Modifying Class Object

[Alf P. Steinbach]

* Chris Rebert:
> On Sun, Feb 7, 2010 at 5:05 PM, T <misceverything at gmail.com> wrote:
>> Ok, just looking for a sanity check here, or maybe something I'm
>> missing.  I have a class Test, for example:
>>
>> class Test:
>>    def __init__(self, param1, param2, param3):
>>        self.param1 = param1
>>        self.param2 = param2
>>        self.param3 = param3
>>
>> Next, I have a dictionary mytest that contains instances of Test.  If
>> I want to modify one of the Test instances within my dictionary, I
>> have to rewrite the entire entry, correct (since Python passes by
>> value, not reference)?
> 
> Incorrect; Python uses neither. See
> http://effbot.org/zone/call-by-object.htm for a excellent explanation
> of what Python does use.

Hm. While most everything I've seen at effbot.org has been clear and to the 
point, that particular article reads like a ton of obfuscation.

Python passes pointers by value, just as e.g. Java does.

There, it needed just 10 words or so. :-) Or perhaps some more words to point 
out that in the Java language spec those reference values are called pointers, 
but that this terminology isn't (apparently) used for Python, and isn't even 
well known among Java programmers. But that's just one extra little para.

One just has to be clear about exactly what it is that's passed by value.

Not Python objects, but references (pointers) to them, the id(o) values.


>> I.e. if I wish to change just param3 of an
>> instance, I would have to do:
>>
>> def changevalue():
>>    for key in mytest.keys():
>>        currentparam = mytest[key]
>>        param1 = currentparam.param1
>>        param2 = currentparam.param2
>>        param3 = currentparam.param3
>>        param3 = "newvalue"
>>        mytest[key] = Test(param1, param2, param3)
>>
>> If there's an easier way to accomplish this that I'm missing, that'd
>> be great!
> 
> def changevalue():
>     for test in mytest.values():
>         test.param3 = "newvalue"

Cheers,

- Alf