Python instances

[henrikpierrou@hotmail.com]

Hi,

How do python instances work?
Why does the code at the end of my posting produce this output:

list in a:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
list in b:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

instead of

list in a:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
list in b:
[]

----------------------------

class MyClass:
list = []

def add(self, x):
self.list.appen d(x)

def printer(self):
print self.list

a = MyClass()
b = MyClass()

for n in range(10):
a.add(n)

print "list in a:"
a.printer()
print "list in b:"
b.printer()

/H

[Roland Heiber]

Re: Python instances

Hi,
[color=blue]
> class MyClass:
> list = [][/color]

you have "list" defined as a classmember, not an instancemember. So
"list" ist defined ONCE for all instances.

Try this instead:

class MyClass:
def __init__(self):
self.list = []
[...]

and use self.list ...

HtH, Roland

[henrikpierrou@hotmail.com]

Re: Python instances

Guess i shouldn't think of the __init__(self) function as a constructor
then.
Thanks.

/H

[Laszlo Zsolt Nagy]

Re: Python instances

[color=blue]
>Guess i shouldn't think of the __init__(self) function as a constructor
>then.
>
>[/color]
__init__ is THE constructor in Python


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IT Consultant mail: gandalf@geochem source.com

Python forever!

[Bengt Richter]

Re: Python instances

On 20 Apr 2005 00:44:53 -0700, henrikpierrou@h otmail.com wrote:
[color=blue]
>Guess i shouldn't think of the __init__(self) function as a constructor
>then.
>Thanks.[/color]
Depends on what you think when you think "constructo r" ;-)
Read about both __new__ and __init__. The former is always
necessary to create an object, and __init__ may take parameters to
define intial state from its parameters, but __new__ does the whole
job for immutables. I.e., "constructo r" translates to combination of
both if both are present, but __new__ must be always be there and
come first. In general there are default methods inherited from
object and/or type, the most primitive classes, so you don't have
to define them except to customize for your purposes.
At least, that's the way I think of it ;-)

Regards,
Bengt Richter

[Kent Johnson]

Re: Python instances

henrikpierrou@h otmail.com wrote:[color=blue]
> Guess i shouldn't think of the __init__(self) function as a constructor
> then.[/color]

No, that's not it. You shouldn't think of variables defined outside of a method as instance variables.

In Java for example you can write something like

public class MyClass {
private List list = new ArrayList();

public void add(Object x) {
list.add(x);
}
}

In this case list is a member variable of MyClass instances; 'this' is implicit in Java.

In Python, if you write something that looks similar, the meaning is different:

class MyClass:
list = []

def add(self, x):
self.list.appen d(x)

In this case, list is an attribute of the class. The Java equivalent is a static attribute. In
Python, instance attributes have to be explicitly specified using 'self'. So instance attributes
have to be bound in an instance method (where 'self' is available):

class MyClass:
def __init__(self):
self.list = []

def add(self, x):
self.list.appen d(x)

Kent

[EuGeNe]

Re: Python instances

henrikpierrou@h otmail.com wrote:[color=blue]
> Hi,
>
> How do python instances work?
> Why does the code at the end of my posting produce this output:
>
> list in a:
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> list in b:
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>
> instead of
>
> list in a:
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> list in b:
> []
>
> ----------------------------
>
> class MyClass:
> list = []
>
> def add(self, x):
> self.list.appen d(x)
>
> def printer(self):
> print self.list
>
> a = MyClass()
> b = MyClass()
>
> for n in range(10):
> a.add(n)
>
> print "list in a:"
> a.printer()
> print "list in b:"
> b.printer()
>
> /H
>[/color]

because list is a class member not an instance member (not sure about
the vocabulary) if in __init__ you set self.list=[] you'll get the
result you want! By declaring list=[] in the class it is shared between
all instances!

--
EuGeNe

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