Understanding how Python Objects and Classes Work

Question

I am trying to follow instructions from Effective Computation in Physics, a Field Guide to Research with Python book (by Anthony Scopatz and Kathryn Duff. In the OOP chapter(specifically page 136 in my version), I tried copying the code and ran the code below:

# import the Particle class from the particle module
from particle import Particle as p
# create an empty list to hold observed particle data
obs = []
# append the first particle
obs.append(p.Particle())
# assign its position
obs[0].r = {'x': 100.0, 'y': 38.0, 'z': -42.0}
# append the second particle
obs.append(p.Particle())
# assign the position of the second particle
obs[1].r = {'x': 0.01, 'y': 99.0, 'z': 32.0}
# print the positions of each particle
print(obs[0].r)
print(obs[1].r)

The result is supposed to give the position typed in. However, the code did not work like this. Instead, I played around with the code and this code worked instead:

# Import the Particle class from the particle Module
from particle import Particle as p

# Create an empty list
obs = []

# Append first element
obs.append(p)

# Assign its position
obs[0].r = {'x': 100.0, 'y': 38.0, 'z': -42.0}

# Append second particle
obs.append(particle())

# Assign second position
obs[1].r = {'x': 0.01, 'y': 99.0, 'z': 32.0}

print(obs[0].r)
print(obs[1].r)

I would love to understand why and what is going on. I am currently reviewing how OOP works and am using Python for now. Please respond! I want to learn how and why does this work!

I am almost certain you have this backwards. The first block of code might work, pretty sure the second one doesn't. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 31, 2021 at 7:30

Kal-1 · Accepted Answer · 2021-03-31 07:44:23Z

I'm not sure I understood everything but if you want to turn your code in POO you can try something like this:

from particle import Particle as p

class Myclass:
    def __init__(self):
        # Create an empty list
        self.obs = []
        # Append first element
        self.obs.append(p)

        # Assign its position
        self.obs[0].r = {'x': 100.0, 'y': 38.0, 'z': -42.0}

        # Append second particle
        self.obs.append(particle())

        # Assign second position
        self.obs[1].r = {'x': 0.01, 'y': 99.0, 'z': 32.0}
    
        #if you want to call getobs from here use :
        #self.getobs1()

    def getobs0(self):
        print(self.obs[0].r)

    def getobs1(self):
        print(self.obs[1].r)
    
if __name__ == '__main__':
    myClass = Myclass() #create an object of Myclass and call __init__()
    myClass.getobs0()
    myClass.getobs1()

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