python create slice object from string

If you want a slice object, why don't you just instantiate one?

s = slice(start, stop, step)

What are you meaning by "creating it from a string"?

slice(*map(lambda x: int(x.strip()) if x.strip() else None, mystring.split(':')))

for single arg slices '-1' or '1' so when mystring.split(':')==1 you just call int(x)

On request, took it out of comment section.

slice(*[{True: lambda n: None, False: int}[x == ''](x) for x in (mystring.split(':') + ['', '', ''])[:3]])