Improve the repr for regular expression match objects

[rhettinger]

BPO	17087
Nosy	@rhettinger, @ezio-melotti, @cjerdonek, @PCManticore, @serhiy-storchaka
Files	sre.patch sre_repr2.patch sre_repr3.patch sre_repr4.patch sre_repr5.patch sre_match_repr.patch sre_repr6.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2013-10-20.10:16:20.894>
created_at = <Date 2013-01-30.23:53:02.426>
labels = ['expert-regex', 'type-feature', 'library']
title = 'Improve the repr for regular expression match objects'
updated_at = <Date 2013-11-25.21:20:38.690>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2013-11-25.21:20:38.690>
actor = 'python-dev'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2013-10-20.10:16:20.894>
closer = 'serhiy.storchaka'
components = ['Library (Lib)', 'Regular Expressions']
creation = <Date 2013-01-30.23:53:02.426>
creator = 'rhettinger'
dependencies = []
files = ['31383', '31744', '31746', '32143', '32144', '32159', '32216']
hgrepos = []
issue_num = 17087
keywords = ['patch']
message_count = 24.0
messages = ['180999', '181001', '181002', '181004', '181043', '195687', '197546', '197579', '197609', '197610', '197618', '200042', '200053', '200055', '200059', '200065', '200111', '200157', '200356', '200378', '200549', '200558', '200559', '204412']
nosy_count = 7.0
nosy_names = ['rhettinger', 'ezio.melotti', 'mrabarnett', 'chris.jerdonek', 'Claudiu.Popa', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue17087'
versions = ['Python 3.4']

[rhettinger]

Experience teaching Python has shown that people have a hard time learning to work with match objects. A contributing cause is the opaque repr:

    >>> import re
    >>> s = 'On 3/14/2013, Python celebrate Pi day.'
    >>> mo = re.search(r'\d+/\d+/\d+', s)
    >>> mo
    <_sre.SRE_Match object at 0x100456100>

They could explore the match object with dir() and help() and the matchobject methods and attributes:

    >>> dir(mo)
    ['__class__', '__copy__', '__deepcopy__', ...
     'end', 'endpos', 'expand', 'group', ... ]
     
    >>> mo.start()
    3
    >>> mo.end()
    12
    >>> mo.group(0)
    '3/14/2013'

However, this gets old when experimenting with alternative regular expressions. A better solution is to improve the repr:

    >>> re.search(r'\d+/\d+/\d+', s)
    <SRE Match object: start=3, stop=12, group(0)='3/14/2013'>

This would make the regular expression module much easier to work with.

[ezio-melotti]

Showing start and stop would be OK, but there might be many groups and they might contain lot of text, so they can't simply be included in the repr as they are.
FWIW there was another issue about changing _sre.SRE_Match to something better, but I can't find it right now.

[cjerdonek]

Is this a duplicate of bpo-13592?

[rhettinger]

Just showing group(0) should be helpful. And perhaps the number of groups. If a string is really long, we can truncate it like reprlib does.

The main goal is to make it easier to work with match objects at the interactive prompt. They are currently too opaque.

[ezio-melotti]

bpo-13592 is indeed the issue I was thinking about, but apparently that's about _sre.SRE_Pattern, so it's not the same thing.

Just showing group(0) should be helpful.

Often the interesting group is group(1), so showing only group(0) seems a bit arbitrary.

And perhaps the number of groups.

If we show only group(0), this might be useful as an indication that there are(n't) other groups.

If a string is really long, we can truncate it like reprlib does.

That's certainly an option.

FWIW I don't usually care about the start/end, and, if included, these values could be included as span=(3,12).

[PCManticore]

Here's my patch attempt. The repr of a match object has the following format:
(groups=\d+, span=(start, end), group0=the entire group or the first X characters, where X is represented by a new constant in sre_constants.h, SRE_MATCH_REPR_SIZE).

[serhiy-storchaka]

What about such output?

    >>> re.search('p((a)|(b))(c)?', 'unpack')
    <SRE Match object: [2: 5]: 'p'(('a')())('c')>

Or may be ('p', [['a'], []], ['c']) if you prefer legal Python expression.

[PCManticore]

Serhiy, at the first glance, that repr doesn't make sense to me, thus it seems a little difficult to comprehend.

[serhiy-storchaka]

Well, then first will commit a simpler patch. I left comments on Rietveld.

[PCManticore]

Here's the new version. I added a few replies on the Rietveld.

[PCManticore]

Added the new version.

[PCManticore]

Serhiy, are there any left issues with my latest patch? It would be nice if we could get this into 3.4.