It seems to me that the desired behavior here is closest to 'str.replace()' out
of all the options discussed, just with the constraint of being limited to
either the start or the end of the string. (Thus the .lreplace() and
.rreplace() option suggested by Glenn.)
The minimal change (which actually also is pretty general?) I think would be to
add 'only_start' and 'only_end' keyword arguments to str.replace(), both
defaulting to False. If, e.g., 'only_start' is passed True, each repetition of
'old' at the start of 's' is replaced by 'new', with the number of replacements
limited by the existing optional 'count'. Similar behavior for 'only_end'=True.
Probably best to raise a ValueError(?) if both 'only_start'=True and
'only_end'=True?
Taking swapping a file extension as an example of a particular transformation
of interest, it would be achieved with something like s.replace(".htm",
".html", only_end=True).
-Brian
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