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2.1.1 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`

- zero of any numeric type, e.g., `0`, `0L`, `0.0`.

- any empty sequence, e.g., `''`, `()`, `[]`.

- any empty mapping, e.g., `{}`.

- instances of user-defined classes, if the class defines a `__nonzero__()` or `__len__()` method, when that method returns zero.

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` for false and `1` for true, unless otherwise stated. (Important exception: the Boolean operations "`or`" and "`and`" always return one of their operands.)