Boolean comparison & PEP8

[Chris Angelico]

On Mon, Jul 29, 2019 at 10:15 AM Richard Damon <Richard at damon-family.org> wrote:
>
> On 7/28/19 7:46 PM, Michael Torrie wrote:
> > On 7/28/19 5:55 AM, Jonathan Moules wrote:
> >> But this appears to be explicitly called out as being "Worse" in PEP8:
> >>
> >> """
> >> Don't compare boolean values to True or False using ==.
> >>
> >> Yes:   if greeting:
> >> No:    if greeting == True:
> >> Worse: if greeting is True:
> >> """
> > Yet the recommended solution to the problem of wanting a default
> > argument of an empty list is something like this:
> >
> > def foo(bar=False);
> >     if bar is False:
> >         bar = []
> >
> >     ....
> >
> > Clearly in this case the expression "not bar" would be incorrect.
> >
> > There's a difference between looking for a particular identity or
> > sentinel, and checking for truthiness or falsiness.
> >
> > So I guess it all depends on what you need to do. If you just need to
> > check truthiness, just do "if greeting"  If you need to know if the
> > variable is some particular sentinel, use the "if greeting is whatever".
>
> I thought the recommended value was None, not False (so you don't need
> to ask why not True). I suppose if None is a valid value for the input
> you would need something else, and False is one possibility.
>

In that situation, you'd be more likely to use a dedicated sentinel object:

_SENTINEL = object()
def foo(bar=_SENTINEL):
    if bar is _SENTINEL: bar = []

But there is a not-too-rare situation where you'd want to check if
something "is False", and that's some form of three-valued logic, eg
"true, false, and unspecified". For instance:

def foo(verbose=None):
    if verbose is False:
        # caller specifically wanted quiet mode

    if verbose is True:
        # caller specifically wanted verbose mode

Of course, if the third value can be simplified away (eg None means
"use the global default"), then you can still just use "if verbose is
None:" and then reassign it. But this is a legit reason to use "is
False".

ChrisA