don't understand behaviour of recursive structure

Dan Davison davison at stats.ox.ac.uk
Sat Mar 14 17:31:35 CET 2009


I'm new to python. Could someone please explain the following behaviour
of a recursive data structure?

def new_node(id='', daughters=[]):
    return dict(id=id, daughters=daughters)

n0 = new_node(id='n0')
n1 = new_node(id='n1')

## Seems OK so far:
n0                                 # {'id': 'n0', 'daughters': []}
n1                                 # {'id': 'n1', 'daughters': []}

## Now try to make n1 a daughter of n0:
n0['daughters'].append(n1)

## But that seems to have made something funny happen to n1:
n1                                 # {'id': 'n1', 'daughters': [{...}]}

## In fact, n1 seems to have become its own daughter
n1['daughters'][0]                 # {'id': 'n1', 'daughters': [{...}]}

## and grand-daughter, etc etc
n1['daughters'][0]['daughters'][0] # {'id': 'n1', 'daughters': [{...}]}

## These changes to n1 are present in n0 (as I would expect)
n0                                 # {'id': 'n0', 'daughters': [{'id':'n1', 'daughters': [...]}]}
n0['daughters'][0]['daughters'][0] # {'id': 'n1', 'daughters': [...]}


Why did the append() operation have this effect? Straight assignment
seems to do what I expected, i.e.

n0['daughters'] = [n1]
n1                                # still the same {'id': 'n1', 'daughters': []}
n0                                # {'id': 'n0', 'daughters': [{'id': 'n1', 'daughters': []}]}

Thanks for any enlightenment,

Dan


~> python --version
Python 2.5.2
~> uname -a
Linux Tichodroma 2.6.27-11-generic #1 SMP Thu Jan 29 19:24:39 UTC 2009 i686 GNU/Linux
Ubuntu 8.10



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