Modifying Class Object

Diez B. Roggisch deets at nospam.web.de
Mon Feb 8 20:04:19 CET 2010


Am 08.02.10 02:51, schrieb 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.

Whao. You just needed 10 words, plus a paragraph to explain something in 
terms of a spec that's just about 600 pages strong. Amazing display of 
conciseness, and certainly the most valuable thing for some programming 
newbie to work with. Thanks!

Diez



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