CODEWITHSUNDEEP
pythonclassconditional-statements
Kelketek
Kelketek

Reputation: 2446

Assign class boolean value in Python

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?

Upvotes: 29

Views: 26668

Answers (2)

interjay
interjay

Reputation: 110202

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__.

Upvotes: 43

Marcin
Marcin

Reputation: 49886

class Foo:
     def __nonzero__(self): return False
     __bool__ = __nonzero__ # this is for python3

In [254]: if Foo():
   .....:     print 'Yeah'
   .....: else: print 'Nay'
   .....:
Nay

Or, if you want to be ultra-portable, you can define __len__ only, which will have the same effect in both languages, but that has the (potential) downside that it implies that your object has a meaningful measure of length (which it may not).

This will work for any instance, depending on the actual logic you put in the method.

Upvotes: 10

Related Questions