Why do function objects evaluate to True in python?

Question

In python it is valid to make a construction like:

def a(): 
    return 0

if a: 
    print \"Function object was considered True\"
else:  
    print \"Funct         


        

Answer 1

In the Truth Value Testing that @Magnus Hoff linked to, I found the most instructive statement to be:

By default, an object is considered true unless its class defines either a _bool_() method that returns False or a _len_() method that returns zero, when called with the object.

I tried defining my own classes, and it seems like _bool_() takes priority over _len_(), which makes sense:

class Falsish(object):
    def __init__(self):
        self.val = "False, even though len(self) > 0"
    def __bool__(self):
        return False
    def __len__(self):
        return 2

class StillFalsish(object):
    def __init__(self):
        self.val = "False, even though len(self) > 0"
    def __len__(self):
        return 2
    def __bool__(self):
        return False

class Truish(object):
    def __init__(self):
        self.val = "True, even though len(self) = 0"
    def __bool__(self):
        return True
    def __len__(self):
        return 0

class StillTruish(object):
    def __init__(self):
        self.val = "True, even though len(self) = 0"
    def __len__(self):
        return 0
    def __bool__(self):
        return True

class NowFalsish(object):
    def __init__(self):
        self.val = "False, with no __bool__(), since len(self) = 0"
    def __len__(self):
        return 0

class NowTruish(object):
    def __init__(self):
        self.val = "True, with no __bool__(), since len(self) > 0"
    def __len__(self):
        return 2

class EvenThisIsTruish(object):
    pass


mybool1 = Falsish()
mybool2 = StillFalsish()
mybool3 = Truish()
mybool4 = StillTruish()
mybool5 = NowFalsish()
mybool6 = NowTruish()
mybool7 = EvenThisIsTruish()
if mybool1: print("mybool1 is true")
if mybool2: print("mybool2 is true")
if mybool3: print("mybool3 is true")
if mybool4: print("mybool4 is true")
if mybool5: print("mybool5 is true")
if mybool6: print("mybool6 is true")
if mybool7: print("mybool7 is true")

The output of the above code is:

mybool3 is true
mybool4 is true
mybool6 is true
mybool7 is true