[Python-Dev] Return type of alternative constructors (original) (raw)

Ethan Furman ethan at stoneleaf.us
Sat May 7 19:14:12 EDT 2016

On 05/07/2016 03:39 PM, Serhiy Storchaka wrote:

Some types have alternative constructors -- class methods used to create an instance of the class. For example: int.frombytes(), float.fromhex(), dict.fromkeys(), Decimal.fromfloat().

But what should return these methods for subclasses? Should they return an instance of base class or an instance of subclass? Almost all alternative constructors return an instance of subclass (exceptions are new in 3.6 bytes.fromhex() and bytearray.fromhex() that return bare bytes and bytearray). But there is a problem, because this allows to break invariants provided by the main constructor.

Please ignore my comments in that issue. I actually prefer that class constructors go through the subclass' new and init. Overriding parent class methods for the sole purpose of getting the subclass's type is quite irritating.

-- Ethan

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