Python Objects and a Simple Web Server

Note

Learning objectives: Explain the three aspects of object orientation. Start a basic Python web server and explain how it works. Prove comprehension by overwriting the GET method.

Objects

Objects are a complex data structure that has three attributes: encapsulation, inheritance, and polymorphism.

(These will appear on a test in high school – I guarantee it.)

• Encapsulation

  • An object is a container for attributes and methods (functions).

  • For example, let’s say I have a class called animal with class animal:…

  • I create an object of that class called Spot with Spot = animal() .

  • Spot is now an object of type animal(). This is called instantiation. Say it with me.

  • I set Spot.legs = 4 and Spot now has an attribute called legs which is a variable containing the integer 4.

>>> class animal:
...     pass
...
>>> Spot = animal()
>>> Spot.legs = 4
>>> Spot.legs
4
>>> Spot.eyes
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'animal' object has no attribute 'eyes'

• Inheritance

  • We can compose objects of other objects.

  • Let’s say I create a class called dog which inherits animal with class dog(animal):… .

  • Now dog has all of the attributes of animal. We say dog is a child class of animal.

class animal:
    # We will set some animal defaults
    legs = 4
    eyes = 2
    claws = False
    tentacles = False

class dog(animal):
    # Even though you can't see it, dog also has legs
    # ... and eyes and a few more attributes.
    tails = 1
    claws = True
    adorable = True
    name = ''

Spot = dog           # We instantiate Spot, an object of class dog
print(Spot.legs)
print(Spot.eyes)
print(Spot.tails)

• Polymorphism

  • The language doesn’t care where it gets a name for an attribute or method.

  • In this example, we have a function called self.istasty() .

  • It does something different for dog() than it does for the parent class animal().

class animal:
    # We will set some animal defaults
    legs = 4
    eyes = 2
    claws = False
    tentacles = False
    name = ''
    edible = True                  # We assume it's edible
    def __init__(self, newname=''):
        if newname:
            self.setname(newname)
        while not self.name:
            self.setname(input('Name this beast:  '))
    def setname(self, newname):
        self.name = str(newname)
    def istasty(self):
        if self.edible:
            print("%s is edible.  Use BBQ sauce!" % self.name)
        else:
            print("Do not eat %s." % self.name)

class dog(animal):
    # Even though you can't see it, dog also has legs
    # ... and eyes and a few more attributes.
    tails = 1
    claws = True
    adorable = False
    edible = False
    name = ''
    def __init__(self, newname=''):
        if newname:
            self.setname(newname)
        while not self.name:
            self.setname(input('Name this doggie:  '))
    def istasty(self):
        if self.edible and not self.adorable:
            print("%s is edible.  Use BBQ sauce!" % self.name)
        elif self.edible and self.adorable:
            print("%s is edible, but too cute!  Do not eat it!" % self.name)
        else:
            print("Do not eat %s." % self.name)

Bessy = animal('Bessy')
Bessy.istasty()

Spot = dog('Spot')           # We instantiate Spot, an object of class dog
Spot.edible = True
Spot.adorable = True
Spot.istasty()               # Notice how self.istasty() has new instructions

Spot is counting on you to get this right. Don’t let Spot be food.

Note

Notice what happens when you just type Spot = dog() . It will make you name your dog.

A Basic Web Server

Let’s create a directory from which we will serve our source code.

cd %userprofile%
md serverroot
cd serverroot

Inside that directory, we will place our program, as stolen from https://docs.python.org/3/library/http.server.html

webserver.py from https://python.org

import http.server
import os
import string

# We create an HTTPRequestHandler class that inherits the BaseHTTPRequestHandler
# ... and lets us play with its attributes and methods
class HTTPServer_RequestHandler(http.server.BaseHTTPRequestHandler):

    # GET method
    def do_GET(self):
        # Send response status code
        self.send_response(200)

        # Send headers
        self.send_header('Content-type','text/html')
        self.end_headers()

        # Send directory listing to the client
        mydir = string.join(list(os.listdir('.')), '<BR>')
        message = str('<HTML>' + mydir + '</HTML>')
        self.wfile.write(bytes(message))
        return

def run():
    print('starting server...')
    server_address = ('', 8008)    # a tuple of ip address and port
    httpd = http.server.HTTPServer(server_address, HTTPServer_RequestHandler)
    print('Starting!')
    httpd.serve_forever()


run()

For more readings on HTTP methods, such as GET and POST, try https://www.w3schools.com/tags/ref_httpmethods.asp .