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References: <CAGGBd_pd9SA7SczanGARFr-+Z9SxP9Mkhfo-RivpcJLrnuexng@mail.gmail.com> <851th2t05t.fsf@benfinney.id.au>
Date: Tue, 23 Jun 2015 17:47:58 -0700
Subject: Re: Looking up a dictionary _key_ by key?
From: Dan Stromberg <drsalists@gmail.com>
To: Ben Finney <ben+python@benfinney.id.au>
Cc: Python List <python-list@python.org>
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On Tue, Jun 23, 2015 at 5:33 PM, Ben Finney <ben+python@benfinney.id.au> wr=
ote:
> Dan Stromberg <drsalists@gmail.com> writes:
>
>> Is there a way of getting the key used by the dictionary, short of
>> storing a reference to it in the value, or using a second dictionary?
>
> The dictionary knows its keys and can provide them on request. Call the
> =E2=80=98dict.keys=E2=80=99 method to get them as a collection.

So it appears:

$ ./pythons --command 'd=3D{1:2, 2:4, 3:8}; print(d.keys())'
/usr/local/cpython-2.4/bin/python good [1, 2, 3]
/usr/local/cpython-2.5/bin/python good [1, 2, 3]
/usr/local/cpython-2.6/bin/python good [1, 2, 3]
/usr/local/cpython-2.7/bin/python good [1, 2, 3]
/usr/local/cpython-3.0/bin/python good <dict_keys object at 0x8514fa4>
/usr/local/cpython-3.1/bin/python good dict_keys([1, 2, 3])
/usr/local/cpython-3.2/bin/python good dict_keys([1, 2, 3])
/usr/local/cpython-3.3/bin/python good dict_keys([1, 2, 3])
/usr/local/cpython-3.4/bin/python good dict_keys([1, 2, 3])
/usr/local/cpython-3.5/bin/python good dict_keys([1, 2, 3])
/usr/local/jython-2.7rc1/bin/jython good [3, 2, 1]
/usr/local/pypy-2.5.0/bin/pypy good [1, 2, 3]
/usr/local/pypy3-2.4.0/bin/pypy3 good dict_keys([1, 2, 3])

But:

$ ./pythons --command 'd=3D{1:2, 2:4, 3:8}; print(d.keys()[1])'
/usr/local/cpython-2.4/bin/python good 2
/usr/local/cpython-2.5/bin/python good 2
/usr/local/cpython-2.6/bin/python good 2
/usr/local/cpython-2.7/bin/python good 2
/usr/local/cpython-3.0/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/cpython-3.1/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/cpython-3.2/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/cpython-3.3/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/cpython-3.4/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/cpython-3.5/bin/python bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object does not support indexing
/usr/local/jython-2.7rc1/bin/jython good 2
/usr/local/pypy-2.5.0/bin/pypy good 2
/usr/local/pypy3-2.4.0/bin/pypy3 bad
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    TypeError: 'dict_keys' object is not subscriptable

Would I have to do an O(n) search to find my key?

Thanks!