Problem with iterators and inheritance

[Yves]

Hi!

I was fiddling around a bit with iterators and I came across some
strange behaviour. I have the following simple code:

--
class A(object):
def __init__(self, n):
self.n = n

def __iter__(self):
return self

def next(self):
if self.n > 0:
self.n -= 1
return "A: %d" % self.n
else:
raise StopIteration()

class B(A):
def __init__(self, n):
super(B,self)._ _init__(n)

def __iter__(self):
return self

def next(self):
if self.n > 0:
self.n -= 1
return "B: %d" % self.n
else:
raise StopIteration()

class C(A):
def __init__(self, n):
super(C,self)._ _init__(n)
self.next = self.mynext

def __iter__(self):
return self

def mynext(self):
if self.n > 0:
self.n -= 1
return "C: %d" % self.n
else:
raise StopIteration()


if __name__=='__ma in__':
a = A(2)
b = B(2)
c = C(2)
for k in [a,b,c]:
# Iterate over the object
for i in k:
print i
print "="*50

--

The output I expected was

A: 1
A: 0
=============== =============== =============== =====
B: 1
B: 0
=============== =============== =============== =====
C: 1
C: 0
=============== =============== =============== =====


but strangely enough I ended up with

A: 1
A: 0
=============== =============== =============== =====
B: 1
B: 0
=============== =============== =============== =====
A: 1
A: 0
=============== =============== =============== =====


Or in other words, it appears that Python does not use 'mynext' when
iterating over an object of class C. When I call c.next() directly
though, it does execute the code from the 'mynext' method. In my
understanding, iterating over a list, simply consists of repeatedly
calling the next() function until a StopIteration is raised, so there
should not really be any difference.
Another thing I noticed is that if I do not let A inherit from 'object'
(removing the calls to super(..) and adding appropriate initalization of
self.n), the result is as expected.
I am completely puzzled, so if anybody could shed some light on this,
I'd appreciate this.

YVES

[Scott David Daniels]

Re: Problem with iterators and inheritance

Yves wrote:
(in surprise because C's __init__ doesn't over-ride next)[color=blue]
> class A(object):
> def __init__(self, n): self.n = n
> def __iter__(self): return self
> def next(self):
> if self.n > 0:
> self.n -= 1
> return "A: %d" % self.n
> else: raise StopIteration()
>
> class C(A):
> def __init__(self, n):
> super(C,self)._ _init__(n)
> self.next = self.mynext
> def __iter__(self): return self
> def mynext(self):
> if self.n > 0:
> self.n -= 1
> return "C: %d" % self.n
> else:
> raise StopIteration()[/color]

The answer is to understand the following code:
class Xyz(object):
def __init__(self): self.x = 23
def x(self): return 42
print Xyz().x

New-style classes control object attribute lookups,
and messages go to the class first (ignoring the instance
dictionary). That is also how "properties " work, which
(if you think about it right) could not otherwise survive
the first assignment of the property.

--Scott David Daniels
scott.daniels@a cm.org