The float constructor can return an instance of float subclass. >>> class FloatSubclass(float): ... pass ... >>> class BadFloat: ... def __float__(self): ... return FloatSubclass(1.2) ... >>> type(float(BadFloat())) <class '__main__.FloatSubclass'> Comparing with other types, complex() always returns complex: >>> class ComplexSubclass(complex): ... pass ... >>> class BadComplex: ... def __complex__(self): ... return ComplexSubclass(1.2, 3.4) ... >>> type(complex(BadComplex())) <class 'complex'> And int() can return an instance of int subclass, but this behavior is deprecated: >>> class BadInt: ... def __int__(self): ... return True ... >>> int(BadInt()) __main__:1: DeprecationWarning: __int__ returned non-int (type bool). The ability to return an instance of a strict subclass of int is deprecated, and may be removed in a future version of Python. True May be we should either deprecate __float__ returning non-float (as for int), or convert the result to exact float (as for complex). The constructor of float subclass always returns an instance of correct type. >>> class FloatSubclass2(float): ... pass ... >>> type(FloatSubclass2(BadFloat())) <class '__main__.FloatSubclass2'>