Introduction to Python

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User-Defined Classes: the class Statement and Object Methods

introduction: classes

A class is a definition for a custom type of object.

At the start of this course, we defined an object:

An object is:
--> a unit of data
--> of a particular type
--> with type-specific functionality

• user-defined classes allow us to create a custom type of object
• the object will have its own behaviors and its own ways of storing data
• many modules use custom objects to facilitate access to module functionality
• we can create our own object types to provide an object-oriented interface to our own module's functionality

the object-oriented interface

All objects have behaviors called methods and data stored in attributes.

mylist = [1, 2, 3]
mylist.append(4)       # list, [1, 2, 3, 4]
print(mylist[-1])      # int, 4

mystr = 'hello'
ustr = mystr.upper()   # str, 'HELLO'

myint = 5
dblint = myint + 5     # int, 10

• we have been working with the OOP interface from the start
• any object of a type we have used (list, str, int, etc) have had data and behaviors designed to work with that data
• the behaviors may take the form of methods, or be other operations such as subscripting or use of operators
• the combination of data plus behaviors is the essence of the OOP interface

example of a class: the date and timedelta object types

Consider any object you encounter in terms of its data and behaviors.

from datetime import date, timedelta

dt = date(2023, 12, 30)         # new 'date' object for 12/30/2023
td = timedelta(days=3)          # new 'timedelta' object:  3 day interval

dt2 = dt + td                   # new date object:  date + timedelta

print(dt)                       # 2024-01-02 (3 days later)
print(type(dt))                 # <class 'datetime.datetime'>

• a date object holds the data for a date
• it is also capable of calculating dates (using math operators)
• this combination of data plus behaviors is the essence of the OOP interface

designing an object interface: the (proposed) server object type

When creating a new object type, consider what you want it to be and to do.

import sysadmin                    # theoretical / proposed module

s1 = sysadmin.Server('work1',
                     username='user',
                     password='pass')

ms = s1.ping()
print(f'{s1.hostname} pinged at {ms}ms')   # 'work1' pinged at 43ms

s1.copyfile_up('myfile.txt')       # copies a file up to the server
s1.copyfile_down('yourfile.txt')   # copies a file down from the server

print(s1.uptime())                 # 7920 ('work1' restarted 2 hours, 12 minutes ago)

s1.restart()                       # restarts the server

print(s1.uptime())                 # 2 (work1 restarted 2 seconds ago)

• when you consider a module you would like to create, think in terms of the OOP interface
• consider what you would want an object to be and to do
• this (proposed) server object represents a server. So we define methods that will give us access to the things a server can do.

the class statement

The class block statement is the blueprint for an object type.

class Me:
    pass          # 'pass' marks an empty block


m = Me()          # construct a new 'Me' object

print(type(m))    # <class '__main__.Me'>

• a class defines what data an object holds and what it does
• with nothing in it (marked by 'pass'), this class statement simply indicates the name
• the name of a class is the same as its type (this is a Me class)
• when we call the class, it produces an instance (object) of the class
• the type() function shows us that this is a Me object
• (the '__main__' part refers to the current script, or when imported would be the name of the module)

Ex. 13.1

defining a method

A method is a function that is part of a class.

class Say:

    def greet(self):
        print('Hello!')

    def make_greeting(self, name):
        return f'Hello, {name}!'

s = Say()

s.greet()                        # Hello!

g = s.make_greeting('Guido')     # 'Hello, Guido!'

• a method is simply a function that belongs to the class
• the method is called through the object with "dot" syntax
• it is in most other ways like a function - it may take arguments and may return return values
• (the 'self' argument will be discussed in our next lesson)

13.2 - 13.2