function arguments passed by reference

[Marcello Pietrobon]

John Roth wrote:

>"Marcello Pietrobon" <teiffel at attglobal.net> wrote in message
>news:mailman.8.1078344338.736.python-list at python.org...
>  
>
>>Hello,
>>
>>My background is C++
>>
>>I would like to know for sure if it is possible to pass a value by
>>reference to a function AND having it changed by the function itself
>>
>>
>>def changeValue( text ):
>>    text = 'c'
>>    print text
>>    return
>>
>> >>> txt = 'a'
>> >>>changeValue( txt )
>>'c'
>> >>>print txt
>>'a'
>>
>>Question 1)
>>I guess that in Python ( differently than in C++)  somehow txt is passed
>>by reference even if its value is not changed.
>>Is this true ?
>>
>>Question 2)
>>How can I define a function that changes the value of txt ( without
>>having to return the changed value ) ?
>>
>>Question 3)
>>Is there some documentation talking extensively about this ?
>>    
>>
>
>The key concept is that everything is an object, and all objects
>are passed by reference in the sense that what's bound to the
>funtion's parameters is the objects that were passed in.
>
>However, you cannot affect the bindings in the calling environment.
>Regardless of what you do (unless you do some deep magic with
>the invocation stack) nothing is going to affect the bindings in the
>caller.
>
>So you can rebind anything you want to the parameters within
>the function, and it will have no effect on the calling environment.
>
>On the other hand, if you mutate a mutable object that was passed
>as a parameter, that change will be visible after you return.
>
>For  example:
>
>def foo(bar):
>    bar = "shazoom"
>
>a = "shazam"
>foo(a)
>
>The result is that a is still "shazam."
>
>However, if you do this:
>
>def foo(bar):
>    bar.append("shazoom")
>
>a = ["shazam"]
>foo(a)
>
>the result is:
>
>["shazam", "shazoom"]
>
>HTH
>John Roth
>
>"shazoom" ::= ["strength", "health", "aptitude", "zeal", "ox, power of",
>"ox, power of another", "money"]
>
>whatever the value you passed in remains.
>  
>

Thank you for the answers,

difficult to get used to these concepts after coming from C++ !
But I am starting to understand:
the main difference between Python and C++ is that I need to keep in 
mind what is mutable and what is not mutable in Python

Cheers,
Marcello