Python - When to create a Class and when to create a Function

Question

Right, so I'm trying to create a Contacts application using Python OOP. I'm fairly new to OOP and still trying to get my head around the concepts.

I understand that a Class is a blueprint for all objects. I like to think of a Class as an entity and each Object is a record of that entity. I am from a Database background so that's why I interpret it like this, feel free to correct me.

Anyways, in the Contacts app I'm making I've created the Class Contacts as outlined below:

 class Contacts():
    def __init__(self, firstName, lastName, address, groupType,
                 telephone, mobile, email, photoField):
        self.firstName = firstName
        self.lastName = lastName
        self.address = address
        self.groupType = groupType
        self.telephone = telephone
        self.mobile = mobile
        self.email = email
        self.photoField = photoField

    def showDetails(self):
        print("First Name:\t", self.firstName)
        print("Last Name:\t", self.lastName)
        print("Address:\t", self.address)
        print("Telephone:\t", self.telephone)
        print("Mobile:\t", self.mobile)
        print("Email:\t", self.email)

Now, if I want to add contacts through the Contacts class into the list I'm using to store each contact object, do I have to create an AddContacts class, or do I create a function instead? I don't know if I'm putting my question across well enough for you to understand what I mean?

What I guess I'm trying to say is that adding contacts is a process and if you look at it from a database point of view you wouldn't create a table called "tbl_AddContacts" since you would do that via a query or a stored procedure, so in my view I would define adding contacts being a function. But asking my colleague who does C# programming he says that adding contacts should be defined by a Class.

This is a confusing concept for me, especially since I don't understand how I would link the AddContacts class with the Contacts class, or even how to define an AddContacts class in the first place!.

Here is the function I defined for adding contacts:

def addContacts():
    firstName = input("First Name: ")
    lastName = input("Last Name: ")
    address = input("Address: ")
    telephone = input("Telephone: ")
    mobile = input("Mobile: ")
    email = input("Email: ")
    print("\n")

    contact = Contacts(firstName, lastName, address, None, telephone, mobile, email, None)

    contactsList.append(contact)

    pickle.dump(contactsList, open("save.p", "wb"))

Please help me out, since I will turning this into a GUI application (uni assignment).

Answer 1

Adding a contact is doing something, rather than being something, so it would make sense as a method/function rather than a class. I would suggest that your functionality should actually be in two separate places.

Creating a new contact from user input should be a class method of Contact:

class Contact(object):

    ...

    @classmethod
    def from_input(cls):
        firstName = input("First Name: ")
        lastName = input("Last Name: ")
        address = input("Address: ")
        telephone = input("Telephone: ")
        mobile = input("Mobile: ")
        email = input("Email: ")
        return cls(firstName, lastName, address, None, 
                   telephone, mobile, email, None)

Adding a new contact to the list of contacts should either be:

1. An instance method of the e.g. AddressBook or ContactList class (or whatever you have holding the contact list); or
2. A separate function if you don't have a class to hold the Contact instances.

For example:

class AddressBook(object):

    ...

    def add_contact(self, contact=None):
        if contact is None:
            contact = Contact.from_input()
        self.contacts.append(contact)

Now your UI can create a Contact and pass it straight to address_book.add_contact().