checking if a list is empty

[Steven D'Aprano]

On Fri, 06 May 2011 16:05:09 -0400, Adam Tauno Williams wrote:

> I'd never accept code like "if not x" as an empty test.

So much the worse for you then.

The point of the "if x" idiom is that it is a polymorphic test which is 
independent of the type. It works with any non-broken object[1], no 
matter what x is, since it allows x to decide for itself whether it is 
empty or not.

Of course, you can write your own polymorphic test:


try:
    flag = len(x) == 0
except TypeError:
    # This *should* succeed, for anything not broken. If not, we can't
    # recover, so just let the exception propagate.
    flag = bool(x)
    # Hilariously, bool(x) may also try len(x) == 0... oh well.
if flag:
    ...

Congratulations! You've now found a way to write "if x:" in five lines, 
53 characters (plus whitespace and comments) and an extraneous variable 
polluting the namespace. If you're being paid per line of code, this is a 
good win.

Of course, sometimes you don't want polymorphism (or at least, not too 
much polymorphism). Sometimes I'll write "if x == 0" or similar if I want 
to emphasize that x is a special case unrelated to the truth/falseness of 
x. But that's quite rare. Normally I trust x to decide for itself whether 
it is empty or not.





[1] Arguably with the exception of iterables. But then the len(x) test 
doesn't work for them either.


-- 
Steven