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

Serhiy Storchaka storchaka at gmail.com
Sat May 7 18:39:13 EDT 2016

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

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.

For example, there are only two instances of the bool class: False and True. But with the from_bytes() method inherited from int you can create new boolean values!

>>> Confusion = bool.from_bytes(b'\2', 'big')
>>> isinstance(Confusion, bool)
True
>>> Confusion == True
False
>>> bool(Confusion)
True
>>> Confusion
False
>>> not Confusion
False

bool is just the most impressive example, the same problem exists with IntEnum and other enums derived from float, Decimal, datetime. [1]

The simplest solution is to return an instance of base class. But this can breaks a code, and for this case we should be use static method (like str.maketrans), not class method.

Should alternative constructor call new and init methods? Thay can change signature in derived class. Should it complain if new or init were overridden?

[1] http://bugs.python.org/issue23640

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