Serializing and deserializing JSON in Python

Introduction

We use JSON to save or send data in a simple text format. Serializing turns data into JSON text. Deserializing turns JSON text back into data.

Saving user settings to a file so they can be loaded later.

Sending data from a web app to a server.

Reading data from a file that was saved in JSON format.

Sharing data between different programs or devices.

Syntax

Python

import json

# Serialize (convert data to JSON string)
json_string = json.dumps(data)

# Deserialize (convert JSON string back to data)
data = json.loads(json_string)

json.dumps() converts Python data to a JSON string.

json.loads() converts a JSON string back to Python data.

Examples

This turns a Python dictionary into a JSON string.

Python

import json

# Serialize a dictionary to JSON string
data = {'name': 'Alice', 'age': 30}
json_str = json.dumps(data)
print(json_str)

This turns a JSON string back into a Python dictionary.

Python

import json

# Deserialize JSON string back to dictionary
json_str = '{"name": "Alice", "age": 30}'
data = json.loads(json_str)
print(data)

Lists can also be converted to JSON strings.

Python

import json

# Serialize a list to JSON string
numbers = [1, 2, 3, 4]
json_str = json.dumps(numbers)
print(json_str)

Sample Program

This program shows how to convert a Python dictionary to a JSON string and back again.

Python

import json

# Original data
person = {'name': 'Bob', 'age': 25, 'city': 'New York'}

# Serialize to JSON string
json_data = json.dumps(person)
print('Serialized JSON:', json_data)

# Deserialize back to Python dictionary
person_data = json.loads(json_data)
print('Deserialized data:', person_data)

Important Notes

JSON only supports basic data types like strings, numbers, lists, and dictionaries.

Python objects like sets or custom classes need special handling before serializing.

Use indent parameter in json.dumps() to make JSON output easier to read.

Summary

Serializing means converting Python data to JSON text.

Deserializing means converting JSON text back to Python data.

Use the json module's dumps() and loads() functions for this.

Practice

1. What does the json.dumps() function do in Python?

easy

A. Reads JSON data from a file

B. Converts Python data into a JSON formatted string

C. Converts JSON string back to Python data

D. Writes Python data directly to a file

Solution

1. Step 1: Understand the purpose of json.dumps()

   This function takes Python objects like dictionaries or lists and turns them into a JSON string.

2. Step 2: Differentiate from other JSON functions

   json.loads() converts JSON strings back to Python objects, while dumps() does the opposite.

3. Final Answer:

   Converts Python data into a JSON formatted string -> Option B

4. Quick Check:

   Serialize = Python to JSON string [OK]

Hint: Remember: dumps() means dump Python to JSON string [OK]

Common Mistakes:

• Confusing dumps() with loads()
• Thinking dumps() writes to a file
• Assuming dumps() reads JSON data

2. Which of the following is the correct syntax to deserialize a JSON string json_str into a Python object?

easy

A. json.load(json_str)

B. json.dumps(json_str)

C. json.loads(json_str)

D. json.deserialize(json_str)

Solution

1. Step 1: Identify the function to convert JSON string to Python

   The correct function is json.loads(), which takes a JSON string and returns Python data.

2. Step 2: Check other options for correctness

   json.dumps() serializes Python to JSON string, json.load() reads JSON from a file object, and json.deserialize() does not exist.

3. Final Answer:

   json.loads(json_str) -> Option C

4. Quick Check:

   loads() = JSON string to Python [OK]

Hint: Use loads() to load JSON string into Python [OK]

Common Mistakes:

• Using dumps() instead of loads()
• Confusing load() with loads()
• Using a non-existent deserialize() function

3. What will be the output of the following code?

import json
py_data = {'name': 'Alice', 'age': 30}
json_str = json.dumps(py_data)
print(type(json_str))

medium

A. <class 'dict'>

B. TypeError

C. <class 'list'>

D. <class 'str'>

Solution

1. Step 1: Understand what json.dumps() returns

   The json.dumps() function converts Python data into a JSON string, so the result is a string type.

2. Step 2: Check the printed type

   The type(json_str) will be <class 'str'> because json_str holds a JSON string.

3. Final Answer:

   <class 'str'> -> Option D

4. Quick Check:

   dumps() output type = str [OK]

Hint: dumps() returns a string, so type is str [OK]

Common Mistakes:

• Thinking dumps() returns a dict
• Confusing dumps() with loads()
• Expecting a list type output

4. The following code raises an error. What is the mistake?

import json
json_str = '{"name": "Bob", "age": 25}'
py_data = json.load(json_str)
print(py_data)

medium

A. json.load() expects a file object, not a string

B. json_str is not a valid JSON string

C. json.loads() should be replaced with json.dumps()

D. Missing import statement for json module

Solution

1. Step 1: Understand the difference between json.load() and json.loads()

   json.load() reads JSON data from a file-like object, not from a string.

2. Step 2: Identify correct function for string input

   To convert a JSON string to Python data, use json.loads() instead of json.load().

3. Final Answer:

   json.load() expects a file object, not a string -> Option A

4. Quick Check:

   load() = file, loads() = string [OK]

Hint: Use loads() for strings, load() for files [OK]

Common Mistakes:

• Using load() on a string instead of loads()
• Assuming json_str is invalid JSON
• Confusing dumps() and loads()

5. You have a Python list data = [{'id': 1}, {'id': 2}, {'id': 3}]. You want to serialize it to JSON but only include dictionaries where id is greater than 1. Which code correctly does this?

hard

A. json.dumps([item for item in data if item['id'] > 1])

B. json.dumps(data.filter(lambda x: x['id'] > 1))

C. json.dumps(filter(lambda x: x['id'] > 1, data))

D. json.dumps([item for item in data if item.id > 1])

Solution

1. Step 1: Filter list with list comprehension

   Use a list comprehension to select dictionaries where id is greater than 1: [item for item in data if item['id'] > 1].

2. Step 2: Serialize filtered list to JSON string

   Pass the filtered list to json.dumps() to get the JSON string.

3. Final Answer:

   json.dumps([item for item in data if item['id'] > 1]) -> Option A

4. Quick Check:

   Filter with list comprehension, then dumps() [OK]

Hint: Filter with list comprehension before dumps() [OK]

Common Mistakes:

• Using filter() without converting to list
• Trying to access dict keys with dot notation
• Using filter() result directly without list()