Question

class Foo(object):
    def __init__(self,x):
        self.x = x
        self.is_bar = False
    def __repr__(self): return str(self.x)

class Bar(object):
    def __init__(self,l = []):
        self.l = l
    def add(self,o):
        self.l += [o]
    def __repr__(self): return str(self.l)

def foo_plus_foo(f1,f2):
    t = Bar()
    if not (f1.is_bar and f2.is_bar):
        f1.is_bar = True
        f2.is_bar = True
        t.add(f1)
        t.add(f2)
        print 'HERE'
    return t

if __name__ == '__main__':
    li = [Foo(1), Foo(2)]
    print foo_plus_foo(li[0],li[1])
    print foo_plus_foo(li[0],li[1])

UNEXPECTED OUTPUT:

HERE
[1, 2]
[1, 2]

EXPECTED OUTPUT:

HERE
[1, 2]
[]

What is happening? What did I do wrong? Why is python using old value? What do I do to avoid this?

Thanks!

Answer 1

Never. Do. This.

def __init__(self,l = []):

Never.

One list object is reused. And it’s Mutable, so that each time it’s reused, the one and only [] created in your method definition is updated.

Always. Do. This.

def __init__( self, l= None ):
    if l is None: l = []

That creates a fresh, new, unique list instance.