[Python-ideas] PEP 4XX: Adding sys.implementation

[Barry Warsaw]

On Apr 30, 2012, at 09:22 PM, Eric Snow wrote:

>> I agree that sequence semantics are meaningless here.  Presumably, a
>> dictionary is proposed because this
>>
>>    cache_tag = sys.implementation.get('cache_tag')
>>
>> is nicer than
>>
>>    cache_tag = getattr(sys.implementation, 'cache_tag', None)
>
>That's a good point.  Also, a dict better reflects a collection of
>variables that a dotted-access object, which to me implies the
>potential for methods as well.
>
>> OTOH, maybe we need a nameddict type!
>
>You won't have to convince _me_. :)

Well, I was being a bit facetious.  You can easily implement those semantics
in pure Python.  5 minute hack below.

Cheers,
-Barry

-----snip snip-----
#! /usr/bin/python3

_missing = object()

import operator
import unittest

class Implementation:
    cache_tag = 'cpython33'
    name = 'CPython'

    def __getitem__(self, name, default=_missing):
        result = getattr(self, name, default)
        if result is _missing:
            raise AttributeError("'{}' object has no attribute '{}'".format(
                self.__class__.__name__, name))
        return result

    def __setitem__(self, name, value):
        raise TypeError('read only')

    def __setattr__(self, name, value):
        raise TypeError('read only')


implementation = Implementation()


class TestImplementation(unittest.TestCase):
    def test_cache_tag(self):
        self.assertEqual(implementation.cache_tag, 'cpython33')
        self.assertEqual(implementation['cache_tag'], 'cpython33')

    def test_name(self):
        self.assertEqual(implementation.name, 'CPython')
        self.assertEqual(implementation['name'], 'CPython')

    def test_huh(self):
        self.assertRaises(AttributeError, operator.getitem,
                          implementation, 'droids')
        self.assertRaises(AttributeError, getattr,
                          implementation, 'droids')

    def test_read_only(self):
        self.assertRaises(TypeError, operator.setitem,
                          implementation, 'droids', 'looking')
        self.assertRaises(TypeError, setattr,
                          implementation, 'droids', 'looking')
        self.assertRaises(TypeError, operator.setitem,
                          implementation, 'cache_tag', 'xpython99')
        self.assertRaises(TypeError, setattr,
                          implementation, 'cache_tag', 'xpython99')
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 836 bytes
Desc: not available
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20120501/8c105337/attachment.pgp>