On Tue, Jul 30, 2019 at 3:46 AM Michael F. Stemper <michael.stem...@gmail.com> wrote: > > On 28/07/2019 19.04, Chris Angelico wrote: > > On Mon, Jul 29, 2019 at 9:48 AM Michael Torrie <torr...@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 -- https://mail.python.org/mailman/listinfo/python-list
