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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