Slicing a list using a variable, in Python

Question

Given a list

a = range(10)

You can slice it using statements such as

a[1]
a[2:4]

However, I want to do th

Answer 1

I ran across this recently, while looking up how to have the user mimic the usual slice syntax of a:b:c, ::c, etc. via arguments passed on the command line.

The argument is read as a string, and I'd rather not split on ':', pass that to slice(), etc. Besides, if the user passes a single integer i, the intended meaning is clearly a[i]. Nevertheless, slice(i) will default to slice(None,i,None), which isn't the desired result.

In any case, the most straightforward solution I could come up with was to read in the string as a variable st say, and then recover the desired list slice as eval(f"a[{st}]").

This uses the eval() builtin and an f-string where st is interpolated inside the braces. It handles precisely the usual colon-separated slicing syntax, since it just plugs in that colon-containing string as-is.