[Tutor] Use of "or" in a lambda expression

[Steven D'Aprano]

On Sat, Apr 04, 2015 at 02:21:19PM -0500, boB Stepp wrote:

> To my mind, would:
> 
> def quit():
>     print('Hello lambda world!')
>     sys.exit()
> 
> and:
> 
> widget = Button(None, text='Hello event world!', command=quit)
> 
> be preferable Python style?

Hell yes!


Using `or` to run functions purely for their side-effects (in this case, 
printing and exiting) makes a nice trick, but I wouldn't use for real.

On the other hand, using `or` for its value is perfectly acceptable. 
E.g. one common idiom might be to iterate over something which might be 
None:

for value in maybe_list or []:
    ...


If `maybe_list` is a non-empty list, it is used; if it is an empty list, 
the second operand (also an empty list) is used, but that's okay since 
they are both empty lists; and if it is None, then the second operand is 
used instead. This is a reasonable idiom to use, and prior to Python 2.5 
it was the closest thing the language had to a "ternary if operator".

These days, the `or` idiom is less common except in old code or code 
that has to run on Python 2.4 or older. Instead, we might write:

for value in (maybe_list if maybe_list is not None else []):
    ...

I'm not entirely sure that's an improvement :-)

-- 
Steve