Class properties and object properties

[SuperZE]

Learning python I was rewriting some of my old programs to see the
pros and cons of python when a steped in some weird (at least for me)
behavior.

Here it is simplified

The code:

>>class Test1:

myList = [4 for n in range(4)]
myInt = 4

>>a = Test1()
>>b = Test1()
>>a.myList

[4, 4, 4, 4]

>>a.myInt

4

>>b.myList

[4, 4, 4, 4]

>>b.myInt

4

>>b.myList[2] = 3
>>b.myInt = 3
>>b.myList

[4, 4, 3, 4]

>>b.myInt

3

>>a.myList

[4, 4, 3, 4]

>>a.myInt

4


I would not expect the second a.myList to have changed as it did
since, for me, I have only changed b.myList. And also, why only the
list changed and not the integer?

One thing i tried was:

>>class Test2:

myList = []
myInt = 4
def __init__(self):
self.myList = [4 for n in range(4)]

>>a = Test2()
>>b = Test2()
>>a.myList

[4, 4, 4, 4]

>>b.myList

[4, 4, 4, 4]

>>b.myList[2] = 3
>>b.myList

[4, 4, 3, 4]

>>a.myList

[4, 4, 4, 4]


And as you see it worked as I expected.

Now the question, why?

[Diez B. Roggisch]

Re: Class properties and object properties

SuperZE wrote:

Learning python I was rewriting some of my old programs to see the
pros and cons of python when a steped in some weird (at least for me)
behavior.
>
Here it is simplified
>
The code:
>

>>>class Test1:

myList = [4 for n in range(4)]
myInt = 4

>>>a = Test1()
>>>b = Test1()
>>>a.myList

[4, 4, 4, 4]

>>>a.myInt

4

>>>b.myList

[4, 4, 4, 4]

>>>b.myInt

4

>>>b.myList[2] = 3
>>>b.myInt = 3
>>>b.myList

[4, 4, 3, 4]

>>>b.myInt

3

>>>a.myList

[4, 4, 3, 4]

>>>a.myInt

4
>
>
I would not expect the second a.myList to have changed as it did
since, for me, I have only changed b.myList. And also, why only the
list changed and not the integer?
>
One thing i tried was:
>

>>>class Test2:

myList = []
myInt = 4
def __init__(self):
self.myList = [4 for n in range(4)]
>

>>>a = Test2()
>>>b = Test2()
>>>a.myList

[4, 4, 4, 4]

>>>b.myList

[4, 4, 4, 4]

>>>b.myList[2] = 3
>>>b.myList

[4, 4, 3, 4]

>>>a.myList

[4, 4, 4, 4]
>
>
And as you see it worked as I expected.
>
Now the question, why?

Because you declare myList to be a *class*-level variable, which means *all*
instances of that class (a and b in your case) *share* it. Python does not
declare *instance* variables the way you do.

Instead, do this:

class Foo(object):
def __init__(self):
self.myList = []

And to avoid a common pitfall you are likely to hit next: default arguments
in functions (and methods) are evaluated only *once*, when defined - not
later!

So

def foo(some_arg=[]):
some_arg.append ("a")
print some_arg

foo()
foo()

will result in

['a']
['a', 'a']

being printed out.

Diez

[SuperZE]

Re: Class properties and object properties

Because you declare myList to be a *class*-level variable, which means *all*

instances of that class (a and b in your case) *share* it. Python does not
declare *instance* variables the way you do.
>
Instead, do this:
>
class Foo(object):
    def __init__(self):
        self.myList = []

Interesting, but that does not explain the difference in the behavior
of myList and myInt

Both were class-level variables, as far as I can see, and therefor a
and b should also share it


And good remind on default arguments :)

[Jerry Hill]

Re: Class properties and object properties

On Mon, Oct 6, 2008 at 9:38 AM, SuperZE <allansuperze@g mail.comwrote:

Interesting, but that does not explain the difference in the behavior
of myList and myInt
>
Both were class-level variables, as far as I can see, and therefor a
and b should also share it

They did share it, until you assigned an instance variable in b, which
shadowed the class variable. Example:

>>class Test1:

myInt = 4

>>a = Test1()
>>b = Test1()
>>a.myInt

4

>>b.myInt

4

>>Test1.myInt

4

>>b.myInt = 3
>>a.myInt

4

>>b.myInt

3

>>Test1.myInt

4

As soon as you bound the name b.myInt to a new value, it created an
instance variable. That hides the value of Test1.myInt.

--
Jerry

[SuperZE]

Re: Class properties and object properties

TYVM Diez and Jerry

Now I understand how this works