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.)