Hi Guido - thanks for your thoughts on this. This came up for me when writing an HL7 library, where the raw data is all bytes - it seemed a little odd that the names went in as bytes and came out as str - especially given the way the re library expects consistency between the patterns and targets - but I also appreciate the point about breaking code.  (Including mine, which has a comment on it that says, "match.groupdict returns a dict with str keys in Python 3.5" :D) Cheers, Gordon -----Original Message----- From: Guido van Rossum <guido@python.org> Reply-to: guido@python.org To: Gordon R. Burgess <gordon@parasamgate.com> Cc: Python-Dev <python-dev@python.org> Subject: Re: [Python-Dev] Possibly inconsistent behavior in re groupdict Date: Sun, 25 Sep 2016 21:36:20 -0700 Hi Gordon, You pose an interesting question that I don't think anyone has posed before. Having thought about it, I think that the keys in the group dict are similar to the names of variables or attributes, and I think treating them always as strings makes sense. For example, I might write a function that allows passing in a pattern and a search string, both either str or bytes, where the function would expect fixed keys in the group dict: def extract_key_value(pattern, target):     m = re.match(pattern, target)     return m and m.groupdict['key'], m.groupdict['value'] There might be a problem with decoding the group name from the pattern, so sticking to ASCII group names would be wise. There's also the backwards compatibility concern: even if we did want to change this, would we want to break existing code (like the above) that might currently work? --Guido On Sun, Sep 25, 2016 at 5:25 PM, Gordon R. Burgess <gordon@parasamgate.com> wrote:

I've been lurking for a couple of months, working up the confidence to ask the list about this behavior - I've searched through the PEPs but couldn't find any specific reference to it.

In a nutshell, in the Python 3.5 library re patterns and search buffers both need to be either unicode or byte strings - but the keys in the groupdict are always returned as str in either case.

I don't know whether or not this is by design, but it would make more sense to me if when searching a bytes object with a bytes pattern the keys returned in the groupdict were bytes as well.

I reworked the example a little just now so it would run it on 2.7 as well; on 2.7 the keys in the dictionary correspond to the mode of the pattern as expected (and bytes and unicode are interconverted silently) - code and output are inline below.

Thanks for your time,

Gordon

[Code]

import sys import re from datetime import datetime

data = (u"first string (unicode)",          b"second string (bytes)")

pattern = [re.compile(u"(?P<ordinal>\\w+) .*\\((?P<type>\\w+)\\)"),            re.compile(b"(?P<ordinal>\\w+) .*\\((?P<type>\\w+)\\)")]

print("*** re consistency check ***\nRun: %s\nVersion: Python %s\n" %       (datetime.now(), sys.version)) for p in pattern:     for d in data:         try:             result = "groupdict: %s" % (p.match(d) and p.match(d).groupdict())         except Exception as e:             result = "error: %s" % e.args[0]         print("mode: %s\npattern: %s\ndata: %s\n%s\n" %               (type(p.pattern).__name__, p.pattern, d, result))

[Output]

gordon@w540:~/workspace/regex_demo$ python3 regex_demo.py *** re consistency check *** Run: 2016-09-25 20:06:29.472332 Version: Python 3.5.2+ (default, Sep 10 2016, 10:24:58) [GCC 6.2.0 20160901]

mode: str pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: first string (unicode) groupdict: {'ordinal': 'first', 'type': 'unicode'}

mode: str pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: b'second string (bytes)' error: cannot use a string pattern on a bytes-like object

mode: bytes pattern: b'(?P<ordinal>\\w+) .*\\((?P<type>\\w+)\\)' data: first string (unicode) error: cannot use a bytes pattern on a string-like object

mode: bytes pattern: b'(?P<ordinal>\\w+) .*\\((?P<type>\\w+)\\)' data: b'second string (bytes)' groupdict: {'ordinal': b'second', 'type': b'bytes'}

gordon@w540:~/workspace/regex_demo$ python regex_demo.py *** re consistency check *** Run: 2016-09-25 20:06:23.375322 Version: Python 2.7.12+ (default, Sep  1 2016, 20:27:38) [GCC 6.2.0 20160822]

mode: unicode pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: first string (unicode) groupdict: {u'ordinal': u'first', u'type': u'unicode'}

mode: unicode pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: second string (bytes) groupdict: {u'ordinal': 'second', u'type': 'bytes'}

mode: str pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: first string (unicode) groupdict: {'ordinal': u'first', 'type': u'unicode'}

mode: str pattern: (?P<ordinal>\w+) .*\((?P<type>\w+)\) data: second string (bytes) groupdict: {'ordinal': 'second', 'type': 'bytes'}

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