dict.get(key, default) evaluates default even if key exists

[Ethan Furman]

On 12/16/20 1:44 AM, Chris Angelico wrote:
> On Wed, Dec 16, 2020 at 8:43 PM Loris Bennett
> <loris.bennett at fu-berlin.de> wrote:
>>
>> Paul Bryan <pbryan at anode.ca> writes:
>>
>>> On Wed, 2020-12-16 at 10:01 +0100, Loris Bennett wrote:
>>>
>>>> OK, I get the point about when the default value is generated and
>>>> that
>>>> potentially being surprising, but in the example originally given,
>>>> the
>>>> key 'a' exists and has a value of '1', so the default value is not
>>>> needed.
>>>
>>> But the function is still called. The get method just doesn't use (or
>>> return) the value it generates because the key exists. Nevertheless,
>>> you're passing the return value of the get_default function as an
>>> argument.
>>>
>>>> Thus, I am still unsurprised when dict.get returns the value of an
>>>> existing key.
>>>
>>> As am I.
>>>
>>>> What am I missing?
>>>
>>> You'll need to tell me at this point.
>>
>> I was just slow and confused, but you helped me get there in the end.  I
>> now realise that issue is not about the result of the dict.get(), but
>> rather about the fact that the method which generates the default value
>> is called, whether or not it is needed.
>>
>> I shall remember that next time I think it might be a good idea to
>> define a computationally massively expensive value as a rarely needed
>> default :-)
>>
> 
> Yep! That's what defaultdict can do - or if you need more flexibility,
> subclass dict and add a __missing__ method.

Or, if the computationally massively expensive call uses potentially
different arguments for each invocation:

     some_var = d.get('a') or cme(arg1, arg2, ...)

--
~Ethan~