Boolean comparison & PEP8
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 <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" )
> print( bar )
> Seems to work just fine.
Works find 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
>>> def foo(bar=):
... return bar
[5, 5, 5]
As far as ways to shoot one's own foot in Python, this is one of the most common.
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order. See above to fix.