bool evaluation

bool.py

class Test(object):
    def __init__(self, var):
        self.var = var
        
    def __bool__(self):
        print 'called it.'
        return bool(self.var)
    
a = Test(10)

if a: # This will call a.__bool__
    pass
# called it.

if not a:
    pass
# called it.

if a is None: # is only ever True if before "a = None". But beware "if not None == True"
    pass
# Nothing happens