overriding bool() for custom class
PythonClassCastingBooleanPython 2.xPython Problem Overview
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 Solutions
Solution 1 - Python
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 - Python
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 - Python
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 - Python
Similar to John La Rooy, I use:
class Test(object):
def __bool__(self):
return False
def __nonzero__(self):
return self.__bool__()
Solution 5 - Python
Solution 6 - Python
[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.