50 Reusability

Let’s look at an example. Here’s a file I created called Penchord_Wizardry.py.

import random

# Class defining a Penchordian
class Penchordian :
    def __init__(self, name):
        self.name = name
        self.is_a_wizard = False

    def write_model(self, type_of_model):
        print (f"{self.name} is now writing a {type_of_model} model.")

    def tell_joke(self, prob_success):
        if random.uniform(0,1) < prob_success:
            print (f"{self.name} attempted a joke.  People loved it!")
        else:
            print (f"{self.name} attempted a joke.  It fell flat.")

# Function to turn someone into a wizard
# Input subject must be an object with a "name" string attribute and a
# "is_a_wizard" boolean attribute
def turn_into_a_wizard(subject):
    subject.is_a_wizard = True
    print (f"{subject.name} is now a wizard.")

Then I’m going to create a new file in the same folder as Penchord_Wizardry.py.

Tip

It is possible to call in functions from a Python file stored in a different folder or subfolder - but it starts getting quite complicated to navigate to them in the code, so we won’t cover that today.

At the beginning of my new file I’m going to import Penchord_Wizardry to get access to the functions from Penchord_Wizardry.py.

# import the entire Penchord_Wizardry module (the Penchordian class and the
# turn_into_a_wizard function)
import Penchord_Wizardry

import random

list_of_penchordian_names = ["Dan", "Sammi", "Kerry", "Mike", "Anna", "Tom",
                             "Amy C", "Amy H", "Chrissie"]

# Randomly select three PenCHORDian names
# random.sample selects three elements from a list without replacement
# (if you want values to be able to be repicked (replacement), use
# random.choices)
chosen_penchordian_names = random.sample(list_of_penchordian_names, 3)

list_of_penchordians = []

# Create some Penchordian objects using the definition from the imported
# module
for name in chosen_penchordian_names:
    list_of_penchordians.append(Penchord_Wizardry.Penchordian(name))

# Call a couple of the class methods on the three created Penchordian objects
for penchordian in list_of_penchordians:
    penchordian.write_model("Discrete Event Simulation")
    penchordian.tell_joke(0.1)

Amy H is now writing a Discrete Event Simulation model.
Amy H attempted a joke.  It fell flat.
Chrissie is now writing a Discrete Event Simulation model.
Chrissie attempted a joke.  It fell flat.
Kerry is now writing a Discrete Event Simulation model.
Kerry attempted a joke.  It fell flat.

I can also choose just to import the bit(s) of the module I need, rather than the whole thing.

Here we don’t import the class Penchordian, but we do import the function turn_into_a_wizard.

Tip

Note that because we’ve imported the specific function by name, we can just then use turn_into_a_wizard.

# just import the turn_into_a_wizard function
from Penchord_Wizardry import turn_into_a_wizard

# Define a new class called HSMA, which has two attributes - a name, and an
# is_a_wizard boolean
class HSMA:
    def __init__(self, name):
        self.name = name
        self.is_a_wizard = False

# Create a new HSMA object, whose name is Gandalf
my_promising_HSMA = HSMA("Gandalf")

# Turn Gandalf into a wizard using the function we imported from the
# Penchord_Wizardry module
turn_into_a_wizard(my_promising_HSMA)

Gandalf is now a wizard.

In comparison, if we imported the whole module instead, our code would look like this:

# just import the turn_into_a_wizard function
import Penchord_Wizardry

# Define a new class called HSMA, which has two attributes - a name, and an
# is_a_wizard boolean
class HSMA:
    def __init__(self, name):
        self.name = name
        self.is_a_wizard = False

# Create a new HSMA object, whose name is Gandalf
my_promising_HSMA = HSMA("Gandalf")

# Turn Gandalf into a wizard using the function we imported from the
# Penchord_Wizardry module
Penchord_Wizardry.turn_into_a_wizard(my_promising_HSMA)

Gandalf is now a wizard.