Improve some re error messages using regex for hints

[serhiy-storchaka]

BPO	22364
Nosy	@rhettinger, @terryjreedy, @pitrou, @ezio-melotti, @stevendaprano, @serhiy-storchaka
Dependencies	bpo-22578: Add additional attributes to re.error bpo-22581: Other mentions of the buffer protocol
Files	re_errors_regex.patch re_errors.patch re_errors_diff.txt re_errors_2.patch regex_errors.diff re_errors_3.patch regex_errors2.diff

^{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 2015-03-25.19:05:44.319>
created_at = <Date 2014-09-08.12:22:56.243>
labels = ['expert-regex', 'type-feature', 'library']
title = 'Improve some re error messages using regex for hints'
updated_at = <Date 2015-03-25.19:05:44.318>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-03-25.19:05:44.318>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-03-25.19:05:44.319>
closer = 'serhiy.storchaka'
components = ['Library (Lib)', 'Regular Expressions']
creation = <Date 2014-09-08.12:22:56.243>
creator = 'serhiy.storchaka'
dependencies = ['22578', '22581']
files = ['37167', '38035', '38036', '38080', '38171', '38229', '38230']
hgrepos = []
issue_num = 22364
keywords = ['patch']
message_count = 27.0
messages = ['226575', '226576', '226599', '226635', '226641', '226790', '226847', '227065', '227084', '227119', '227130', '227132', '228599', '230464', '230481', '230965', '231057', '235532', '235534', '235678', '236188', '236201', '236257', '236549', '236551', '236954', '239279']
nosy_count = 9.0
nosy_names = ['rhettinger', 'terry.reedy', 'pitrou', 'ezio.melotti', 'mrabarnett', 'steven.daprano', 'BreamoreBoy', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22364'
versions = ['Python 3.5']

[serhiy-storchaka]

In some cases standard re module and third-party regex modules raise exceptions with different error messages.

1. re.match(re.compile('.'), 'A', re.I)

re: Cannot process flags argument with a compiled pattern
regex: can't process flags argument with a compiled pattern

2. re.compile('(?P<foo_123')

re: unterminated name
regex: missing >

3. re.compile('(?P<foo_123>a)(?P=foo_123')

re: unterminated name
regex: missing )

4. regex.sub('(?P<a>x)', r'\g<a', 'xx')

re: unterminated group name
regex: missing >

5. re.sub('(?P<a>x)', r'\g<', 'xx')

re: unterminated group name
regex: bad group name

6. re.sub('(?P<a>x)', r'\g<a a>', 'xx')

re: bad character in group name
regex: bad group name

7. re.sub('(?P<a>x)', r'\g<-1>', 'xx')

re: negative group number
regex: bad group name

8. re.compile('(?P<foo_123>a)(?P=!)')

re: bad character in backref group name '!'
regex: bad group name

9. re.sub('(?P<a>x)', r'\g', 'xx')

re: missing group name
regex: missing <

10. re.compile('a\\')
    re.sub('x', '\\', 'x')

re: bogus escape (end of line)
regex: bad escape

11. re.compile(r'\1')

re: bogus escape: '\1'
regex: unknown group

12. re.compile('[a-')

re: unexpected end of regular expression
regex: bad set

13. re.sub(b'.', 'b', b'c')

re: expected bytes, bytearray, or an object with the buffer interface, str found
regex: expected bytes instance, str found

14. re.compile(r'\w', re.UNICODE | re.ASCII)

re: ASCII and UNICODE flags are incompatible
regex: ASCII, LOCALE and UNICODE flags are mutually incompatible

15. re.compile('(abc')

re: unbalanced parenthesis
regex: missing )

16. re.compile('abc)')

re: unbalanced parenthesis
regex: trailing characters in pattern

17. re.compile(r'((.)\1+)')

re: cannot refer to open group
regex: can't refer to an open group

Looks as in one case re messages are better, and in other cases regex messages are better. In any case it would be good to unify error messages in both modules.

[serhiy-storchaka]

18. re.compile(r'.???')

re: multiple repeat
regex: nothing to repeat at position 3

[stevendaprano]

I'm dubious about this issue. It suggests that the wording of the exceptions is part of the API of the two modules.

If the idea is just to copy the best error messages from one module to the other, then I guess there is no harm. But if the idea is to guarantee to keep the two modules' messages in sync, then I think it is unnecessary and harmful.

[serhiy-storchaka]

Yes, the idea of this issue is to enhance the re module (and the regex module if Matthew will) be picking the best error messages (or writing a new one).

[mrabarnett]

re: Cannot process flags argument with a compiled pattern
regex: can't process flags argument with a compiled pattern

Error messages usually start with a lowercase letter, and I think that all the other ones in the re module do.

By the way, which is preferred, "cannot" or "can't"? The regex module always uses "can't", but re module uses "cannot" except for "TypeError: can't use a bytes pattern on a string-like object", I think.

Also, you said that one of the re module's messages was better, but didn't say which! Did you mean this one?

re: expected bytes, bytearray, or an object with the buffer interface, str found
regex: expected bytes instance, str found

[serhiy-storchaka]

By the way, which is preferred, "cannot" or "can't"? The regex module always
uses "can't", but re module uses "cannot" except for "TypeError: can't use
a bytes pattern on a string-like object", I think.

It's interesting question. Grepping in CPython sources got results:

Cannot 210
cannot 865
Can't 216
can't 796

Lowercase wins uppercase with score 4:1 and short and long forms are
equivalent.

I left the decision to English speakers.

Also, you said that one of the re module's messages was better, but didn't
say which! Did you mean this one?
> re: expected bytes, bytearray, or an object with the buffer interface,
> str found
> regex: expected bytes instance, str found

Both are not good. re variant is too verbose, but it is more correct.

May be 6, 7, 8, 10, 11, 16, 18 are better in re.

[terryjreedy]

I prefer "cannot" for error messages. "Can't" is an informal version of "cannot", used in speech, dialog representing speech, and 'informal' writing. It looks wrong to me in this context.

[BreamoreBoy]

How can anything that's in the stdlib be unified with something that's not in the stdlib and currently has no prospects of getting in the stdlib?

[serhiy-storchaka]

The regex module is potential candidate for replacement of the re module.

[BreamoreBoy]

The key word is "potential". I do not believe that any changes should be made to the re module until such time as there is a fully approved PEP for the regex module and that work has actually started on getting it into the stdlib. Surely backward compatibility also comes into this?

[terryjreedy]

Steven and Mark are correct that a tracker patch cannot change a 3rd party module. On the other hand, we are free to improve error messages in new versions. And we are free to borrow ideas from 3rd part modules. I changed the title accordingly.

(Back compatibility comes into play in not making message enhancements in bugfix releases even though message details are not part of the documented API. People who write code that depends on those details, and doctexts need not so depend, should expect to revise for new versions. I expect that some of our re tests would need to be changed.)

Re and regex are a bit special in that regex is the only re replacement (that I know of) and is (almost) a drop-in replacement. So some people *are*, on their own, replacing re with regex by installing regex (easy with pip) and adding 'import regex as re' at the top of their code.

Serhiy suggested either picking the best or writing a new one, I think a new one combining both would be best in many of the cases. As a user, I like "name missing terminal '>'" for #2 (is there an adjective for a name in this context?) and for #4, "group name missing terminal '>'". (Note that we usually quote literals, as in #8.) For #12, I would like a parallel construction "set expression missing terminal ']'" if that is possible. But the currently vague re message "unexpected end of regular expression" might be raised as a point where the specific information is lost and only the general version is correct.

As for #14, either UNICODE and LOCALE *are* compatible (for re) or this is buggy.
>>> import re
>>> re.compile(r'\w', re.UNICODE | re.LOCALE)
re.compile('\\w', re.LOCALE|re.UNICODE)