[issue22493] Deprecate the use of flags not at the start of regular expression

[Ales Kvapil]

Ales Kvapil  added the comment:

See also https://bugs.python.org/issue33658

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Ales Kvapil]

Ales Kvapil  added the comment:

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))

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Donald Stufft]

Changes by Donald Stufft :


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[issue22493] Deprecate the use of flags not at the start of regular expression

[Roundup Robot]

Roundup Robot added the comment:

New changeset c04a56b3a4f2 by Serhiy Storchaka in branch '3.6':
Issue #22493: Updated an example for fnmatch.translate().
https://hg.python.org/cpython/rev/c04a56b3a4f2

New changeset ded9a3c3bbb6 by Serhiy Storchaka in branch 'default':
Issue #22493: Updated an example for fnmatch.translate().
https://hg.python.org/cpython/rev/ded9a3c3bbb6

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Patch LGTM (but I changed tests a little). Thanks Tim!

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Roundup Robot]

Roundup Robot added the comment:

New changeset c35a528268fd by Serhiy Storchaka in branch '3.6':
Issue #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 #22493: Warning message emitted by using inline flags in the middle of
https://hg.python.org/cpython/rev/9d0f4da4d531

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Tim Graham]

Tim Graham added the comment:

Yes, I found that Django needs an update to support that syntax in URLpatterns. 
Thanks.

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Matthew Barnett]

Matthew Barnett added the comment:

I downloaded Python 3.6.0b1 not long after it was released and it works for me:

>>> re.match('(?i:CaseInsensitive)', 'caseinsensitive')
<_sre.SRE_Match object; span=(0, 15), match='caseinsensitive'>

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Tim Graham]

Tim Graham added the comment:

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?

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Added file: http://bugs.python.org/file44680/better-warning-2.diff

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

In tests you can either add re.escape(), or escape special characters manually 
(r'\(\?i\)'). What you prefer.

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

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).

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue22493] Deprecate the use of flags not at the start of regular expression

[Tim Graham]

Tim Graham added the comment:

Looks like we could remove the '^', but it doesn't resolve the deprecation 
warnings. The inline flags in `pattern` still need to be moved before `_prefix`.

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Matthew Barnett]

Matthew Barnett added the comment:

@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.

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Tim Graham]

Tim Graham added the comment:

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 `pattern` and 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.

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Tim Graham]

Tim Graham added the comment:

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)

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Added file: http://bugs.python.org/file44675/better-warning.diff

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue22493] Deprecate the use of flags not at the start of regular expression

[Roundup Robot]

Roundup Robot added the comment:

New changeset 31f8af1c3567 by Serhiy Storchaka in branch 'default':
Issue #22493: Inline flags now should be used only at the start of the
https://hg.python.org/cpython/rev/31f8af1c3567

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Updated patch that deprecates flags not at the start. fnmatch.translate() now 
uses scoped flags (issue433028).

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Added file: http://bugs.python.org/file44526/re_deprecate_nonstart_flags2.patch

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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versions: +Python 3.6 -Python 3.5

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 That sounds a bit random. It wouldn't totally address the discrepancy with 
 regex, would it?

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

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

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.

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Added file: http://bugs.python.org/file36845/re_deprecate_nested_flags.patch

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Antoine Pitrou]

Antoine Pitrou added the comment:

 Here is alternative, much simpler, patch. It deprecates only flags in nested 
 subpatterns.

That sounds a bit random. It wouldn't totally address the discrepancy with 
regex, would it?
MRAB, what do you think on this topic?

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Matthew Barnett]

Matthew Barnett added the comment:

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!

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[issue22493] Deprecate the use of flags not at the start of regular expression

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

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).

--
components: Library (Lib), Regular Expressions
files: re_deprecate_nonstart_flags.patch
keywords: patch
messages: 227520
nosy: ezio.melotti, mrabarnett, pitrou, serhiy.storchaka
priority: normal
severity: normal
stage: patch review
status: open
title: Deprecate the use of flags not at the start of regular expression
type: enhancement
versions: Python 3.5
Added file: http://bugs.python.org/file36721/re_deprecate_nonstart_flags.patch

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