15. JSON and PYTHON

By Bernd Klein. Last modified: 01 Feb 2022.

Introduction

JSON stands for JavaScript Object Notation. JSON is an open standard file and data interchange format. The content of a JSON file or JSON data is human-readable. JSON is used for storing and exchanging data. The JSON data objects consist of attribute–value pairs. The data format of JSON looke very similar to a Python dictionary, but JSON is a language-independent data format. The JSON syntax is derived from JavaScript object notation syntax, but the JSON format is text only. JSON filenames use the extension .json.

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dumps and load

It is possible to serialize a Python dict object to a JSON formatted string by using dumps from the json module:

import json

d = {"a": 3, "b": 3, "c": 12}

json.dumps(d)

'{"a": 3, "b": 3, "c": 12}'

The JSON formatted string looks exactly like a Python dict in a string format. In the followoing example, we can see a difference: "True" and "False" are turned in "true" and "false":

d = {"a": True, "b": False, "c": True}

d_json = json.dumps(d)
d_json

'{"a": true, "b": false, "c": true}'

We can transform the json string back in a Python dictionary:

json.loads(d_json)

{'a': True, 'b': False, 'c': True}

Differences between JSON and Python Dictionaries

If you got the idea that turning dictionaries in json strings is always structure-preserving, you are wrong:

persons = {"Isabella": {"surname": "Jones",
                       "address": ("Bright Av.", 
                                   34, 
                                   "Village of Sun")},
           "Noah": {"surname": "Horton",
                    "address": (None, 
                                None, 
                                "Whoville")}
          }


persons_json = json.dumps(persons)                                
print(persons_json)                          

{"Isabella": {"surname": "Jones", "address": ["Bright Av.", 34, "Village of Sun"]}, "Noah": {"surname": "Horton", "address": [null, null, "Whoville"]}}

We can see that the address tuple is turned into a list!

json.loads(persons_json)

{'Isabella': {'surname': 'Jones',
  'address': ['Bright Av.', 34, 'Village of Sun']},
 'Noah': {'surname': 'Horton', 'address': [None, None, 'Whoville']}}

You can prettyprint JSON by using the optinional indent parameter:

persons_json = json.dumps(persons, indent=4)                                
print(persons_json) 

{
    "Isabella": {
        "surname": "Jones",
        "address": [
            "Bright Av.",
            34,
            "Village of Sun"
        ]
    },
    "Noah": {
        "surname": "Horton",
        "address": [
            null,
            null,
            "Whoville"
        ]
    }
}

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Relationship between Python dicts and JSON Objects

import json

d = {"d": 45, "t": 123}
x = json.dumps(d)
print(x)

lst = [34, 345, 234]
x = json.dumps(lst)
print(x)

int_obj = 199
x = json.dumps(int_obj)
print(x)

{"d": 45, "t": 123}
[34, 345, 234]
199

There is another crucial difference, because JSON accepts onls keys str, int, float, bool or None as keys, as we can see in the following example:

board = {(1, "a"): ("white", "rook"),
         (1, "b"): ("white", "knight"),
         (1, "c"): ("white", "bishop"),
         (1, "d"): ("white", "queen"),
         (1, "e"): ("white", "king"),
         # further data skipped
        }

Calling json.dumps with board as an argument would result in the exeption TypeError: keys must be str, int, float, bool or None, not tuple.

To avoid this, we could use the optional key skipkeys:

board_json = json.dumps(board, 
                       skipkeys=True)

board_json

'{}'

We avoided the exception, but the result is not satisfying, because the data is missing!

A better solution is to turn the tuples into string, as we do in the following:

board2 = dict((str(k), val) for k, val in board.items())
board2

{"(1, 'a')": ('white', 'rook'),
 "(1, 'b')": ('white', 'knight'),
 "(1, 'c')": ('white', 'bishop'),
 "(1, 'd')": ('white', 'queen'),
 "(1, 'e')": ('white', 'king')}

board_json = json.dumps(board2)
board_json

'{"(1, \'a\')": ["white", "rook"], "(1, \'b\')": ["white", "knight"], "(1, \'c\')": ["white", "bishop"], "(1, \'d\')": ["white", "queen"], "(1, \'e\')": ["white", "king"]}'

 

board2 = dict((str(k[0])+k[1], val) for k, val in board.items())
board2

{'1a': ('white', 'rook'),
 '1b': ('white', 'knight'),
 '1c': ('white', 'bishop'),
 '1d': ('white', 'queen'),
 '1e': ('white', 'king')}

board_json = json.dumps(board2) board_json

board2 = dict((str(key[0])+key[1],