Metaclasses - magic functions

Ethan Furman ethan at
Tue Dec 20 20:51:15 EST 2016

On 12/20/2016 03:39 PM, Ben Finney wrote:
> "Mr. Wrobel" writes:

>> Quick question, can anybody tell me when to use __init__ instead of
>> __new__ in meta programming?
> Use ‘__new__’ to do the work of *creating* one instance from nothing;
> allocating the storage, determining the type, etc. — anything that will
> be *the same* for every instance. ‘__new__’ is the constructor.
> Use ‘__init__’ to do the work of *configuring* one instance, after it
> already exists. Any attributes that are special to an instance should be
> manipulated in the ‘__init__’ method. ‘__init__’ is the initialiser.

That sounds like general object creation/class advice, which as a general
guideline is okay, but don't feel like it's the most important thing.

I only use `__new__` when the object being created is (or is based on)
an immutable type; otherwise I use `__init__`.  Likewise, if I'm using
`__new__` then I do all my configuration in `__new__` unless I have a
really good reason not to (such as making it easier for subclasses to
modify/skip `__init__`).

As far as metaclasses go... the only time I recall writing an `__init__`
for a metaclass was to strip off the extra arguments so `type.__init__`
wouldn't fail.


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