overriding bool() for custom class

Question

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)

Solution 1

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

Solution 2

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__

Solution 3

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

Solution 4

Similar to John La Rooy, I use:

class Test(object):
    def __bool__(self):
        return False

    def __nonzero__(self):
        return self.__bool__()

Solution 5

[this is a comment to the answer from @john-la-rooy but I cannot comment yet :) ]

For Python3 compatibility you can do (I was looking for this)

class test(object):
    def __bool__(self):
        return False

    __nonzero__=__bool__

only problem is that you need to repeat the __nonzero__ = __bool__ everytime you change __bool__ in subclasses. Otherwise __nonzero__ will be kept from the superclass. You can try

from builtins import object  # needs to be installed !

class test(object):
    def __bool__(self):
        return False

    __nonzero__=__bool__

which should work (not confirmed) or write a metaclass :) yourself.

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Comments

[Ponkadoodle]

That's handy. Thanks for this alternate solution.

[TrinitronX]

Thanks for this... I was looking for a way to test the __nonzero__ class method for testing False here

[jhp]

Or use the approach from tknickman to avoid this issue. It will add another function call though!

[Callam Delaney]

This is actually the best solution as it is compatible with both Python3 and Python2 to my knowledge. Even if it is not an iterable you could implement something to mock it.

[IceArdor]

__nonzero__ = __bool__ is the preferable way of aliasing __bool__ because it's less code (quicker to read, less to maintain), less likely to hide a bug (what happens if __bool__ later becomes an async function?), but more importantly 1 less stack frame for a frequently called function that's also implicit! The difference here is whether you're defining 2 functions or just 1 function that's referenced twice.

[Leo Ufimtsev]

It would be of benefit to mention that bool is python3 and code to make a class more portable across python2/3 as mentioned by John below.

[Leo Ufimtsev]

The nonzero__=__bool seems to how an error in python 2 because bool is not defined.

[Zhang Fan]

@LeoUfimtsev have you moved "__nonzero__=__bool__" to a smaller line number than "def __bool__(self):" ? in that case it is undefined, written order matters here.

[LMCuber]

Wouldn't this create a class variable instead of an instance variable?