[Python-Dev] PEP 285: Adding a bool type (original) (raw)

Alex Martelli aleax@aleax.it
Sun, 31 Mar 2002 21:06:25 +0100

On Saturday 31 March 2002 9:45, Andrew Koenig wrote: ... """ Liskov substitutibility would seem to suggest deriving int from bool, not the other way around. That is: If I have a program that uses bool values, I can change it so that it uses int values without affecting the behavior of the program. The reverse is not true.

I wonder if this is the circle-ellipse problem over again? """

I think not, because circle/ellipse, square/rectangle and bag-of-bananas/ bag-of-fruit are problems only under mutability. If an object is immutable (either because of general language rules -- a FP language, say -- or because of specific considerations, such as a const reference in C++) you CAN pass a reference to an immutable circle wherever a reference to an immutable ellipse is expected (etc) and keep Liskov happy.

And Python numbers are immutable... so I don't think the problem is related to the well-known category you mention.

Python's int and the proposed bool can't satisfy Liskov anyway, either way, due to str and/or repr. If I "know" that x is one of the proposed bool objects, I could "assert str(x).istitle()" (or repr(x).istitle(), depending on which function is meant to return 'True' and 'False') -- if x is an int, that fails. If I know that x is an int, I can similarly asserts tr(x).isdigit() -- but if x is a bool, that fails.

[Some would say this is too strict a reading of Liskov for practical use... do you think it is?]

Apart from str and repr issues, I cannot find any assertion I can make under the proposed PEP, that will work for any x that is an int, but breaks if x is a bool. So (net of str/repr issue), any instance of bool IS-A int, so inheriting bool from int seems OK (net of str/repr issues). OTOH, if I know that x is one of the proposed bool values, I can assert 0<=x<2, and that will fail in many cases if x is an int instead. So, inheriting int from bool seems unwarranted. Or, am I missing something?

Alex