This is kinof of a simple question but not

I am trying to understand if there are any string functions that can allow myself to alter strings by passing the index's of characters (alot of the python string functions take characters or strings as parameters not the indexes):
- Change the text with in a string from x index to y index to the string z...
so for example; if I have the string
a = "hello"
a.replace( 1, 4, "ummer" ) # change the characters from a[1] to a[4] to "ummer"
print a # so a will now equal "hummero"
- insert a string into another string...
for example
a = hello
a.insert( 1, "aaaaaa" )
print a # so a will now equal "heaaaaaallo"
I know I can replace stuff saying this... a.replace( 'ell', 'ummer' ) but I need to pass the index of the string rather than the characters as the parameters