Working with JSON

JSON

JSON = JavaScript Object Notation → Pronounced like the first name 'Jason'

• Language-independent and lightweight data format for data exchange between servers and (web) applications
• Derived from JavaScript
• Used for storing and transporting data
• Designed to be ‘human-readable’
• REST very often works with JSON
• File format: .json

JSON is made up by key-value pairs

• A key is always a string, enclosed by double quotes
• A semicolon (:) seperates the key and value
• A value must be one of the following data types:
  • A string
  • A number
  • An object
  • An array
  • A boolean
  • null
    
  • Pay attention: string values must be enclosed by double quotes

{
   "course":"API Development"
}

{
   "key":value
}

A JSON file usually contains multiple key-value pairs

• Seperated by a comma
• No comma after the last key-value pair

Correct: key-value pairs seperated by a comma

{
   "course":"API Development",
   "semester":1
}

The order of key-value pairs doesn't matter

• For JSON, both objects/files (above and below this text) contain the same data

Correct: order of key-value pairs changed

{
   "semester":1,
   "course":"API Development"
}

A key can only occur once within an object, but can occur again in child objects

Wrong: the same key twice within the same object

{
   "course":"API Development",
   "course":"Python"
}

Notation of data types

• A string
• A number
• An object
• An array
• A boolean
• null

{
   "string":"a string value",
   "wholeNumber":17,
   "decimalNumber":7.37,
   "object":{
      "course":"API Development",
      "hoursPerWeek":3
   },
   "array":[
      {"name":"Els Depaepe", "class":"2 CCS 01"},
      {"name":"Simon Vervoort", "class":"2 CCS 02"},
      {"name":"Bram Verbiest","class":"2 IOT"}
   ],
   "boolean":true,
   "empty":null
}

Example 1: from table to JSON

Table: results

course	student	grade
Python	Laurien Stas	12
SQL	Robbe Van Amont	4
Python	Michaël Aerts	14

{
   "results":[
      {
         "course":"Python",
         "student":"Laurien Stas",
         "grade":12
      },
      {
         "course":"SQL",
         "student":"Robbe Van Amont",
         "grade":4
      },
      {
         "course":"Python",
         "student":"Michaël Aerts",
         "grade":14
      }
   ]
}

Example 2: nested structure

Table: course

courseId	name	semester
1	Python	1
2	SQL	2

Table: result

resultId	courseId	student	grade
1	1	Laurien Stas	12
2	2	Robbe Van Amont	4
3	1	Michaël Aerts	14

{
   "courses":[
      {
         "courseId":1,
         "name":"Python",
         "semester":1,
         "results":[
            {"resultId":1,"student":"Laurien Stas","grade":12},
            {"resultId":3,"student":"Michaël Aerts","grade":14}
         ]
      },
      {
         "courseId":2,
         "name":"SQL",
         "semester":2,
         "results":[
            {"resultId":2,"student":"Robbe Van Amont", "grade":4}
        ]
      }
   ]
}

The structure of JSON should always be correct, otherwise it can not be parsed
Check whether the structure is correct or not: https://jsonformatter.curiousconcept.com/
Enter your JSON and click on 'Process'

Scroll down and check the validity

Made a mistake? Check the error message(s)!

Exercise: convert the following table data into JSON

Table: song

name	artist	durationInSeconds	hasAlbum
Something	Lasgo	221	true
Phoenix	Enemy Inside	208	true
Expectations	Lauren Jauregui	204	false

JSON in Python

Python has a build in package to work with JSON data
To use it, import the 'json' package:

import json

The structure of JSON is very similar to a dictionary in Python → Time for a small reminder!

Dictionary

• A dictionary is an ordered, changeable collection of key-value pairs
• The key is unique so you can use it to retrieve any value
• You can imagine a dictionary as a table with two columns

letters = {'e': 1, 'h': 1, 'n': 1, 'o': 2, 't': 4}

A dictionary is created via curly braces: {}
Key and value are separated by a :
Key/value pairs are separated by a ,

dictionary = {key 1: value 1, ..., key n: value n}

Looking up a value is done by the key

First method

The key is given 'as an index' in square brackets

letters = {'e': 1, 'h': 1, 'n': 1, 'o': 2, 't': 4}
print('Occurrences of letter t:', letters['t'])

Second method

A dictionary also has a method to retrieve the value associated with a key.
The advantage of the method get()is that you can decide what is returned if the key is not found.

letters = {'e': 1, 'h': 1, 'n': 1, 'o': 2, 't': 4}

my_choice = input('Letter? ')
print(letters.get(my_choice, 'This character does not occur'))

Parsing JSON

json.loads()

• Function that can be used to parse a valid JSON string and convert it into a Python dictionary
• Expects a string value as parameter, not a file
• Useful if you already have a JSON object in Python
  
  From JSON to Python dictionary
  

import json

# a JSON course object, put in a Python variable:
json_course = '{"name":"API Development","semester":1}'

# parse the JSON object using the loads() function:
course = json.loads(json_course)

# print the data:
print("Course name:", course["name"])
print("Semester:", course["semester"])

json.load()

• Function that can be used to parse the data from a .json file to a JSON object
• Then converts the JSON object into a Python dictionary
• Expects a file location as parameter
  
  From JSON file to Python dictionary
  

course.json

{
   "name":"API Development",
   "semester":1
}

import json

with open("course.json") as json_file:
    # read out the file and parse the JSON object using the load() function:
    course = json.load(json_file)
    print(course)
    
    # print the data:
    print("Course name:", course["name"])
    print("Semester:", course["semester"])

json.dumps()

• Function that can be used to convert a Python dictionary (or other object) into a JSON string
• Expects a dictionary or other Python object as parameter
  
  From Python dictionary to JSON
  

import json

# a Python dictionary:
course = {'name': 'API Development', 'semester': 1}

# convert into JSON using the dumps function:
json_course = json.dumps(course)

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

json.dump()

• Function that can be used to write data of a Python dictionary to a .json file
• Writes in the memory and then the command for writing to disk is executed separately
• Expects 2 parameters: first the Python dictionary, then the file to write to
• Pay attention:
  • The file should be opened before the json.dump() function is called
  • The file should be closed after calling the json.dump() function
    
    From Python dictionary to JSON file
    

import json

# a Python dictionary:
course = {'name': 'API Development', 'semester': 1}

with open("course.json", "w", encoding="UTF-8") as file:
    json.dump(course, file)

Indent

• A big advantage of JSON is readability
• By default, Python generates one line in a .json file
• Solution to make it even more readable: add indents
  

import json

# a Python dictionary:
course = {'name': 'API Development', 'semester': 1}

with open("course.json", "w", encoding="UTF-8") as file:
      json.dump(course, file, indent=3)