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.

.. code-block:: python

    >>> 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.

.. code-block:: python
    :linenos:

    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().

.. code-block:: python
    :linenos:

    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.**

.. image:: images/sadpuppy.png

.. 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.

.. code-block:: bat

    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

.. code-block:: python
    :linenos:
    :caption: 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 .