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 valueFalse
.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 orFalse
for false and 1 orTrue
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.