Creating slice notation from string
zuo at chopin.edu.pl
Thu Sep 3 00:33:29 CEST 2009
03-09-2009 o 00:11:17 MRAB <python at mrabarnett.plus.com> wrote:
> bvdp wrote:
>> I'm trying to NOT create a parser to do this .... and I'm sure that
>> it's easy if I could only see the light!
>> Is it possible to take an arbitrary string in the form "1:2", "1",
>> ":-1", etc. and feed it to slice() and then apply the result to an
>> existing list?
>> For example, I have a normal python list. Let's say that x = [1,2,3,4]
>> and I have a string, call it "s', in the format "[2:3]". All I need to
>> do is to apply "s" to "x" just like python would do.
>> I can, of course, convert "x" to a list with split(), convert the 2
>> and 3 to ints, and then do something like: x[a:b] ... but I'd like
>> something more general. I think the answer is in slice() but I'm lost.
> >>> x = [1,2,3,4]
> >>> s = "[2:3]"
> >>> # Using map.
> >>> x[slice(*map(int, s.strip("").split(":")))]
> >>> # Using a list comprehension.
> >>> x[slice(*[int(i) for i in s.strip("").split(":")])]
Of course, you could also do something like this:
eval('x' + s)
eval(str(x) + s)
-- but it's worse: less secure (e.g. if s could be user-typed) and most
probably much more time-consuming (especially the latter).
Jan Kaliszewski (zuo) <zuo at chopin.edu.pl>
More information about the Python-list