[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library

Ethan Furman ethan at stoneleaf.us
Sun Apr 28 08:37:24 CEST 2013

On 04/27/2013 08:59 PM, Greg Ewing wrote:
> Ethan Furman wrote:
>> Overriding __init__ is a PITA because __init__ is also called when you do
>>   Planet(3)  # get EARTH
>> and __init__ was expecting a gravitational constant and radius (or something like that).
>> A couple ways around that:
>>   1) have the metaclass store the args somewhere special
>>   2) have a `_init` that the metaclass calls with the args instead of __init__
> I don't much like either of those. It would be much nicer if
> one could just write an ordinary __init__ method and have it
> work as expected.

Agreed, on both counts.

> It's possible to make it work, I think. The __call__ method
> of the metaclass is going to have to do something special
> anyway, so that Planet(3) can look up and return an existing
> instance instead of making a new one. And if it doesn't make
> a new instance, it's not going to call the __init__ method.

So far I've had that logic in __new__ (which, of course, has no control on whether __init__ is called); I'll check out 
__call__ as soon as I can.

Thanks for the tip!


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