Question

I was testing a list to see if it’s empty or not. Normally I use len(list) == 0 and I vaguely remembered reading a little while ago that the correct way to test if a list is empty was whether it was True or false.

So I tried list is False, and that returned False. Maybe I’m suppose to be using == ?
Nope, that also returned false. list is True, returned false as did list == True.

Now I’m confused so I do a quick google and end up at: Best way to check if a list is empty

The top answer is:

if not a:
    print "List is empty"

So I search around some more and end up in the python manual where 4.1 states:

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

any empty sequence, for example, ”, (), [].

Now I’m plain confused. If I test a list as if not list, it works fine. But if an empty list is false, then why can’t I just do if list is False or if list == False?

Thanks

Answer 1

An empty list is not False, but when you convert it to a boolean, it converts to False. Likewise for dicts, tuples, strings, etc.:

>>> [] == False
False
>>> bool([]) == False
True
>>> {} == False
False
>>> bool({}) == False
True

When you put something in the condition of an if clause, it is its boolean value that is used for testing the if. That’s why if someList is the same as if bool(someList). Likewise, not foo does a boolean not, so not [] equals True.