[Python-Dev] For review: PEP 285: Adding a bool type

2002年3月10日 19:38:50 -0500

> But True - 1 won't equal False. Should it?

No. My logic for deciding the type of "x <op> y" (and for "<op> x")
is based exclusively on the types of x and y. For a given <op>, the
type of "x <op> y" should be derived from the types of x and y and the
properties of <op>. The values of x and y don't enter into it (except
implicitly, insofar as their type limits the possible values). Thus,
x&y, x|y, and x^y yield a bool when both x and y are bools, because
they yield bool values whenever the arguments are both bools; but x+y
is an int, even when both x and y are bools and the outcome is 0 or 1
(representable as a bool): this is because there's also a case
(True+True) where the outcome (2) is not representable as a bool.
The only exception is +x: this returns an int result for a bool
argument. That's a matter of taste -- the unary + screams "number" to
me. I could be convinced that it should return x unchanged. But in
any case the type of the result still only depends on the *type* of
the argument.
This is a very general rule that I like a lot: that the type of a
result should only depend on the type of the arguments, not on their
values. I expect that this rule will make reasoning about programs
(as in PsyCo or PyChecker) easier to do.
I recently caved in and allowed an exception: 2**2 returns an int, but
2**-2 returns a float. This was a case of "practicality beats
purity". I don't see True-1 in the same league though.
--Guido van Rossum (home page: http://www.python.org/~guido/)