Object in List : how?

[Khairil Sitanggang]

Thank you.

On Sun, Jul 24, 2022 at 2:23 AM Peter Otten <__peter__ at web.de> wrote:

> On 23/07/2022 06:28, 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
>
> This just overwrites the attribute; the previous value 10 is lost.
>
> > NODELIST.append(NODE)
> >
> > NODE.NO = 30
>
> This againoverwrites the attribute; the previous value 20 is lost.
>
> > NODELIST.append(NODE)
>
>
> You are three times appending the *same* node to the list.
> To create a new node you need to invoke the initializer:
>
> [I'm following a common convention and use lowercase names in my examples]
>
> nodelist = []
>
> # first node
> node = Node()
> node.no = 10
> nodelist.append(node)
>
> # second node
> node = Node()  # this is crucial
> node.no = 20
> nodelist.append(node)
>
> ... and so on. However, usually object creation and initialization is
> combined by allowing arguments to the initializer:
>
> class Node:
>      def __init__(self, no, a):
>          self.no = no
>          self.a = a
>
> nodelist = []
> for no in [10, 20, 30]:
>      nodelist.append(Node(no, 20))
>
> > NO1 = 20
> > if NO1 not in NODELIST[:].NO  ???
>
> You are checking if the list contains an item with a specific attribute
> value, so you cannot use the nodelist directly, you need an intermediate
> list that contains the attribute values:
>
> no1 = 20
> nos = [node.no for node in nodelist]
> if no1 not in nos:
>      print("not found")
>
> There's one disadvantage to this approach. If the node list is huge
> another huge list with the attribute values is built even though the
> first item in the nodelist may already have the searched-for attribute
> value. To avoid the overhead you could write a function:
>
> def contains_no(nodes, no):
>      for node in nodes:
>          if node.no == no:
>              return True
>      return False
>
> if not contains_no(nodelist, 20):
>      print("not found")
>
> But Python has something more elegant, a kind of /lazy/ /list/ called
> "generator expression" where each item is calculated on demand. With
> that you can write
>
> if 20 not in (node.no for node in nodelist):
>      print("not found")
>
> and your script will stop inspecting further nodes as soon as a matching
> node is found.
>