About Python truth value test

If you are using Python, you probably will write something like this to check if a list is empty:

if list:
    print("not empty!")
else:
    print("empty list")

That's because empty list evaluates to False, as described on official docs,.

But, if you type [] == False in a Python interactive shell and hit enter, the result is False!

>>> [] == False
False

If empty list evaluates to False in a if statement, then why a direct comparision between empty list and False evaluates to False? It should be True, right? What's happening here?

Because of this, I am using len(some_list) == 0 to check empty-ness since the beginning, which is the least confusing and most explict way to do this.

But that doesn't resolve the problem: I don't know why it works, which didn't really bother me, until today.

== operator

To know how things work, its better to have a look at the official documents, here is the page for == operator.

And this page makes it very clear:

Objects of different types, except different numeric types, never compare equal.

Back to [] == False, since these two objects are of different types: list and bool respectively, the result is False.

You can check this by [] == {}, which evaluates to False, because even if they are both empty and will evaluates to True in a if statement, they are not of the same type.

if {}:
    # {} evaluates to False

if []:
    # [] evaluates to False
    
{} == []
# although they both evaluates to False above, the result here is False

if statement

This part is described by Truth Value Testing.

Any object can be tested for truth value, for use in an if or while condition

So inside if statement, the truth value of variables like [] is defined by its __bool__ functions:

By default, an object is considered true unless its class defines either a __bool__() method that returns False or a __len__() method that returns zero, when called with the object.

That explains why empty list evaluates to False in a if statement. Now I am more comfortable to use if some_list: xxx in my code!