Python string replace the values

[Peter Otten]

Steve D'Aprano wrote:

> On Sat, 2 Sep 2017 03:13 am, Ganesh Pal wrote:
> 
>> Example :
>> 
>>   "a0000" and "1" => a0001
>> 
>>   "a0000" and "aa" => c00aa
> 
> Why does the leading 'a' change to a 'c'? Is that a mistake? I'll assume
> its a typo.
> 
> You want string slicing.
> 
> base = 'a0000'
> extra = 'ab'

assert len(extra)

> print( base[:-len(extra)] + extra )

or

base[:-len(extra) or None] + extra

if you need to handle the extra = "" case.

Another option is a template:

>>> "a{:0>4}".format("")
'a0000'
>>> "a{:0>4}".format("xy")
'a00xy'
>>> "a{:0>4}".format("xyztu")
'axyztu'

This can be modified to limit its arg by specifying a precision:

>>> "a{:0>4.4}".format("xyztu")
'axyzt'