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>>> import re
>>> p = re.compile(r'abc(?P<n>def)')
>>> p.sub(r'\g<n>', 'abcdef123abcdef')
'def123def'
>>> p.groupindex['n'] = 2
>>> p.sub(r'\g<n>', 'abcdef123abcdef')
'def123def'
>>> p.groupindex
{'n': 2}
>>>

The re module creates the dict purely for the benefit of the user, and as it's a normal dict, it's mutable.
An alternative would to use an immutable dict or dict-like object, but Python doesn't have such a class, and it's probably not worth writing one just for this use-case.

Matthew Barnett wrote:
> The re module creates the dict purely for the benefit of the user
this dict affects on regex.sub()
>>> import re
>>> p = re.compile(r'abc(?P<n>def)')
>>> p.groupindex
{'n': 1}
>>> p.groupindex['n'] = 2
>>> p.sub(r'\g<n>', 'abcdef')
Traceback (most recent call last):
 File "/usr/local/lib/python3.2/sre_parse.py", line 811, in expand_template
 literals[index] = s = g(group)
IndexError: no such group
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/local/lib/python3.2/re.py", line 286, in filter
 return sre_parse.expand_template(template, match)
 File "/usr/local/lib/python3.2/sre_parse.py", line 815, in expand_template
 raise error("invalid group reference")
sre_constants.error: invalid group reference
>>>

It appears I was wrong. :-(
The simplest solution in that case is for it to return a _copy_ of the dict.

But regex.sub is affected only if you manually muck with the dict, right? If so, then it looks like a case of "it hurts when I do this" (the doctor’s reply: "Don’t do this.")

the first message shows how it can work with a broken dict
Éric Araujo wrote:
> But regex.sub is affected only if you manually muck with the dict, right?
I can get code from anywhere

> I can get code from anywhere
I am afraid I don’t understand. Could you start again and explain what bug you ran into, i.e. what behavior does not match what the docs say? At present this report looks like it is saying "when I put random things in an internal data structures then bad things happen", and I don‘t think Python promises to not break when people do random editions to internal data structures.

I take someone's code
make tests for its behavior
all tests say "the code is working"
I continue to write the code
make tests for its behavior
all tests say "the code is working"
I install it somewhere and it crashes
now it is depending on the cache, when this exception is raised
Éric Araujo wrote:
>and I don‘t think Python promises to not break when people do random editions
when people do something wrong, python should raise an exception

Looks like a case for a read-only dict/dictproxy :)

I fully agree with Éric. Just don't do this.

I'm not so sure. If dicts or classes are used for configuration
or informational purposes, I prefer them to be locked down.
An example of the first is the decimal context, where it was possible
to write context.emax = 9 instead of context.Emax = 9 without getting
an error. This is an easy mistake to make and can be hard to track
down in a large program.
The mistake here is maybe less likely, but I agree with Georg that
it's a case for a read-only dict/dictproxy.

I propose using a MappingProxy type in 3.4 and add an example to the docs for stable versions.

Copy or proxy may affect performance. We will need to make benchmarks to see how much.

Here are two patches which implement two alternative solutions. They are based on regex code.
Dict copying patch matches current regex behavior and needs modifying other code to avoid small slowdown. Artificial example:
$ ./python -m timeit -s 'import re; n = 100; m = re.match("".join("(?P<g%d>.)" % g for g in range(n)), "x" * n); t = ",".join(r"\g<g%d>" % g for g in range(n))' -- 'm.expand(t)'
Without patch: 7.48 msec per loop
With re_groupindex_copy.patch but without modifying _expand: 9.61 msec per loop
With re_groupindex_copy.patch and with modifying _expand: 7.41 msec per loop
While stdlib code can be modified, this patch can cause small slowdown of some third-party code.
Dict proxying patch has no performance effect, but it is slightly less compatible. Some code can accept dict but not dict-like object.

Ping.

What approach looks better, a copy or a read-only proxy?

@Serhiy Storchaka
> What approach looks better, a copy or a read-only proxy?
ISTM, your proxy patch is better, because it expects an exception rather than silence.

New changeset 4d5826fa77a1 by Serhiy Storchaka in branch 'default':
Issue #14260: The groupindex attribute of regular expression pattern object
https://hg.python.org/cpython/rev/4d5826fa77a1