Why does [] and bool return []? [duplicate]

Question

I am trying to return a boolean in a function like this:

return mylist and any(condition(x) for x in mylist)

The behavior should be to return True if the list is empty or if any element in it meets the condition. I am using the first operand as a shortcircuit since any would return True if the list was empty, which is not what I am after.

I would expect [] and boolval to return False since the list is empty, but to my surprise it returns [] whether boolval is True or False. I would expect the first operand to be automatically evaluated as a boolean since it is involved in a comparison operation, and not whatever is happening.

I am not really asking how to solve my problem, which is easily done by an explicit type conversion: bool(mylist), but rather asking what is happening and why.

edit: when I ask "why" this is happening I am not looking for the "facts" only, as they are already explained in the linked duplicate question, but also the reasons behind the implementation of this behavior.

Answer 1

The and and or operators do not return True/False. They return the last thing evaluated (that's the case in other dynamic languages too, eg. javascript).

The official documentation describes that

• for and, the first falsy value, or the last operand
• for or, the first truthy value, or the last operand

That's by design, so you can create expressions like return username or 'guest'. So, if you want guarantee that a boolean value is returned, you have to

return bool(x or y)

instead of

return x or y