Lecture 31

The json module

MCS 260 Fall 2020
David Dumas

Reminders

pip and pytest

On Friday we'll talk about testing and pytest, a module that is not always included with Python.

Python's packaging tool pip can install it.

If your interpreter name is python, run:

python -m pip install pytest

If your interpreter name is python3, run:

python3 -m pip install pytest

Is pytest installed?

Run the command

python -m pytest

====================== test session starts =======================
platform win32 -- Python 3.8.3, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: C:\Users\ddumas\example
collected 0 items

===================== no tests ran in 0.02s ======================
    

JSON

JSON stands for JavaScript object notation. It is a format for storing various types of data in text files. Many languages can read and write this format.

Supported basic types:

• string — must use double quotes.
• number — float, int, other? Up to reader.
• boolean — lower case names true, false.
• null — similar to Python's None.

Supported composite types:

• array — ordered sequence of values like Python list. Surrounded by square brackets, values separated by commas.
• object — associative array like Python dict. Surrounded by curly braces, comma separator. Keys must be strings.

A JSON file must contain a single value. Most often it is an object or array.

Writing JSON

Use json.dump(val,f) to write val to file object f as JSON.

Conversion table:

• dict → object
• list or tuple → array
• int or float → number
• bool → boolean
• None → null

Or use json.dumps(val) to get JSON string.

Reading JSON

Use json.load(f) to interpret contents of file object f as JSON and return the decoded result.

json.loads(text) will instead process string text as JSON.

Not supported in JSON

• Complex numbers
• Date/time types
• Distinctions between:
  • int and float^*
  • tuple and list
• Comments

* But Python's json module will try to guess when reading.

Example

The US federal government offers a free JSON API for retrieving federal spending data.

API means Application Programming Interface; essentially, a way for programs to talk to one another.

This is a HTTP API, so there are URLs you can retrieve to get a JSON document with spending data.

Question: What fraction of the federal budget goes to the National Science Foundation?

The URL

https://api.usaspending.gov/api/v2/references/toptier_agencies/

returns data about many "top tier" federal agencies as JSON.