Print an array of object attributes from an array of objects

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I have a Node class with some attributes

I then have an array of Node objects

nodes = [
    Node(some attributes),
    Node(some attributes),
    Node(some attributes),
]

I want to do something like this

for i, node in enumerate(nodes):
    arr[i] = node.attribute

print(arr)

by typing something like

print(nodes.attribute)

or 

print([nodes].attribute)

or 

print(nodes[*].attribute)

etc.

and then have it return something like

print(nodes)

but with the specific attribute instead of returning the objects

I'm somewhat new to python, and it seems like this should be easier than iterating through the array.

is there?

object class arrays python
1 Answers

It's not that easy because in python the square brackets define a list, not an array. A list doesn't force you to have the same type of element (in your case Node) throughout the list.

Some options you have:

Iterate through the list

The same thing you did in the question.

attributes = []
for node in nodes:
     attributes.append(node.attr)

List comprehension

A more pythonic syntax for the previous.

attributes = [node.attr for node in nodes]

Map a function on this list

This requires you to define a function that receives a node and returns an attribute of that node.

def get_attr(node)
    return node.attr

# or alternatively:
get_attr = lambda node: node.attr

attributes = map(getattr, nodes)

Vectorize this function and pass an array as an argument

This is probably the closest to what you want to do. It requires two things: to vectorize the previous function and to convert nodes to an array.

import numpy as np
get_attr_vec = np.vectorize(get_attr)
nodes = np.array(nodes)

attributes = get_attr_vec(nodes)

To reproduce this example you need to define the list of nodes first:

class Node:
    def __init__(self, a):
        self.attr = a

n1 = Node(1)
n2 = Node(2)
n3 = Node(3)

nodes = [n1, n2, n3]

Also you can use the built-in function getattr instead of the dot syntax.

# These two are the same thing:
a = node.attr
a = getattr(node, 'attr')
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