[Python-Dev] Possibly inconsistent behavior in re groupdict (original) (raw)

Guido van Rossum guido at python.org
Mon Sep 26 00:36:20 EDT 2016

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 at 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\w+) .*\((?P\w+)\)"), re.compile(b"(?P\w+) .*\((?P\w+)\)")] 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 at w540:~/workspace/regexdemo$ python3 regexdemo.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\w+) .*((?P\w+)) data: first string (unicode) groupdict: {'ordinal': 'first', 'type': 'unicode'} mode: str pattern: (?P\w+) .*((?P\w+)) data: b'second string (bytes)' error: cannot use a string pattern on a bytes-like object mode: bytes pattern: b'(?P\w+) .*\((?P\w+)\)' data: first string (unicode) error: cannot use a bytes pattern on a string-like object mode: bytes pattern: b'(?P\w+) .*\((?P\w+)\)' data: b'second string (bytes)' groupdict: {'ordinal': b'second', 'type': b'bytes'} gordon at w540:~/workspace/regexdemo$ python regexdemo.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\w+) .*((?P\w+)) data: first string (unicode) groupdict: {u'ordinal': u'first', u'type': u'unicode'} mode: unicode pattern: (?P\w+) .*((?P\w+)) data: second string (bytes) groupdict: {u'ordinal': 'second', u'type': 'bytes'} mode: str pattern: (?P\w+) .*((?P\w+)) data: first string (unicode) groupdict: {'ordinal': u'first', 'type': u'unicode'} mode: str pattern: (?P\w+) .*((?P\w+)) data: second string (bytes) groupdict: {'ordinal': 'second', 'type': 'bytes'}

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-- --Guido van Rossum (python.org/~guido)

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