[Python-Dev] [NPERS] Re: a feature i'd like to see in python #2: indexing of match objects (original) (raw)

Alastair Houghton alastair at alastairs-place.net
Fri Dec 8 12:11:00 CET 2006

On 7 Dec 2006, at 21:47, Josiah Carlson wrote:

Alastair Houghton <alastair at alastairs-place.net> wrote:

On 7 Dec 2006, at 02:01, Josiah Carlson wrote:

Alastair Houghton <alastair at alastairs-place.net> wrote:

On 7 Dec 2006, at 01:01, Josiah Carlson wrote:

If we don't want slicing, or if prodicing a slice would produce a semantically questionable state, then lets not do it.

...if you return match objects from slicing, you have problems like m [::-1].groups(). I don't know what that should return. I would argue that any 'step' != 1 has no semantically correct result for slicing on a match object, so we shouldn't support it. OK, but even then, if you're returning a match object, how about the following: m = re.match('(A)(B)(C)(D)(E)', 'ABCDE') print m[0] ABCDE n = m[2:5] print list(n) ['B', 'C', 'D'] print n[0] B print n.group(0) B The problem I have with it is that it's violating the invariant that match objects should return the whole match in group(0). If we were going to go with slicing, then it would be fairly trivial to include the whole match range. Some portion of the underlying structure knows where the start of group 2 is, and knows where the end of group 5 is, so we can slice or otherwise use that for subsequent sliced groups.

But then you're proposing that this thing (which looks like a tuple,
when you're indexing it) should slice in a funny way. i.e.

m = re.match('(A)(B)(C)(D)(E)', 'ABCDE') print m[0] ABCDE print list(m) ['ABCDE', 'A', 'B', 'C', 'D', 'E']

n = m[2:5] print list(n) ['BCD', 'B', 'C', 'D'] print len(n) 4

p = list(m)[2:5] print p ['B', 'C', 'D'] print len(p)

Or are you saying that

m[2:5][0] != m[2:5].group(0)

but

m[0] == m.group(0)

??

Either way I think that's really counter-intuitive.

Honestly, I don't think that slicing should be supported if it's
going to have to result in match objects, because I can't see a way
to make them make sense.

I think that's Frederik's objection also, but unlike me he doesn't
feel that the slice operation should return something different (e.g.
a tuple).

Kind regards,

Alastair.

-- http://alastairs-place.net

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