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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: count strangeness
Date: Sun, 22 May 2011 09:13:44 +0200
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James Stroud wrote:

> Peter Otten wrote:
>> James Stroud wrote:
>>> WTF?
>> 
>> Put the code into a file, run it -- and be enlightened ;)
> 
> 
> tal 72% python2.7 eraseme.py
> 1
> 2
> 4
> 8tal 73% cat eraseme.py
> #! /usr/bin/env python
> 
> class C:
>    def __init__(self):
>      self.data = []
>    def doit(self, count=0):
>      for c in self.data:
>        count += c.doit(count)
>      count += 1
>      print count
>      return count
> 
> c = C()
> c.data.extend([C() for i in xrange(10)])
> c.doit()
> tal 74% python2.7 eraseme.py
> 1
> 2
> 4
> 8
> 16
> 32
> 64
> 128
> 256
> 512
> 1024
> 
> 
> Hmmm. It's still 1024.
> 
> What am I missing?
> 
> James

Like Chris I assumed that you wondered why 1024 appeared twice. It turns out 
WTF is not a got a good problem description.

Now for a complete run-through, the inner c.doit() has len(c.data) == 0, so 
count += c.doit(count) just adds count + 1 and you get 10 iterations:

(1) count=0, print 1, add 1
(2) count=1, print 2, add 2
(3) count=3, print 4, add 4
(4) count=7, print 8, add 8
(5) you get the idea

Another way to look at it: in your special case the inner loop can be 
rewritten as

for c in self.data:
    print count + 1
    count += count + 1