An interesting python problem

[Johnny Lee]

Hi,
Look at the follow command in python command line, See what's
interesting?:)

class A: i = 0 a = A()
b = A()
a.i = 1
print a.i, b.i 1 0

---------------------------------------
class A: arr = [] a = A()
b = A()
a <__main__.A instance at 0x00C96698> b <__main__.A instance at 0x00CA0760> A <class __main__.A at 0x00B314B0> a.arr.append("haha")
print a.arr , b.arr ['haha'] ['haha'] a.arr = ["xixi"]
print a.arr , b.arr ['xixi'] ['haha'] A.arr ['haha'] A.arr.append("xx")
A.arr ['haha', 'xx'] a.arr ['xixi'] b.arr ['haha', 'xx'] b.arr.pop() 'xx' b.arr ['haha'] A.arr ['haha']

-------------------------------------
class X: def __init__(self):
self.arr = [] m = X()
n = X()
m.arr.append("haha")
print m.arr, n.arr

['haha'] []

[bruno modulix]

Johnny Lee wrote:

Hi,
Look at the follow command in python command line, See what's
interesting?:)

class A:
i = 0
a = A()
b = A()
a.i = 1
print a.i, b.i
1 0
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.
---------------------------------------

class A:
arr = []
a = A()
b = A()
a
<__main__.A instance at 0x00C96698>
b
<__main__.A instance at 0x00CA0760>
A
<class __main__.A at 0x00B314B0>
a.arr.append("haha")
print a.arr , b.arr
['haha'] ['haha']
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

a.arr = ["xixi"]
Then you add an instance variable arr to a, shadowing A.arr
print a.arr , b.arr
['xixi'] ['haha']

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

(snip)

class X:
def __init__(self):
self.arr = []
m = X()
n = X()
m.arr.append("haha")
print m.arr, n.arr

['haha'] []

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 'o****@xiludom.gro'.split('@')])"

[Johnny Lee]

bruno modulix wrote:

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 ?

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

[bruno modulix]

Johnny Lee wrote:

bruno modulix wrote:

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 ?

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

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/p...sses-pycon.pdf

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