Introduction to Python

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The JSON File Format and Multidimensional Containers

the json file format

JavaScript Object Notation is a simple "data interchange" format for sending or storing structured data as text.

• JSON is used by many web APIs and other programs for communcating data
• it is "lightweight", meaning it does not require overhead as with a database server
• it is text-based, so it can be easily stored to file or transmitted over a network
• it was originally created for JavaScript, but became popular for use with any language
• it is more flexible than CSV in describing many-to-one relationships
• it uses lists and dictionaries to describe data

a sample json file

Fortunately for us, JSON resembles Python in many ways, making it easy to read and understand.

{
   "key1":  ["a", "b", "c"],
   "key2":  {
              "innerkey1": 5,
              "innerkey2": "woah"
            },
   "key3":  false,
   "key4":  null
}

• JSON numbers, strings, lists and dictionaries use the same format as Python objects
• JSON requires double quotes around strings
• the format uses true and false (lowercased) as boolean values
• it uses null as its None value
• when converted, the file will become a Python container with all values converted to Python objects

reading a structure from a json file

The json.load() function decodes the contents of a JSON file.

Here's a program to read the structure shown earlier, read from a file that contains it:

import json                 # we use this module to read JSON

fh = open('sample.json')
mys = json.load(fh)         # load from a file, convert into Python container
fh.close()

print((type(mys)))            # dict (the outer container of this struct)

print(mys['key2']['innerkey2'])     # woah

reading a structure from a json string

The json.loads() function decodes the contents of a JSON string.

In this example, we show what we must do if we don't have access to a file, but instead receive the data as a string, as in the case with web request.

import json                 # we use this module to read JSON
import requests             # use 'pip install' to install


response = requests.get('https://davidbpython.com/mystruct.json')

text = response.text        # file data as a single string

mys = json.loads(text)      # load from a file, convert into Python container

print((type(mys)))            # dict (the outer container of this struct)

print((mys['key2']['innerkey2']))     # woah

The requests module allows your Python program to act like a web browser, making web requests (i.e., with a URL) and downloading the response. If you try this program and receive a ModuleNotFoundError, you must run pip install at the command line to install it.

printing a complex object readably: writing to a string

A nested object can be confusing to read.

If we have an multidimensional object that is squished together and hard to read, we can use .dumps() with indent=4

import json

obj = {'a': {'x': 1, 'y': 2, 'z': 3}, 'b': {'x': 1, 'y': 2, 'z': 3}, 'c': {'x': 1, 'y': 2, 'z': 3} }

print((json.dumps(obj, indent=4)))

this prints:

{
    "a": {
        "x": 1,
        "y": 2,
        "z": 3
    },
    "b": {
        "x": 1,
        "y": 2,
        "z": 3
    }
}

sidebar: writing an object to json file

We can use json.dump() write to a JSON file.

Dumping a Python structure to JSON

import json

wfh = open('newfile.json', 'w')  # open file for writing

obj = {'a': 1, 'b': 2}

json.dump(obj, wfh)

wfh.close()