Normally, you would just do:
s = s[:-3] + s[-2:]
The s[:-3]
gives you a string up to, but not including, the comma you want removed ("this is a string"
) and the s[-2:]
gives you another string starting one character beyond that comma (" a"
).
Then, joining the two strings together gives you what you were after ("this is a string a"
).