Serializing and deserializing JSON in Python - Mini Project: Build & Apply

Serializing and deserializing JSON

📖 Scenario: You are working on a simple contact list app. You want to save the contacts to a file and later read them back.

🎯 Goal: Learn how to convert a Python dictionary to a JSON string (serialize) and then convert it back from JSON string to a Python dictionary (deserialize).

📋 What You'll Learn

Create a Python dictionary with exact contact details

Create a JSON string from the dictionary using json.dumps()

Convert the JSON string back to a dictionary using json.loads()

Print the final dictionary to verify it matches the original

💡 Why This Matters

🌍 Real World

Many apps save data in JSON format because it is easy to share and read by different programs.

💼 Career

Understanding JSON serialization and deserialization is essential for backend development, APIs, and data exchange between systems.

1

Create the contacts dictionary

Create a dictionary called contacts with these exact entries: 'Alice': '[e-mail]', 'Bob': '[e-mail]', 'Charlie': '[e-mail]'

Python

# Create the contacts dictionary with exact entries
# Your code here

Hint

Use curly braces {} to create a dictionary and separate keys and values with colons.

2

Import the json module

Add the line import json at the top of your code to use JSON functions

Python

# Import the json module
# Your code here

contacts = {
    'Alice': '[e-mail]',
    'Bob': '[e-mail]',
    'Charlie': '[e-mail]'
}

Hint

Use import json to access JSON functions in Python.

3

Serialize the dictionary to JSON string

Create a variable called contacts_json and set it to the JSON string of contacts using json.dumps(contacts)

Python

# Convert contacts dictionary to JSON string
# Your code here

Hint

Use json.dumps() to convert a dictionary to a JSON string.

4

Deserialize the JSON string and print

Create a variable called contacts_loaded and set it to the dictionary obtained by deserializing contacts_json using json.loads(contacts_json). Then print contacts_loaded

Python

# Convert JSON string back to dictionary and print it
# Your code here

Hint

Use json.loads() to convert JSON string back to a dictionary. Then use print() to show the result.

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()