How the getattr() method works on this HackerRank case?

Question

I recently began to learn programming. During a HackerRank problem solving session, this one came up:

You are given a set A and N number of other sets. These N number of sets have to perform some specific mutation operations on set A. Your task is to execute those operations and print the sum of elements from set A.

Here is a sample input (always in str format):

16 # number of elements on set A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 24 52 # set A

4 # number of operations to realize on set A

intersection_update 10 # first input with operation and number of elements on the incoming set

2 3 5 6 8 9 1 4 7 11 # first set to realize operation on set A

update 2 # second input with operation and number of elements on the incoming set

55 66 # second set

The number of inputs is as big as N.

Here is my inefficient solution (1):

num_elem_a = int(input())
set_a = set(map(int, input().split()))
num_oper = int(input())
for i in range(0, num_oper):
    entry = input().split()
    if entry[0] == 'intersection_update':
        set_a.intersection_update(set(map(int, input().split())))
    elif entry[0] == 'update':
        set_a.update(set(map(int, input().split())))
    elif entry[0] == 'difference_update':
        set_a.difference_update(set(map(int, input().split())))
    elif entry[0] == 'symmetric_difference_update':
        set_a.symmetric_difference_update(set(map(int, input().split())))
        
        
print(sum(set_a))

I am trying to learn how to code without all these "if's", and found a shorter and apparently cleverly solution (2):

(_, A) = (int(input()),set(map(int, input().split())))
B = int(input())
for _ in range(B):
    (command, newSet) = (input().split()[0],set(map(int, input().split())))
    getattr(A, command)(newSet)

print (sum(A))

Here is my doubt:

In my code (1) I had to call a method this way:

set_a.symmetric_difference_update(set(map(int, input().split())))

How is the getattr(A, command)(newSet) line doing the same job in (2)? I don't get the logic. I understood which elements the command is calling, but I don't see how this command produces the same effect as my version in (1).

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. — Community, Sep 18 '21 at 15:04

score 0 · Answer 1 · answered Sep 12 '21 at 15:31

This question seems to be a duplicate, please refer to What is getattr() exactly and how do I use it?

Regarding your need to remove the "if's", please consider that, when working with other people, readibility is more important than unecessary if statements. For example, I could easily understand how the code sample (1), whilst code sample (2) is harder to read, besides that, you are opening doors to unwated code injections.

By this I mean that you are supposed to only apply specific permutation operations, which means that not all operations should be allowed. For example, you would not want the user to be able to call the set's clear method, which the code in solution (2) allows.

Of course this isn't a problem when working through code samples in HackerRank, just some food for thought to motivate your programming journey.

Thank you for your feed back Rui! As a learner this hints on how to think when programming are very helpful. — dgr, Sep 13 '21 at 22:05

How the getattr() method works on this HackerRank case?

1 Answers1