Boolean comparison & PEP8

[Michael F. Stemper]

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 <torriem at 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.