Repository navigation
Deprecate the use of flags not at the start of regular expression #66683
Description
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
Show more details
GitHub fields:
assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2016-09-16.22:33:54.254>
created_at = <Date 2014-09-25.10:29:00.775>
labels = ['expert-regex', 'type-feature', 'library']
title = 'Deprecate the use of flags not at the start of regular expression'
updated_at = <Date 2018-05-27.08:53:56.829>
user = 'https://github.com/serhiy-storchaka'bugs.python.org fields:
activity = <Date 2018-05-27.08:53:56.829>
actor = 'aleskva'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-09-16.22:33:54.254>
closer = 'serhiy.storchaka'
components = ['Library (Lib)', 'Regular Expressions']
creation = <Date 2014-09-25.10:29:00.775>
creator = 'serhiy.storchaka'
dependencies = []
files = ['36721', '36845', '44526', '44675', '44680']
hgrepos = []
issue_num = 22493
keywords = ['patch']
message_count = 21.0
messages = ['227520', '228841', '228930', '228931', '228959', '275601', '275758', '276559', '276562', '276566', '276583', '276620', '276621', '276648', '276650', '276746', '276757', '276758', '279567', '317786', '317788']
nosy_count = 7.0
nosy_names = ['pitrou', 'ezio.melotti', 'mrabarnett', 'python-dev', 'serhiy.storchaka', 'Tim.Graham', 'aleskva']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22493'
versions = ['Python 3.6']
Activity
serhiy-storchaka commentedon Sep 25, 2014
The meaning of inline flags not at the start of regular expression is ambiguous. Current re implementation and regex in the V0 mode enlarge the scope to all expression. In V1 mode in regex they affect only the end of the expression.
I propose to deprecate (and then forbid in 3.7) the use of inline flags not at the start of regular expression. This will help to change the meaning of inline flags in the middle of the expression in future (in 3.8 or later).
serhiy-storchaka commentedon Oct 9, 2014
Here is alternative, much simpler, patch. It deprecates only flags in nested subpatterns. No changes needed in tests and other stdlib modules. It is very unlikely that it is used in third party code.
pitrou commentedon Oct 9, 2014
That sounds a bit random. It wouldn't totally address the discrepancy with regex, would it?
MRAB, what do you think on this topic?
mrabarnett commentedon Oct 9, 2014
I think the simplest and clearest approach from the user's point of view is just to deprecate inline flags that are not at the start of the pattern.
In practice, they almost invariably occur at the start anyway, although I do remember once seeing a pattern in which the inline flag was at the end!
serhiy-storchaka commentedon Oct 10, 2014
No, it will not totally address the discrepancy with regex, but at least it will allow as to change the behavior of flags in subpatterns. And we always can convert a pattern to a subpattern (surround by "(?:" and ")").
For now Python re module is only one regular expression implementation in which flags in the middle of the expression affect all expression. [1]
[1] http://www.regular-expressions.info/modifiers.html
serhiy-storchaka commentedon Sep 10, 2016
Updated patch that deprecates flags not at the start. fnmatch.translate() now uses scoped flags (bpo-433028).
python-dev commentedon Sep 11, 2016
New changeset 31f8af1c3567 by Serhiy Storchaka in branch 'default':
Issue bpo-22493: Inline flags now should be used only at the start of the
https://hg.python.org/cpython/rev/31f8af1c3567
timgraham commentedon Sep 15, 2016
Could we include the offending pattern in the deprecation message? I'm attaching a proposed patch. With that patch I can more easily find the offending pattern, whereas before I had no idea:
django/django/urls/resolvers.py:101: DeprecationWarning: Flags not at the start of the expression ^(?i)test/2/?$
compiled_regex = re.compile(regex, re.UNICODE)
timgraham commentedon Sep 15, 2016
And on further investigation, I'm not sure how to fix the deprecation warnings in Django. We have a urlpattern like this:
url(r'^(?i)CaseInsensitive/(\w+)', empty_view, name="insensitive"),
The regex string r'^(?i)CaseInsensitive/(\w+)' is later substituted in this line in Django's URL resolver as the
pattern:if re.search('^%s%s' % (re.escape(_prefix), pattern), candidate_pat % candidate_subs, re.UNICODE):
It seems Django would need to extract any flags from
patternand put them at the start of the '^%s%s' string that's constructed for re.search(). I'm not sure if this can be done easily.mrabarnett commentedon Sep 15, 2016
@tim: Why are you using re.search with '^'? Does the pattern that's passed in ever contain '(?m)'? If not, re.match without '^' is better.
timgraham commentedon Sep 15, 2016
Looks like we could remove the '^', but it doesn't resolve the deprecation warnings. The inline flags in
patternstill need to be moved before_prefix.serhiy-storchaka commentedon Sep 15, 2016
Thanks Tim, this is great idea! I consider this as usability bug and going to apply a fix to 3.6.
But regular expression can be generated and be very long. I think it should be truncated before including in a warning message.
As for Django, you can use (?i:CaseInsensitive) in 3.7 (unless _prefix is case sensitive, but you want to make it case sensitive if pattern is case sensitive).
serhiy-storchaka commentedon Sep 15, 2016
In tests you can either add re.escape(), or escape special characters manually (r'\(\?i\)'). What you prefer.
timgraham commentedon Sep 16, 2016
Adding an updated patch.
I guess the (?i:CaseInsensitive) syntax isn't merged yet? I tried it but it didn't work. It might be premature to proceed with this deprecation if that alternative isn't already present. Is there an issue for it?
mrabarnett commentedon Sep 16, 2016
I downloaded Python 3.6.0b1 not long after it was released and it works for me:
timgraham commentedon Sep 16, 2016
Yes, I found that Django needs an update to support that syntax in URLpatterns. Thanks.
python-dev commentedon Sep 16, 2016
New changeset c35a528268fd by Serhiy Storchaka in branch '3.6':
Issue bpo-22493: Warning message emitted by using inline flags in the middle of
https://hg.python.org/cpython/rev/c35a528268fd
New changeset 9d0f4da4d531 by Serhiy Storchaka in branch 'default':
Issue bpo-22493: Warning message emitted by using inline flags in the middle of
https://hg.python.org/cpython/rev/9d0f4da4d531
serhiy-storchaka commentedon Sep 16, 2016
Patch LGTM (but I changed tests a little). Thanks Tim!
python-dev commentedon Oct 27, 2016
New changeset c04a56b3a4f2 by Serhiy Storchaka in branch '3.6':
Issue bpo-22493: Updated an example for fnmatch.translate().
https://hg.python.org/cpython/rev/c04a56b3a4f2
New changeset ded9a3c3bbb6 by Serhiy Storchaka in branch 'default':
Issue bpo-22493: Updated an example for fnmatch.translate().
https://hg.python.org/cpython/rev/ded9a3c3bbb6
aleskva commentedon May 27, 2018
Maybe there should be introduced some method to merge patterns as just piping patterns will not work:
pats = [r'(?m)^line.continues$', r'(?s)begin.*?end']
re.compile('|'.join(pats))
aleskva commentedon May 27, 2018
See also https://bugs.python.org/issue33658