Python: Want to use a string as a slice specifier

Question

Suppose I have a variable S with the string "1:3" (or for that matter, "1", or "1:" or ":3") and I want to use that as a slice specifier on list L. You cannot simply do L[S] since the required args for a slice are "int:int".

Now, I current have some ugly code that parses S into its two constituent ints and deals with all the edge cases (4 of them) to come up with the correct slice access but this is just plain ugly and unpythonic.

How do I elegantly take string S and use it as my slice specifier?

Answer 1

Here's another solution

eval("L[%s]" % S) 

warning - It's not safe if S is coming from an external(unreliable) source.

Answer 2

This can be done without much hacking by using a list comprehension. We split the string on :, passing the split items as arguments to the slice() builtin. This allows us to quite nicely produce the slice in one line, in a way which works in every case I can think of:

slice(*[int(i.strip()) if i else None for i in string_slice.split(":")])

By using the slice() builtin, we neatly avoid having to deal with too many edge cases ourselves.

Example usage:

>>> some_list = [1, 2, 3]
>>> string_slice = ":2"
>>> some_list[slice(*[int(i.strip()) if i else None for i in string_slice.split(":")])]
[1, 2]
>>> string_slice = "::-1"
>>> some_list[slice(*[int(i.strip()) if i else None for i in string_slice.split(":")])]
[3, 2, 1]