6'
>>> hashlist = list(hash)
>>> hashlist.insert(4, '-')
>>> ''.join(hashlist)
'3558-79ACB6'

I am not sure how this compares as far as performance, but I do feel it's easier on the eyes than the other solutions. ;-)