python create slice object from string

Question

I'd like to create a slice object from a string; right now the only way seems through a cumbersome hacky eval statement

class getslice:
    def __getitem__(self, idx): return idx[0]
eval("getslice()[%s, 1]" %(":-1"))

thanks in advance.

Edit: Sorry if the original prompt was not clear, the input in this case was ":-1". The point was to parse the string. Ignacio Vazquez-Abrams's response at least solved the problem (and seems to work with reverse indexing as well), but I think my solution above is still more clear if not conceptually clean (and will work correctly if Python ever changes slicing syntax).

Answer 1

use operator module:

in Python 2.5 or earlier: operator.getslice(obj,start,end)

in Python 2.6 or later: operator.getitem(obj,[start:end])

Answer 2

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"?

Answer 3

A slice object is usually created using subscript notation, this notation uses slice() internally, as stated on the slice() documentation. What you want to do is:

your_string[start:end]

From the python tutorial:

Strings can be subscripted (indexed); like in C, the first character of a string has subscript (index) 0. There is no separate character type; a character is simply a string of size one. Like in Icon, substrings can be specified with the slice notation: two indices separated by a colon.

>>> word = 'Help' + 'A' 
>>> word[4]
'A'
>>> word[0:2]
'He'
>>> word[2:4]
'lp'

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.

>>> word[:2]    # The first two characters
'He'
>>> word[2:]    # Everything except the first two characters
'lpA'

Answer 4

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