overriding bool() for custom class

user_216292 0
Viewed 28708

All I want is for bool(myInstance) to return False (and for myInstance to evaluate to False when in a conditional like if/or/and. I know how to override >, <, =)

I've tried this:

class test:
    def __bool__(self):
        return False

myInst = test()
print bool(myInst) #prints "True"
print myInst.__bool__() #prints "False"

Any suggestions?

(I am using Python 2.6)

python-2.x boolean casting class python
6 Answers

Is this Python 2.x or Python 3.x? For Python 2.x you are looking to override __nonzero__ instead.

class test:
    def __nonzero__(self):
        return False
user_488370

If you want to keep your code forward compatible with python3 you could do something like this

class test:
    def __bool__(self):
        return False
    __nonzero__=__bool__
user_1747280

If your test class is list-like, define __len__ and bool(myInstanceOfTest) will return True if there are 1+ items (non-empty list) and False if there are 0 items (empty list). This worked for me.

class MinPriorityQueue(object):
    def __init__(self, iterable):
        self.priorityQueue = heapq.heapify(iterable)
    def __len__(self):
        return len(self.priorityQueue)

>>> bool(MinPriorityQueue([])
False
>>> bool(MinPriorityQueue([1,3,2])
True
user_26833990
Ignacio_Vazquez-Abrams_208620
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