Python string replace the values

[Ganesh Pal]

In the fixed length string i.e "a0000",last 4 bits i.e "0000" should be
replaced by the user provided value ( the value is between 0001 + f f f f )
. I intend to form  final_string  using a fixed_string and an user provided
 user_string


Example:

fixed_string = "a0000"
user_string ='1'

final string = fixed_string  and  user_string

Example :

  "a0000" and "1" => a0001

  "a0000" and "aa" => c00aa


PS :  "and" this is not logical and it's just for example

If I concatenation using + or +=  or it's append the value at the end of
the string

>>> "a0000" + "1"    ===> expected was a0001
'a00001'



 I am on python 2.7 and Linux ,  any ideas that you recommend  to handle
this


Regards,
Ganesh