Object in List : how?

[Khairil Sitanggang]

dn,

Thanks for the good analogy and explanation.  I need some time to digest it.

Regards,
-Irfan


On Sat, Jul 23, 2022 at 6:55 PM dn <PythonList at danceswithmice.info> wrote:

> On 24/07/2022 09.57, MRAB wrote:
> > On 23/07/2022 05:28, Khairil Sitanggang wrote:
> >> Hello Expert:
> >>
> >> I just started using python. Below is a simple code.  I was trying to
> >> check
> >> if, say, NO1 is not in the NODELIST[:].NO
> >> How can I achieve this purpose?
> >>
> >> Regards,
> >> -Irfan
> >>
> >>
> >> class Node:
> >>      def __init__(self):
> >>          self.NO = 0
> >>          self.A = 20
> >>
> >> NODE = Node()
> >> NODELIST = []
> >>
> >> NODE.NO = 10
> >> NODELIST.append(NODE)
> >>
> >> NODE.NO = 20
> >> NODELIST.append(NODE)
> >>
> >> NODE.NO = 30
> >> NODELIST.append(NODE)
> >>
> >>
> >> NO1 = 20
> >> if NO1 not in NODELIST[:].NO  ???
> >
> > No, you can't do it that way. You have to iterate through the list and
> > check each member individually:
> >
> >     if any(NO1 == N.NO for N in NODELIST):
> >
> > And another thing: you've created only 1 node, and you're changing it
> > each time before adding it to the list, so the list ends up with 3
> > references to the _same_ object.
>
> +1
>
>
> Imagine the object (Node) was instead a person, and a bunch of us-people
> get together in a room. You could shout (above the polite conversation)
> "is Fred here?" and Fred will reply - or if Fred is elsewhere, silence
> will reign. That works - but only because everyone knows their own name.
>
> Now imagine something like (my) grade school, where the teacher takes a
> roll/register to note who is present for (or absent from) class. In this
> case another staff-member could enter the room and instead of shouting
> (which would be rude), can ask the teacher "is Fred here?". The teacher
> is able to answer from the roll.
>
> The former case is (sort of) the solution proposed above - in looking
> for Fred, you walk through the room, asking each person in-turn "are you
> Fred?".
>
> The latter is the case for Node and what you were hoping to implement.
>
> Thus, an alternate approach is to keep a register of nodes. Note that
> this is more than a list, because each element of the list (each node)
> is also identified (on the list) by its name. So, two pieces of
> (related) data: the id of the node, and the node itself - the name of
> the person and the person him-/her-self.
>
> Assuming you only have one ID that will be used to access a node,
> Python's built-in dict[ionary] data-structure will provide the advantage
> of direct-access (instead of going through (on average) half the nodes,
> asking each one in-turn, are you ...
>
> So:
>
> > NODE = Node()
> > NODELIST = []
> >
> > NODE.NO = 10
> > NODELIST.append(NODE)
>
> becomes:
>
>     graph = dict{}      # could be written as: graph = {}
>     node10 = Node( 10 )
>     graph[ node.id ] = node10
>
> or even:
>
>     graph[ 20 ] = Node( 20 )
>
> if you don't need lots of nodes 'hanging around' outside of the 'list'
> (actually a dict) representing the graph.
>
>
> Then:
>
> > NO1 = 20
> > if NO1 not in NODELIST[:].NO  ???
>
> becomes a single line:
>
>     the_node_required = graph[ 20 ]
>
> where "20" is the search-criteria.
>
> Does that make sense?
>
>
> If it does, and when you realise that you'd like to do more with the
> graph than retrieve single nodes (and more-importantly, should you want
> (or dare) to delve further into the depths of Object-Oriented
> Programming) you could declare a second class (graph) containing a
> number of "methods". If one of the methods implemented is __contains__()
> then you could indeed ask:
>
>     if 20 in graph:
>
> ie "is Fred in-class today?"
> but just as we would ask "is Fred here?" rather than "is someone
> identified by the name 'Fred' here?", it would seem better form to write:
>
>     if node( 20 ) in graph:
>
> ie is there a node with the id of 20, somewhere within the graph?
>
>
> Such might involve sub-classing OrderedDict, UserDict, or even UserList,
> from the Collections library in the (provided/"batteries-included")
> Python Standard Library:
> https://docs.python.org/3/library/collections.html
>
> If needed, the extra methods you choose to implement might include such
> functionality as connecting nodes 10 and 20 with a path/edge, being able
> to traverse edges, and so-on...
>
> Thus, we've covered two categories of class/object: one, a 'data-class'
> which contains data about a single node; the other a 'collection-class'
> which contains multiple nodes making-up the graph. A useful distinction
> and a common related-pair of data-constructs!
> --
> Regards,
> =dn
> --
> https://mail.python.org/mailman/listinfo/python-list
>