truncating match in regular expression match objects repr

[sethtroisi]

BPO	39949
Nosy	@rhettinger, @ericvsmith, @ezio-melotti, @sethtroisi, @serhiy-storchaka, @qwenger
PRs	bpo-39949: Add ... to truncated match in repr(match) #20922

^{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 = None
closed_at = None
created_at = <Date 2020-03-12.21:49:27.009>
labels = ['expert-regex', 'type-feature', '3.9']
title = 'truncating match in regular expression match objects repr'
updated_at = <Date 2020-06-19.11:11:33.869>
user = 'https://github.com/sethtroisi'

bugs.python.org fields:

activity = <Date 2020-06-19.11:11:33.869>
actor = 'matpi'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Regular Expressions']
creation = <Date 2020-03-12.21:49:27.009>
creator = 'Seth.Troisi'
dependencies = []
files = []
hgrepos = []
issue_num = 39949
keywords = ['patch']
message_count = 17.0
messages = ['364052', '364053', '371618', '371626', '371641', '371645', '371648', '371650', '371698', '371699', '371700', '371843', '371864', '371867', '371868', '371869', '371878']
nosy_count = 7.0
nosy_names = ['rhettinger', 'eric.smith', 'ezio.melotti', 'mrabarnett', 'Seth.Troisi', 'serhiy.storchaka', 'matpi']
pr_nums = ['20922']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39949'
versions = ['Python 3.9']

[sethtroisi]

Following on https://bugs.python.org/issue17087

Today I was mystified by why a regex wasn't working.

    >>> import re
    >>> re.match(r'.{10}', 'A'*49+'B')
    <_sre.SRE_Match object; span=(0, 10), match='AAAAAAAAAA'>

    >>> re.match(r'.{49}', 'A'*49+'B')
    <_sre.SRE_Match object; span=(0, 49), match='AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA>

    >>> re.match(r'.{50}', 'A'*49+'B')
    <_sre.SRE_Match object; span=(0, 50), match='AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA>

I became confused on why the B wasn't matching in the third example; It is matching just
in the interactive debugger it doesn't fit on the line and doesn't show

My suggestion would be to truncate match (in the repr) and append '...' when it's right quote wouldn't show

with short matches (or exactly enough space) there would be no change

    >>> re.match(r'.{48}', string.ascii_letters)
    <_sre.SRE_Match object; span=(0, 48), match='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUV'>

when not all of match can be displayed

    >>> re.match(r'.{49}', string.ascii_letters)
    <_sre.SRE_Match object; span=(0, 49), match='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW>
    <_sre.SRE_Match object; span=(0, 49), match='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRS'...>

I'm happy to help out by writing tests or impl if folks thing this is a good idea.

I couldn't think of other examples (urllib maybe?) in Python of how this is handled but I could potentially look for some if that would help

[ericvsmith]

I think the missing closing quote is supposed to be your visual clue that it's truncated. Although I'll grant you that it's pretty subtle.

[rhettinger]

+1 for adding an ellipsis. It's a conventional way to indicate that the displayed data is truncated.

Concur with Eric that missing close quote is too subtle (and odd, and unexpected).