On 29/07/2019 12.56, Rob Gaddi wrote: > On 7/29/19 10:44 AM, Michael F. Stemper 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:
>>>> 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 > Works fine right up until you do anything that modifies bar, and find > that bar always points not to a new empty list each time but to the same > empty list on each call. > >>>> def foo(bar=[]): > ... bar.append(5) > ... return bar > ... >>>> foo() > [5] >>>> foo() > [5, 5] >>>> foo() > [5, 5, 5] > > As far as ways to shoot one's own foot in Python, this is one of the > most common. I knew there had to be a good reason. -- Michael F. Stemper Always use apostrophe's and "quotation marks" properly. -- https://mail.python.org/mailman/listinfo/python-list
