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pythonobjectboolean

Evaluation of boolean expressions in Python

What truth value do objects evaluate to in Python?

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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.