Python Library Reference

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3.1 Truth Value Testing

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.^3.1

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.)

Footnotes

...False.^3.1

Additional information on these special methods may be found in the Python Reference Manual.

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Python Library Reference

Previous: 3. Built-in Types Up: 3. Built-in Types Next: 3.2 Boolean Operations

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Release 2.5.2, documentation updated on 21st February, 2008.