How to implement pointers in Python? (or any similar solution else)

Question

I have a Class, it will be implemented to be many instances. I want to keep connections between some of the instances, and pass messages among them. In C++, I can do this like:

class A
{
   A (*connections)[];
   int sum;

public:
   void pass(int index)
   {
       connections[index] -> receive(sum);
   }
   void receive(int input)
   {
       sum += input;
   }
}

Then I only need to add pointers of other instances to connections[], I can pass messages among them with each other.

Currently I have to use Python doing this, but Python doesn't support pointers. I wonder what is the proper solution, or design pattern, to solve this problem?

Thank you in advance.

I can't quite follow your C++ code, could you provide a Python usage example? — Rik Poggi
– Rik Poggi, Commented Mar 22, 2012 at 11:48
@RikPoggi , I don't know how to implement this in Python, that's why I ask here... — timothy
– timothy, Commented Mar 22, 2012 at 11:50
I wasn't looking for the class A implementation, but for the API of its instances. — Rik Poggi
– Rik Poggi, Commented Mar 22, 2012 at 11:54
connections[index] -> receive(sum); is similar to connections[index].receive(sum) in Python, but connections[index] should be a pointer (or say a reference, not a value). — timothy
– timothy, Commented Mar 22, 2012 at 11:57

obmarg · Accepted Answer · 2012-03-22 12:20:38Z

7

Python doesn't need pointers in order to achieve this as every variable is a reference to an object. These references are slightly different from C++ references, in that they can be assigned to - much like pointers in C++.

So to achieve what you're looking for, you'd just need to do something like this:

class A(object):
    def __init__( self, connections, sum ):
        self.connections = connections
        self.sum = sum

    def passToConnections( self, index ):
        self.connections[ index ].receive( self.sum )

    def receive( self, input ):
       self.sum += input

And just to prove that this works as expected:

>>> a1 = A( [], 0 )
>>> a2 = A( [], 0 )
>>> a3 = A( [ a1, a2 ], 10 )
>>> a3.passToConnections( 0 )
>>> a3.passToConnections( 1 )
>>> a3.passToConnections( 1 )
>>> print a1.sum
10
>>> print a2.sum
20

So as you can see we have altered the original objects a1 and a2 by calling through to them via the references in a3

edited Mar 22, 2012 at 12:20

answered Mar 22, 2012 at 12:00

obmarg

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1 Comment

quamrana Over a year ago

@obmag: The pass method should pass self.sum into the call to receive. but +1 anyway.

Janne Karila · Accepted Answer · 2012-03-22 12:45:23Z

3

In Python we have names and containers (such as list) that refer to objects, similar to references in C++. Example -- a and b refer to the same object:

>>> a = object()
>>> b = a
>>> a is b
True

edited Mar 22, 2012 at 12:45

answered Mar 22, 2012 at 11:55

Janne Karila

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5 Comments

Rik Poggi Over a year ago

In which way is your answer answering the OP question?

rplnt Over a year ago

That any variable is just a pointer. So when you save "object" somewhere it is just saving its pointer.

Rik Poggi Over a year ago

@rplnt: And now that OP knows this (which he also seemed to already know) how would his problem be solved? IMO this is a very poor answer, without even a link to the doc or some in-depth explanation.

rplnt Over a year ago

Explanation is often better than complete solution I guess. And connections[index] is not a value, it's a reference. When you change it, referenced object changes. Except the case when it is immutable (such as Number or str).

Rik Poggi Over a year ago

@rplnt: Nothing that you just said is in this answer, you're making my point that it's a poor answer.

Andreas Florath · Accepted Answer · 2012-03-22 12:03:16Z

Python standard way of handling things supports you. In python every variable is a reference.

class A:

    def __init__(self, name, lsum):
        self.__name = name
        self.__sum = lsum
        self.__connections = []

    def add_connection(self, con):
        self.__connections.append(con)

    def send_signal(self, cidx):
        print("Send signal from [%s] to [%s]" % 
              (self.__name, self.__connections[cidx].__name))
        self.__connections[cidx].receive(self.__sum)

    def receive(self, lsum):
        print("Add [%s] to [%s] in [%s]" % (lsum, self.__sum, self.__name))
        self.__sum += lsum

    def get_sum(self):
        return self.__sum

a = A("Obj1", 10)
b = A("Obj2", 20)
c = A("Obj3", 30)

a.add_connection(b)
a.add_connection(c)

a.send_signal(0)
a.send_signal(1)

print("Sum A [%s]" % a.get_sum())
print("Sum B [%s]" % b.get_sum())
print("Sum C [%s]" % c.get_sum())

Output:

Send signal from [Obj1] to [Obj2]
Add [10] to [20] in [Obj2]
Send signal from [Obj1] to [Obj3]
Add [10] to [30] in [Obj3]
Sum A [10]
Sum B [30]
Sum C [40]

Rik Poggi · Accepted Answer · 2012-03-22 12:24:55Z

1

I'd say that you might be looking for a class attribute.

Would something like this work for you?

class A(object):
    connection = []
    def send(self, num):
        self.connection.append(num)
    def receive(self):
        return self.connection.pop(-1)

Example:

>>> x = A()
>>> y = A()
>>> x.send(10)
>>> y.receive()
10

Anyway you might want to implement this with the queue module.

Edit: I'd guess that you want the receive method to look like:

def receive(self):
    s = sum(self.connection)
    self.connection[:] = []    # empty the list
    return s

Beware that you should never assign anything to the connection attribute or you'll loose the reference.

edited Mar 22, 2012 at 12:24

answered Mar 22, 2012 at 11:58

Rik Poggi

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