[Python-3000] Detecting conflicts in dict displays

[Fredrik Lundh]

Guido van Rossum wrote:

>> Wouldn't the behavior of the above be undefined for the case where a ==
>> b?  I'd much rather get an exception than undefined behavior.
>
> There's nothing undefined in the language currently about {a: 1, b:
> 2}. It creates an empty dict d, and then does d[a] = 1; d[b] = 2. If a
> and b are equal, you end up with {a: 2}.

for the formal definition, see "dictionary displays" in the language reference:

    The key/datum pairs are evaluated from left to right to define the
    entries of the dictionary: each key object is used as a key into the
    dictionary to store the corresponding datum. /.../

    Clashes between duplicate keys are not detected; the last datum
    (textually rightmost in the display) stored for a given key value pre-
    vails.

    ( http://pyref.infogami.com/dictionary-displays )

</F>