Boolean comparison & PEP8

[Chris Angelico]

On Tue, Jul 30, 2019 at 3:46 AM Michael F. Stemper
<michael.stemper at gmail.com> wrote:
>
> On 28/07/2019 19.04, Chris Angelico wrote:
> > On Mon, Jul 29, 2019 at 9:48 AM Michael Torrie <torriem at gmail.com> 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.
> >
> > This is a fairly unusual case, though. More commonly, the default
> > would be None, not False, and "if bar is None:" is extremely well
> > known and idiomatic.
>
> That's certainly how I would have done it until I read your post. But
> reading it immediately raised the question of why not:
>
>  def foo( bar=[] ):
>    if len(bar)==0:
>      print( "Pretty short" )
>    else:
>      print( bar )
>    return
>
> Seems to work just fine.
>

If you're not mutating it, then fine. But if you're going to append to
the list, this is falling into the classic trap of mutable default
arguments.

ChrisA