Varibles -- copies and references

[Chris Rebert]

On Mon, Feb 2, 2009 at 10:02 PM, Ferdinand Sousa
<ferdinandsousa at gmail.com> wrote:
> Hi
>
> Some weeks back I had been following the thread "Why can't assign to
> function call". Today, I saw the "function scope" thread, and decided I
> should ask about the behaviour below:
>
>>>>                                                                       #
>>>> Simple variables
>>>>p=55
>>>> q=p
>>>> q
> 55
>>>> q=44
>>>> p
> 55
>>>>
>>>>                                                                       #
>>>> In a function
>>>> def dd(d):
>     del d['key']
>     return d
>
>>>> adict={'pramod':'goodboy', 'ferdi':'badboy', 'key':'to be deleted'}
>>>> dd(adict)
> {'ferdi': 'badboy', 'pramod': 'goodboy'}
>>>> adict
> {'ferdi': 'badboy', 'pramod': 'goodboy'}                          #
> undesirable?
>>>>
>>>>                                                                       #
>>>> In a class
>>>>class AA:
>     a=111
>
>
>>>> x=AA()
>>>> x.a
> 111
>>>> y=x
>>>> y.a
> 111
>>>> y.a=222
>>>> x.a
> 222                                                                       #
> undesirable?
>>>> z=copy.deepcopy(x)
>>>> z.a
> 222
>>>> z.a=444
>>>> x.a
> 222
>>>> y.a
> 222
>>>>
>
> Guess the simple types show the expected behaviour (even though they are
> technically instances of existing classes). The user defined classes seem to
> be references/shallow copies. The "function scope" thread refers to
> http://www.python.org/doc/faq/general/#why-are-default-values-shared-between-objects
> to explain why default parameters should not be modified. But, in the
> function example above, the dictionary passed as a variable *in the calling
> scope* gets modified.
>
> How can you avoid the dictionary being changed?
> Assigning one object to another always creates references? Can you modify
> anything in the class def to get a copy? [I have a tingling feeling this is
> how the default types (integers, strings etc) are defined]

Stephen gave a great summary of the issue; here's another popular
write-up about it: http://effbot.org/zone/call-by-object.htm

Cheers,
Chris

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