[Python-Dev] RE: Re: PEP 285: Adding a bool type

[Gerald S. Williams]

Mark McEahern wrote:
> If, as Fredrik says, there's already one obvious way to return True (and
> that's the int 1), then we already have the problem that people will be
> inclined to compare something to this canonical truth in conditional
> statements.

Actually, he wrote "preferred", not "obvious".

I.e.:
  def bool(x): return not not x
will return 0 or 1.

but (), [], "", and None also evaluate as false in a logical
context, while most other things evaluate as true.

Having canonical True and False values would lead newcomers
to compare to them. This would work, for example:

  DONE = y and (x > y)
  ...
  if DONE is False:
     ...

On the other hand, this wouldn't (unless y is a boolean):

  DONE = (x > y) and y
  ...
  if DONE is False:
     ...

Of course, it works either way if you replace "DONE is False"
with "not DONE". Or if you replace "y" with "bool(y)".

Since there aren't canonical True/False values, there is
currently an obvious solution: use "not DONE". But "native"
boolean support will lead to mistakes like this unless you
start taking steps to introduce type-safety with operations
involving boolean values. This is definitely going farther
than was intended by the PEP.

-Jerry

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