An interesting python problem

[Johnny Lee]

Hi,
Look at the follow command in python command line, See what's
interesting?:)
[color=blue][color=green][color=darkred]
>>> class A:[/color][/color][/color]
i = 0[color=blue][color=green][color=darkred]
>>> a = A()
>>> b = A()
>>> a.i = 1
>>> print a.i, b.i[/color][/color][/color]
1 0

---------------------------------------
[color=blue][color=green][color=darkred]
>>> class A:[/color][/color][/color]
arr = [][color=blue][color=green][color=darkred]
>>> a = A()
>>> b = A()
>>> a[/color][/color][/color]
<__main__.A instance at 0x00C96698>[color=blue][color=green][color=darkred]
>>> b[/color][/color][/color]
<__main__.A instance at 0x00CA0760>[color=blue][color=green][color=darkred]
>>> A[/color][/color][/color]
<class __main__.A at 0x00B314B0>[color=blue][color=green][color=darkred]
>>> a.arr.append("h aha")
>>> print a.arr , b.arr[/color][/color][/color]
['haha'] ['haha'][color=blue][color=green][color=darkred]
>>> a.arr = ["xixi"]
>>> print a.arr , b.arr[/color][/color][/color]
['xixi'] ['haha'][color=blue][color=green][color=darkred]
>>> A.arr[/color][/color][/color]
['haha'][color=blue][color=green][color=darkred]
>>> A.arr.append("x x")
>>> A.arr[/color][/color][/color]
['haha', 'xx'][color=blue][color=green][color=darkred]
>>> a.arr[/color][/color][/color]
['xixi'][color=blue][color=green][color=darkred]
>>> b.arr[/color][/color][/color]
['haha', 'xx'][color=blue][color=green][color=darkred]
>>> b.arr.pop()[/color][/color][/color]
'xx'[color=blue][color=green][color=darkred]
>>> b.arr[/color][/color][/color]
['haha'][color=blue][color=green][color=darkred]
>>> A.arr[/color][/color][/color]
['haha']

-------------------------------------
[color=blue][color=green][color=darkred]
>>> class X:[/color][/color][/color]
def __init__(self):
self.arr = [][color=blue][color=green][color=darkred]
>>> m = X()
>>> n = X()
>>> m.arr.append("h aha")
>>> print m.arr, n.arr[/color][/color][/color]
['haha'] []

[bruno modulix]

Re: An interesting python problem

Johnny Lee wrote:[color=blue]
> Hi,
> Look at the follow command in python command line, See what's
> interesting?:)
>
>[color=green][color=darkred]
>>>>class A:[/color][/color]
>
> i = 0
>[color=green][color=darkred]
>>>>a = A()
>>>>b = A()
>>>>a.i = 1
>>>>print a.i, b.i[/color][/color]
>
> 1 0[/color]

Quite what I would expect. First you declare i as being a *class*
attribute of A, with value 0. Then you create 2 instances a and b of A.
Then you add to a an *instance* variable named i (that then shadows the
class variable of the same name), with value 1. Then you print a.i,
wihci is the instance variable i of a, and b.i, which is the class
variable i of A, with value 0.
[color=blue]
> ---------------------------------------
>
>[color=green][color=darkred]
>>>>class A:[/color][/color]
>
> arr = []
>[color=green][color=darkred]
>>>>a = A()
>>>>b = A()
>>>>a[/color][/color]
>
> <__main__.A instance at 0x00C96698>
>[color=green][color=darkred]
>>>>b[/color][/color]
>
> <__main__.A instance at 0x00CA0760>
>[color=green][color=darkred]
>>>>A[/color][/color]
>
> <class __main__.A at 0x00B314B0>
>[color=green][color=darkred]
>>>>a.arr.appen d("haha")
>>>>print a.arr , b.arr[/color][/color]
>
> ['haha'] ['haha'][/color]

Now you create a class A with a *class* variable arr which is an empty
list, and 2 instances a and b of A. Then you append to a.arr - which is
A.arr, so when you print a.arr and b.arr, you in fact print A.arr
[color=blue][color=green][color=darkred]
>>>>a.arr = ["xixi"][/color][/color][/color]

Then you add an instance variable arr to a, shadowing A.arr
[color=blue][color=green][color=darkred]
>>>>print a.arr , b.arr[/color][/color]
>
> ['xixi'] ['haha'][/color]

So now you print a.arr and A.arr (accessed thru b)

(snip)

[color=blue]
>[color=green][color=darkred]
>>>>class X:[/color][/color]
>
> def __init__(self):
> self.arr = []
>[color=green][color=darkred]
>>>>m = X()
>>>>n = X()
>>>>m.arr.appen d("haha")
>>>>print m.arr, n.arr[/color][/color]
>
> ['haha'] []
>[/color]

Here you define a class X with an *instance* variable arr, and two
instances m and n of X, then append to m.arr, which of course has no
impact on n.

I dont see anything interesting nor problematic here. If you understand
the difference between class attributes and instance attributes, the
difference between mutating an object and rebinding a name, and the
attribute lookup rules in Python, you'll find that all this is the
normal and expected behavior.

Or did I miss something ?

--
bruno desthuilliers - is Python much more readable than Perl ???
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Johnny Lee]

Re: An interesting python problem


bruno modulix wrote:[color=blue]
>
> I dont see anything interesting nor problematic here. If you understand
> the difference between class attributes and instance attributes, the
> difference between mutating an object and rebinding a name, and the
> attribute lookup rules in Python, you'll find that all this is the
> normal and expected behavior.
>
> Or did I miss something ?
>[/color]

No, you didn't miss anything as I can see. Thanks for your help:)

[bruno modulix]

Re: An interesting python problem

Johnny Lee wrote:[color=blue]
> bruno modulix wrote:
>[color=green]
>>I dont see anything interesting nor problematic here. If you understand
>>the difference between class attributes and instance attributes, the
>>difference between mutating an object and rebinding a name, and the
>>attribute lookup rules in Python, you'll find that all this is the
>>normal and expected behavior.
>>
>>Or did I miss something ?
>>[/color]
>
>
> No, you didn't miss anything as I can see. Thanks for your help:)[/color]

You're welcome !-)

Ok, I guess all this is not that intuitive, specially when comes from
less dynamic languages. Python's object model is quite powerful, but one
need to have a good understanding of it to understand *why* it works
that way. There's an interesting slideshow about metaclasses and
descriptors that may help (if your brain is robust enough !-) :

http://www.python.org/pycon/dc2004/papers/24/metaclasses-pycon.pdf

HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"