[Python-Dev] Another case for frozendict (original) (raw)

MRAB python at mrabarnett.plus.com
Wed Jul 16 04:27:23 CEST 2014

On 2014-07-16 00:48, Russell E. Owen wrote:

In article <CAPTjJmoZHLfT3G4eqV+=ZCVbpf65fkcmah9h8p162UHA7fQLA at mail.gmail.com>, Chris Angelico <rosuav at gmail.com> wrote:

On Mon, Jul 14, 2014 at 12:04 AM, Jason R. Coombs <jaraco at jaraco.com> wrote: > I can achieve what I need by constructing a set on the ‘items’ of the dict. > >>>> set(tuple(doc.items()) for doc in res) > > {(('n', 1), ('err', None), ('ok', 1.0))}

This is flawed; the tuple-of-tuples depends on iteration order, which may vary. It should be a frozenset of those tuples, not a tuple. Which strengthens your case; it's that easy to get it wrong in the absence of an actual frozendict. I would love to see frozendict in python. I find myself using dicts for translation tables, usually tables that should not be modified. Documentation usually suffices to get that idea across, but it's not ideal. frozendict would also be handy as a default values for function arguments. In that case documentation isn't enough and one has to resort to using a default value of None and then changing it in the function body. I like frozendict because I feel it is expressive and adds some safety. Here's another use-case.

Using the 're' module:

import re

Make a regex.

... p = re.compile(r'(?P\w+)\s+(?P\w+)')

What are the named groups?

... p.groupindex {'first': 1, 'second': 2}

Perform a match.

... m = p.match('FIRST SECOND') m.groupdict() {'first': 'FIRST', 'second': 'SECOND'}

Try modifying the pattern object.

... p.groupindex['JUNK'] = 'foobar'

What are the named groups now?

... p.groupindex {'first': 1, 'second': 2, 'JUNK': 'foobar'}

And the match object?

... m.groupdict() Traceback (most recent call last): File "", line 2, in IndexError: no such group

It can't find a named group called 'JUNK'.

And with a bit more tinkering it's possible to crash Python. (I'll leave that as an exercise for the reader! :-))

The 'regex' module, on the other hand, rebuilds the dict each time:

import regex

Make a regex.

... p = regex.compile(r'(?P\w+)\s+(?P\w+)')

What are the named groups?

... p.groupindex {'second': 2, 'first': 1}

Perform a match.

... m = p.match('FIRST SECOND') m.groupdict() {'second': 'SECOND', 'first': 'FIRST'}

Try modifying the regex.

... p.groupindex['JUNK'] = 'foobar'

What are the named groups now?

... p.groupindex {'second': 2, 'first': 1}

And the match object?

... m.groupdict() {'second': 'SECOND', 'first': 'FIRST'}

Using a frozendict instead would be a nicer solution.

More information about the Python-Dev mailing list