[Python-Dev] Return type of alternative constructors

[Serhiy Storchaka]

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