Terry J. Reedy added the comment:
I do not agree with the patch. A summary of my view: Range objects support the
'in' operator and they are an intended option for choices, and, as I said
before, are exactly the right option for arithmetic sequences with more than a
few items. The problem is that they are now, in effect, special-cased relative
to other builtins by having their compact representation replaced by an
expanded tuple display. Moreover, the iteration required to do this introduces
a discrepancy relative to the doc. This bug might be better fixed by a code
change.
(The OP did not pass a (x)range object but a list. That was unnecessary and
easily fixed in itself. But such a fix leaves the issue above. Condensing long
sequences is a somewhat separate issue.)
As to intent:
"The choices keyword argument may be more convenient for type checkers that
simply check against a range of values:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=int, choices=range(5, 10))
>>> parser.parse_args('7'.split())
Namespace(foo=7)
>>> parser.parse_args('11'.split())
usage: PROG [-h] {5,6,7,8,9}
PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)"
Note the expansion instead of the normal representation. It is not a big deal
here, but obviously can be.
">>> parser.add_argument(
... 'integers', metavar='int', type=int, choices=range(10),
... nargs='+', help='an integer in the range 0..9')"
As to tuple display expansion: the link points to
2284 def _check_value(self, action, value):
2285 # converted value must be one of the choices (if specified)
2286 if action.choices is not None and value not in action.choices:
2287 args = {'value': value,
2288 'choices': ', '.join(map(repr, action.choices))}
2289 msg = _('invalid choice: %(value)r (choose from %(choices)s)')
2290 raise ArgumentError(action, msg % args)
In 2288 "', '.join(map(repr, action.choices))" produces a tuple display without
parentheses. It essentially reproduced str/repr for tuples, lists, frozensets,
sets, dicts, dict views, etc., leaving off the irrelevant fence characters. In
doing so, by iteration, it introduces a bug --see below.
For range objects, the tuple representation is a drastic change from the normal
representation. In that sense, it special cases range among built-ins, and in a
bad way when the range represents many values. (Help messages apparently do the
same.) I consider this to be something of a bug. So the code as it is would
have to special case range objects to avoid special-casing them in the sense
above.
The same would apply to any custom objects that have a succinct description for
a large, possible infinite set. Here are two examples: "a word containing no
'e's" and "a 'word' containing only the letters a, b, c, d, e". Objects
representing such infinite sets of strings could easily have a __contains__
method but not an __iter__ method.
The code above requires the choices object to be iterable as well as supporting
'in'. This contradicts the doc statement "Any object that supports the in
operator can be passed as the choices value,". That discrepancy is a bug. It
should be fixed by either adding the restriction to the doc or removing it from
the code. I recommend the latter.
The code could simply use the str or repr of the choice object without trying
to be fancy with a custom, fence-stripped, representation that does not work
correctly or at all for all possible choice objects. In other words
if action.choices is not None and value not in action.choices:
msg = "invalid choice: %r (choose from %r)" % (value, action.choices)
raise ArgumentError(action, msg)
If the custom representation for non-range builtins is desired, then they are
the ones that should be special-cased to not use their default representation.
----------
assignee: docs@python ->
keywords: -easy
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue16418>
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