Attached patch implements rich comparison for _sre.SRE_Pattern objects created by re.compile().

Comparison between patterns is used in the warnings module to not add duplicated filters, see issue #18383:

New changeset f57f4e33ba5e by Martin Panter in branch '3.5':
Issue #18383: Avoid adding duplicate filters when warnings is reloaded
https://hg.python.org/cpython/rev/f57f4e33ba5e

For the warnings module, it became a problem in test_warnings since the Python test runner started to clear all caches. When re.purge() is called, re.compile() creates a new object, whereas with the cache it returns the same object and so the two patterns are equal since it's the same object. => see issue #28688

Ten subtest in test_re fail with

TypeError: unhashable type: '_sre.SRE_Pattern'

> Ten subtest in test_re fail with: TypeError: unhashable type: '_sre.SRE_Pattern'

Oops, right. Updated patch implements also hash() on patterns.

Added comments on Rietveld.

I hope you make it clear what you mean by 'equal', i.e. that it's comparing the pattern and the flags (AFAICT), so re.compile('(?x)a') != re.compile('(?x) a ').

Patch version 3:

* pattern_hash() includes isbytes, I also shifted flags by 1 bit to not erase the isbytes bit (FYI maximum value of flags is 256)
* pattern_richcompare() avoids calling PyObject_RichCompareBool() if flags or isbytes is different
* unit test ensures that no BytesWarning warning is raised
* checks hash() in unit tests
* fix also the unit test with a different flag (use the same pattern)
* document also in the unit test that the comparison is case sensitive

Ok, I hope that it's the last attempt: patch 5

* Remove hash(b) != hash(a): only keep tests on hash(b)==hash(a) when b==a
* Replace re.ASCII flag with no flag to test two patterns with different flags

There is a problem with locale-depending flags. The same pattern may be compiled with the LOCALE flag to different pattern objects in different locales. Perhaps we should compare the compiled code instead of pattern strings. Or both.

Serhiy: "There is a problem with locale-depending flags. The same pattern may be compiled with the LOCALE flag to different pattern objects in different locales."

Oh, I didn't know and you are right.

"Perhaps we should compare the compiled code instead of pattern strings. Or both."

PatternObject contains many fields. I used the following two fields which come from re.compile():

* pattern
* flags

I considered that they were enough because pattern_repr() only displays these ones. Other fields:

* groups
* groupindex
* indexgroup
* weakreflist
* isbytes
* codesize, code

weakreflist can be skipped, isbytes is already tested in my patch.

Would it be possible to only compare code instead of pattern? What are groups, groupindex and indexgroup: should we also compare them?

Maybe I can start from  pattern_compare-4.patch and only add a test comparing code?

'[abc]' and '[cba]' are compiled to the same code. Do we want to handle them as equal? This is implementation defined.

> '[abc]' and '[cba]' are compiled to the same code. Do we want to handle them as equal?

Comparison must be fast. If the comparison is just memcmp(code1,
code2, codesize), it's ok.

I agree that we must put a similar somewhere to say that some parts
are implementation details. Maybe split the test in two parts, and
mark the second part with @cpython_only.

I see two options:

* Compare flags, isbytes and code. This makes some different patterns be compiled to equal objects. For example spaces in verbose mode are ignored. But this don't make equal all equivalent patterns. '[aA]' would be equal to '(?:a|A)' but still would be not equal to '(i?a)' with current implementation.

* Compare flags, isbytes, code and pattern. This makes literally different patterns be compiled to not equal objects even if the difference is not significant. '[abc]' would be different from '[cba]' despites the fact that matching both always returns the same result.

Since this issue becomes a little ambiguous, I would target the patch to 3.7 only. Maybe we will find other subtle details or will decide to change the meaning of equality of pattern objects before releasing 3.7.

New approach: patch 5 now compares indexgroup, groupindex and code instead of comparing pattern, to handle correctly two equal pattern strings compiled with the re.LOCALE flag and two different locales.

The patch also converts indexgroup list to a tuple to be able to hash it. (It also prevents modification, but this is just a side effect, and groupindex remains a mutable dictionary.)

_sre.compile() checks types which helps to identify a bug in unit tests.

This looks too complicated. groups, indexgroup and groupindex are unambiguously derived from pattern string. If caching works different pattern strings are compiled to different pattern objects. Currently they are not equal, even if their codes are equal. And I don't see large need to consider them equal.

Back to basis, patch 6:

* revert changes on indexgroup and groupindex types: I will fix this later, once this issue is fixed
* pattern_richcompare() and pattern_hash() also uses pattern, but don't use groups, indexgroup nor groupindex anymore

I removed the @cpython_only unit test and rewrote test_pattern_compare_bytes() to make it easier to follow.

re.compile('abc', re.IGNORECASE) is different than re.compile('ABC', re.IGNORECASE), but it's a deliberate choice to not test it. I consider that the behaviour can change depending on the Python implementation and in a future version.

pattern_compare-6.patch LGTM.

New changeset 5e8ef1493843 by Victor Stinner in branch '3.6':
Implement rich comparison for _sre.SRE_Pattern
https://hg.python.org/cpython/rev/5e8ef1493843

Serhiy Storchaka: "pattern_compare-6.patch LGTM."

Thank you very much for your very useful reviews! I pushed the change.

For stricter checks on _sre.compile() arguments, I created the issue #28765: "_sre.compile(): be more strict on types of indexgroup and groupindex".

+           && left->codesize && right->codesize);

There is a typo. Should be:

+           && left->codesize == right->codesize);

While we are here, it perhaps worth to add a fast path for self == other.

New changeset 6b43d15fd2d7 by Victor Stinner in branch '3.6':
Issue #28727: Fix typo in pattern_richcompare()
https://hg.python.org/cpython/rev/6b43d15fd2d7

New changeset c2cb70c97163 by Victor Stinner in branch '3.6':
Issue #28727: Optimize pattern_richcompare() for a==a
https://hg.python.org/cpython/rev/c2cb70c97163

Serhiy Storchaka: "&& left->codesize && right->codesize)"

Ooops! Fixed!

"While we are here, it perhaps worth to add a fast path for self == other."

Done.