JSON

JSON is a common format for data. It looks a lot like a Python dictionary, and has support across lots of different languages and tools.

In this lesson is a brief primer on using JSON in Python.

Quick Facts

JSON stands for "JavaScript Object Notation", but as you can see, it's not just used in JavaScript
JSON is commonly used on the Web, as the format for different applications to communicate
JSON is also commonly used for configuration files for applications

Handling JSON in Python

Python has a built in package called json to work with JSON-formatted data.

A JSON file looks a lot like a Python dictionary:

{
  "class A": {
    "students": 35,
    "location": "campus x"
  },
  "class B": {
    "students": 33,
    "location": "campus y"
  }
}

Converting from JSON to Python datatypes

When you access a JSON file, it's text - a string. But, that string represents data that has more structure and meaning. It might have numbers, strings, lists, and dictionaries within it.

In order to work with the data in Python, you need to parse the JSON string and convert it to Python datatypes.

Python's built-in json module makes it pretty easy:

import json
# some JSON:
json_text =  '{ "name":"John", "age":30, "city":"New York"}'
# parse it using loads
parsed = json.loads(json_text)
# the result is a Python dictionary:
print(parsed) # { "name": "John", "age": 30, "city": "New York" }
print(parsed["age"]) # 30

The difference is a little bit subtle, because the string and the dictionary are visually similar!

However, when you have converted a JSON-formatted string to a python dict, you can access the values inside.

What result would you get if you tried to access json_text["age"]?

Python to JSON

You can also go the other way and turn a Python object into JSON:

import json

# a Python object (dict):
student = {
  "name": "John",
  "age": 30,
  "city": "New York"
}

# convert into JSON:
json_text = json.dumps(student)

# the result is a JSON string:
print(json_text)

The type of json_text is a string. It's formatted as JSON, which other programs might be able to use, like if you saved it as a config file, or sent it over the network to another application.

JSON Files

If you have JSON data in a file, you can load it like this example:

import json
data_file = open("data.json", "r")
data = json.load(data_file)
data_file.close()
data # Parsed data from the file

Or, if you wanted to write to a file, there's a corresponding function:

import json
data = { "name": "John" }
target_file = open("target.json", "w")
json.dump(data, target_file)
target_file.close()

Programming 1