Dict when defining not returning multi value key error

Emile van Sebille emile at fenx.com
Thu Jul 31 23:29:50 CEST 2014


On 7/31/2014 2:16 PM, Ian Kelly wrote:
> On Thu, Jul 31, 2014 at 1:17 PM, Terry Reedy <tjreedy at udel.edu> wrote:
>> On 7/31/2014 7:24 AM, Dilu Sasidharan wrote:
>>
>>> I am wondering why the dictionary in python not returning multi value
>>> key error when i define something like
>>>
>>> p = {'k':"value0",'k':"value1"}
>>
>>
>> This is documented behavior: "you can specify the same key multiple times in
>> the key/datum list, and the final dictionary’s value for that key will be
>> the last one given." I am not sure whether this is an accident of the
>> initial design, never changed since, or intended for certain uses.  It may
>> partly be because this choice is slightly simpler or, since keys are
>> expressions, not constants, that the check can only come at runtime.
>
> I don't know either, but I think that it's generally expected that the
> above would be equivalent to:
>
> p = dict()
> p['k'] = "value0"
> p['k'] = "value1"
>
> which of course will not throw an error since you're just updating the
> value in that case.

And which is also how it works:

Python 2.5 (r25:51908, Dec 18 2009, 14:22:21)
[GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>>
 >>> def test():p = {'k':"value0",'k':"value1"}
...
 >>> from dis import dis
 >>> dis(test)
   1           0 BUILD_MAP                0
               3 DUP_TOP
               4 LOAD_CONST               1 ('value0')
               7 ROT_TWO
               8 LOAD_CONST               2 ('k')
              11 STORE_SUBSCR
              12 DUP_TOP
              13 LOAD_CONST               3 ('value1')
              16 ROT_TWO
              17 LOAD_CONST               2 ('k')
              20 STORE_SUBSCR
              21 STORE_FAST               0 (p)
              24 LOAD_CONST               0 (None)
              27 RETURN_VALUE


Emile





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