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How to store the reference of a dictionary element ?

Terry Reedy tjreedy at udel.edu
Thu Dec 19 17:38:34 EST 2002 
"Alfredo P. Ricafort" <alpot at mylinuxsite.com> wrote in message
news:mailman.1040293705.17998.python-list at python.org...

> On Thu, 2002-12-19 at 15:46, Terry Reedy wrote:
> > inputData['childKey'] = [attribute1,
attribute2,inputData['parentKey']]

> This won't do.  If the value list of inputData['parentKey'] is
changed
> the inputData['childKey'] will not see the changes.

Any newbie who wants to write correct code should ignore this.  Two
references to the same object are two references to the *same object*.
If the object is mutable and is changed, then the change is visible no
matter how you access it.  This is a common 'gotcha' for newbies who
mistakenly think that assignment creates a copy, which it does not.
To illustrate:

>>> d={}
>>> d['p']=[1,2]
>>> d['c']=[3,4,d['p']] # equivalent to what I wrote and quoted above
>>> id(d['p']), id(d['c'][2])
(7953824, 7953824)
>>> d['p'].append(5) # a change to the parent value list
>>> d['c'][2] # access via the child *does* see the change
[1, 2, 5]

Terry J. Reedy
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