DIT Python Lecture

This lecture will aim to cover some very basic REST principles while also introducing us to the Python programming language.

But first...

Who/what uses Python:

• Script writing and automation
• Backend development in:
  • Instagram
  • Dropbox
  • Quora
• Data science and machine learning
  • Seaborn
  • Matplotlib
• Internet of Things (IoT)
  • Go to language for many Raspberry Pi projects

Some Language Properties:

• Interpreted, not compiled
• Dynamically typed
• Easy to read syntax
• High level language
• Free and open source
• Object orientated
• Huge range of third party libraries

Lets begin:

We will be creating a simple client and server in python, but before we begin, ensure Python 3.6.1 is installed on your system. Verify by running the following command in your terminal:

$ python -V
Python 3.6.1

Sometimes you may need to explicitly say you are using Python 3, so if the above command returns Python 2.7.x you may need to run as:

$ python3 -V
Python 3.6.1

If running Python3 still returns you an older version of Python, update it using the instructions outlined at the bottom of this page. If it's the case where you need to use python3, make sure to run the client and server run as python3 client.py and python3 server.py. Go to the bottom of the page for some help on installing Python.

Once Python has been setup, we will be looking at the following sections:

• Interacting with a mock REST service.
• Building a Python client for the above mock REST service.
• Building a Python server that serves data to the client in order to replace the mock service.

Step 1 - Interacting With Mock REST Service

The mock REST service uses https://www.mockable.io to serve static dummy data. This can be very useful for developers building clients before any backend systems exist. Ensure it is working for you by running a curl command on the following endpoints:

• http://demo4866252.mockable.io/users
• http://demo4866252.mockable.io/users/234298
• http://demo4866252.mockable.io/users/234342

When interacting with these, can you:

• Tell what HTTP method is used (GET/POST/PUT/PATCH)?
• List all the headers returned?

Step 2 - Build a client

Now that we can pull data from the above mock endpoints, we can start creating a simple Python client. Lets break this into a number of small tasks. In this step we are going to be using two Python 3 modules, HTTPConnection and the json library. Start implementing your code in the client.py file

• Import the HTTPConnection module, and use it to make a request to the above URLs. (Solution)
• When you can successfully make a request and print the raw response as a string, use the json module to decode that into a native python data type (as in list and dict). (Solution)
• Now we figured out how to represent the data to something meaningful, create a client class called UsersClient to encapsulate the logic for making remote calls. (Solution)
• Finally, instead of having the data represented as native Python data types, lets create a User class to encapsulate the returned data from the API. (Solution)

Step 3 - Build a server

Now lets build a server to serve the same data. Just like when creating the client, we can break this into small tasks. We will be making use of the HTTPServer and once again the json library. Start implementing your code in the server.py file

• To create a Http server, you will need to import HTTPServer to handle the actual server instance, and the BaseHTTPRequestHandler to handle incoming requests. Get something running so that you can make curl requests to it. (Solution)
• Once we have something being served, lets implement the list all users endpoint (the /users endpoint). Ensure to also set the Content-Type: application/json header. Return the same static data the dummy endpoints return. Use the json library to encode Python's data types to JSON. (Solution)
• Next see if we can implement the endpoint where we can get users by their Id. As in the users/234298 and users/234342 endpoint. Remember the Id should be dynamic, and if we don't have that Id, we should return 404 - Not Found. There are a number of ways to extract the Id within the URL path, one being a regex pattern match. (Solution)

Python Installation

Mac OSX

See more here or here, but basically using brew is probably the easiest option.

$ brew install python3

If you run into linking issues, run:

$ brew link --overwrite python3

Windows

Download and install Python from here. Once installed you should be able to run a Python shell. You then need to add Python to the system path, you can do this by:

• Opening My Computer > Properties > Advanced System Settings > Environment Variables
• Then edit the Path environment variable by appending %USERPROFILE%\AppData\Local\Programs\Python\Python36-32 (or where ever your Python is installed on your PC). Ensure that each path entry is separated by a ;.
• Test to ensure it works by opening a command prompt and typing python -V

If using windows, it is also recommend to use cygwin over the native windows command prompt, but for the purpose of this lecture, using the native command prompt should be ok.

Anything Else

Other unix like sytems should have python installed by default, just check the version is correct. When running python3 -V, the version may be a little older, perhps 3.4.x. For the purpose of this lecture it may be ok, however it is reccomented to use 3.6.1 as this material was tested using that version.