If statements in Python allow you to do something like:
if not x:
print "X is false."
This works if you're using an empty list, an empty dictionary, None, 0, etc, but what if you have your own custom class? Can you assign a false value for that class so that in the same style of conditional, it will return false?
Since Python 3, you need to implement the __bool__
method on your class. This should return True or False to determine the truth value:
class MyClass:
def __init__(self, val):
self.val = val
def __bool__(self):
return self.val != 0 #This is an example, you can use any condition
x = MyClass(0)
if not x:
print('x is false')
If __bool__
has not been defined, the implementation will call __len__
and the instance will be considered True if it returned a nonzero value. If __len__
hasn't been defined either, all instances will be considered True.
For further reading:
In Python 2, the special method __nonzero__
was used instead of __bool__
.