Editing Community Data Science Course (Spring 2015)/Day 4 Lecture

[[File:Highfivekitten.jpeg|200px|thumb|In which you learn how to use Python and web APIs to meet the likes of her!]]
== Lecture Slides ==

* [http://mako.cc/teaching/2014/cdsw-autumn/lecture2-web_apis.pdf Slides (PDF)] — For viewing
* [http://mako.cc/teaching/2014/cdsw-autumn/lecture2-web_apis.odp Slides (ODP Libreoffice Slides Format)] — For editing and modification
== Resources ==

* Encoding: 
** [http://nedbatchelder.com/text/unipain.html Pragmatic Unicode]
** [https://docs.python.org/2/howto/unicode.html Official Python Unicode documentation]


== Lecture Outline ==
;Introduction and context

* You can write some tools in Python now. Congratulations!
* Today we'll learn how to find/create data sets
* Next week we'll get into data science (asking and answering questions)


;Outline: 

* What is an API?
* How do we use one to fetch interesting datasets?
* How do we write programs that use the internet? 
* How can we use the placekitten API to fetch kitten pictures?
* Introduction to structured data (JSON)
* How do we use APIs in general?


;What is a (web) API?

* API: a structured way for programs to talk to each other (aka an interface for programs)
* Web APIs: like a website your programs can visit (you:a website::your program:a web API)


; How do we use an API to fetch datasets?

Basic idea: your program sends a request, the API sends data back
* Where do you direct your request? The site's API endpoint.
** For example: Wikipedia's web API endpoint is http://en.wikipedia.org/w/api.php
* How do I write my request? Put together a URL; it will be different for different web APIs.
** Check the documentation, look for code samples
* How do you send a request? 
** Python has modules you can use, like <code>requests</code> (they make HTTP requests)
* What do you get back?
** Structured data (usually in the JSON format)
* How do you understand (i.e. parse) the data? 
** There's a module for that!


; How do we write Python programs that make web requests?

To use APIs to build a dataset we will need:
* all our tools from last session: variables, etc
* the ability to open urls on the web
* the ability to create custom URLS
* the ability to save to files 
* the ability to understand (i.e., parse) JSON data that APIs usually give us


; New programming concepts:

* interpolate variables into a string using % and %()s
* requests
* open files and write to them


; How do we use an API to fetch kitten pictures?

[http://placekitten.com/ placekitten.com]
* API that takes specially crafted URLs and gives appropriately sized picture of kittens
* Exploring placekitten in a browser:
** visit the API documentation
** kittens of different sizes
** kittens in greyscale or color
* Now we write a small program to grab an arbitrary square from placekitten by asking for the size on standard in: [http://mako.cc/teaching/2014/cdsw-autumn/placekitten_raw_input.py placekitten_raw_input.py]


; Introduction to structured data (JSON, JavaScriptObjectNotation)

* what is json: useful for more structured data
* import json; json.loads()
* like Python (except no single quotes)
* simple lists, dictionaries
* can reflect more complicated data structures
* Example file at http://mako.cc/cdsw.json
* You can parse data directly with <code>.json()</code> on a <code>requests</code> call

; Using other APIs

* every API is different, so read the documentation!
* If the documentation isn't helpful, search online
* for popular APIs, there are python modules that help you make requests and parse json

Possible issues:
* rate limiting
* authentication
* text encoding issues