Python string replace the values

[MRAB]

On 2017-09-01 18:13, Ganesh Pal wrote:
> 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
> 
How long is user_string? len(user_string)
Truncate fixed_string by that many characters: fixed_string[ : 
-len(user_string)]

Then append the user_string: fixed_string[ : -len(user_string)] + 
user_string

Example:

 >>> fixed_string = "a0000"
 >>> user_string ='1'
 >>> len(user_string)
1
 >>> fixed_string[ : -len(user_string)]
'a000'
 >>> fixed_string[ : -len(user_string)] + user_string
'a0001'

Note: this won't work properly if len(user_string) == 0, so if that 
could happen, just skip it. (Appending an empty string has no effect 
anyway!)