On 2014-07-16 00:48, Russell E. Owen wrote:
In article
<CAPTjJmoZHLfT3G4eqV+=zcvbpf65fkcmah9h_8p162uha7f...@mail.gmail.com>,
Chris Angelico <ros...@gmail.com> wrote:
On Mon, Jul 14, 2014 at 12:04 AM, Jason R. Coombs <jar...@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<first>\w+)\s+(?P<second>\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 "<stdin>", line 2, in <module>
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<first>\w+)\s+(?P<second>\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.
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