Evaluation of boolean expressions in Python

boolean, object, python

Solution

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. Operations and built-in functions that have a Boolean result always return 0 or `False` for false and 1 or `True` for true, unless otherwise stated. (Important exception: the Boolean operations "or" and "and" always return one of their operands.)

https://docs.python.org/2/library/stdtypes.html#truth-value-testing

And as mentioned, you can override with custom objects by modifying nonzero.

Problem

Original source

Related problems

• Boolean value of objects in Python
• How do "and" and "or" act with non-boolean values?
• What is Truthy and Falsy? How is it different from True and False?
• How do "and" and "or" act with non-boolean values?