What do boolean operations on lists mean?

Question

I was going through the source of pyftpdlib and I found this:

if self.rejected_users and self.allowed_users: raise AuthorizerError("rejected_users and allowed_users options are mutually exclusive")

rejected_users and allowed_users are lists. What's confusing me is how the and operator operates on two lists. I'd appreciate it if someone helped me out.

Answer 1

All objects in Python have a boolean 'value'; they are either true or false in a boolean context.

Empty lists are false. This applies to all sequences and containers, including tuples, sets, dictionaries and strings.

Numeric 0 is false too, so 0, 0.0, 0j are all false as well, as are None and of course False itself:

>>> bool([])
False
>>> bool([1, 2])
True
>>> bool(0)
False
>>> bool('')
False

Everything else is considered true in a boolean context; so a list that is not empty is true, and two non-empty lists together with and is considered true as well.

You can make custom types look like empty containers by implementing __len__() and returning 0, or look like a number by implementing __nonzero__()^* and returning False when the instance is to be the boolean equivalent of numeric zero.

Just remember that and and or shortcircuit; if the first expression locks in the result then that value is returned and the second expression is ignored altogether. For and, that means that in the expression x and y, y is ignored if x is a false value (like an empty list), because the whole expression can never be true in that case. For x or y, y is ignored if x is a true value.

These rules are all covered by the Boolean operations reference documentation.

^{*_{In Python 3, use __bool__ instead}}.

Answer 2

Empty list evaluates to False and non-empty list evaluates to True.

if list1 and list2:

is equivalent to:

if list1 is not empty and list2 is not empty:

---

List of falsy values in python:

• None

• False

• zero of any numeric type, for example, 0, 0L, 0.0, 0j.

• any empty sequence, for example, '', (), [].

• any empty mapping, for example, {}.

• instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.

All other values are considered true — so objects of many types are always true.