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Date: Fri, 29 May 2015 10:17:30 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: What is considered an "advanced" topic in Python?
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On 05/29/2015 10:03 AM, sohcahtoa82@gmail.com wrote:
> On Friday, May 29, 2015 at 9:02:06 AM UTC-7, Mike Driscoll wrote:

>> I've been asked on several occasions to write about intermediate or advanced topics
>>  in Python and I was wondering what the community considers to be "intermediate" or
>>  "advanced". I realize we're all growing in our abilities with the language, so this
>>  is going to be very subjective, but I am still curious what my fellow Python
>>  developers think about this topic.
>
> Metaclasses.
>
> I've read about them.  I still don't understand them, why you would want them, and what you gain from them.

Metaclasses change the way a class behaves.

For example, the new (in 3.4) Enum class uses a metaclass.

   class SomeEnum(Enum):
      first = 1
      second = 2
      third = 3

The metaclass changes normal class behavior to:

   - support iterating: list(SomeEnum) --> [SomeEnum.first, SomeEnum.second, SomeEnum.third]
   - support a length:  len(SomeEnum) --> 3
   - not allow new instances to be created:  --> SomeEnum(1) is SomeEnum(1)  # True

--
~Ethan~