Question

Can I be sure that '' always will be considered False while anything else isn’t?

>>> if '':
...    print('bah')
...
>>> if 'x':
...    print('bah')
...
bah

Why or why not? What mechanism in Python defines this behavior?

If '' is evaluated as False, why do I get this result:

>>> if '' == False:
...    print('bah')
...
>>>

Answer 1

From the documentation:

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:

• 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.