Object containing a list of objects.

[Chris Rebert]

On Sat, Aug 28, 2010 at 10:48 AM, cocolombo <cocolombo at gmail.com> wrote:
> Hello.
>
> I am putting objects (test) into a container object (tests) and the
> test object is also a container for a another list of object
> (scores).
>
> Problem is that all instances of class tests have the same value.
>
> To illustrate:
>
> class score(object):
>    val = 0
The previous line does nothing useful; delete it.

>    def __init__(self, val):
>        self.val = val
>    def __str__(self):
>        return str(self.val) + "\n"
>
> class test(object):
>    listOfScores = []
No! This makes the list a class/static variable *shared between all
instances*. Delete the previous line and define a proper initializer:

    def __init__(self):
        self.listOfScores = []

>    def __str__(self):
>        ret = ""
>        for s in self.listOfScores:
>            ret += str(s)
>        return ret
>
> class tests(object):
>    listOfTest = []
Again, same problem.

    def __init__(self):
        self.listOfTest = []

>    def __str__(self):
>        ret = ""
>        for t in self.listOfTest:
>            ret += str(t)
>        return ret

That is more efficiently+concisely written as:
    return "".join(str(t) for t in self.listOfTest)


> Now I run the script
> :
> ======================
> score1 = score(10)
> score2 = score(20)
> score3 = score(30)
> score4 = score(40)
>
> test1 = test()
> test2 = test()
>
>
> test1.listOfScores.append(score1)
> test1.listOfScores.append(score2)
> test2.listOfScores.append(score3)
> test2.listOfScores.append(score4)
>
> theTests = tests()
> theTests.listOfTest.append(test1)
> theTests.listOfTest.append(test2)
>
> print theTests.listOfTest[0]
> print theTests.listOfTest[1]
>
> ==============
>
> This is the data structure I am EXPECTING:
<snip>
> But what I get is this:
<snip>
> What is wrong ?

Python is not Java/C# and has no instance variable declarations. You
just assign to an attribute of self in __init__ and *that* is what
creates instance variables.

Any variables you assign to directly in the class body (as you were
doing with listOfScores and listOfTest) are made class variables (Java
lingo: "static variables"), and are /shared between all instances/,
which is rarely what one actually wants.
To get regular instance variables, define a proper __init__() and
assign the variables to self therein.

Also, per PEP 8 (http://www.python.org/dev/peps/pep-0008/ ):
- Classes are conventionally CapitalizedWords, so name your classes
Score, Test, and Tests rather than score, test, and tests.
- Variables/methods are conventionally underscored_between_words, so
list_of_test rather than listOfTest.

Cheers,
Chris
--
http://blog.rebertia.com