Boolean comparison & PEP8

[Rob Gaddi]

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" )
>     else:
>       print( bar )
>     return
> 
> 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 
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.

-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.