Serializing and deserializing JSON in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JSON serialization code?

Consider this Python code that serializes a dictionary to a JSON string. What will be printed?

Python

import json

data = {'name': 'Alice', 'age': 30, 'is_student': False}
json_str = json.dumps(data)
print(json_str)

A {"name": "Alice", "age": 30, "is_student": True}

B {'name': 'Alice', 'age': 30, 'is_student': False}

C {"name": "Alice", "age": "30", "is_student": false}

D {"name": "Alice", "age": 30, "is_student": false}

💡 Hint

Remember that JSON uses double quotes and lowercase true/false for booleans.

The json.dumps function converts Python objects to JSON strings. It uses double quotes for keys and string values, and converts Python False to JSON false.

❓ Predict Output

intermediate

What is the value of 'data' after deserializing this JSON string?

Given this JSON string, what Python object results from deserializing it?

Python

import json

json_str = '{"fruits": ["apple", "banana"], "count": 2}'
data = json.loads(json_str)
print(data)

A {'fruits': ['apple', 'banana'], 'count': 2}

B {"fruits": ["apple", "banana"], "count": 2}

C {fruits: ['apple', 'banana'], count: 2}

D ['apple', 'banana', 2]

💡 Hint

json.loads converts JSON strings to Python dictionaries and lists.

The JSON string represents an object with a list and a number. json.loads converts it to a Python dict with a list and int.

❓ Predict Output

advanced

What error does this code raise when serializing?

What error will this code raise when trying to serialize the object to JSON?

Python

import json

class Person:
    def __init__(self, name):
        self.name = name

p = Person('Bob')
json_str = json.dumps(p)

A TypeError: Object of type Person is not JSON serializable

B ValueError: Invalid JSON format

C AttributeError: 'Person' object has no attribute 'name'

D No error, outputs '{}'

💡 Hint

json.dumps can only serialize basic Python types by default.

Custom objects like Person are not serializable by default and cause a TypeError.

❓ Predict Output

advanced

What is the output of this code using json.loads with an invalid JSON string?

What happens when you run this code?

Python

import json

invalid_json = '{"name": "Alice", age: 30}'
data = json.loads(invalid_json)

A {'name': 'Alice', 'age': 30}

B json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes

C SyntaxError: invalid syntax

D TypeError: string indices must be integers

💡 Hint

JSON keys must be double quoted strings.

The key 'age' is not enclosed in double quotes, so json.loads raises JSONDecodeError.

🧠 Conceptual

expert

Which option correctly serializes a Python dictionary with non-string keys to JSON?

Given a Python dictionary with integer keys, which option produces a valid JSON string with string keys?

Python

import json

data = {1: 'one', 2: 'two'}

A json.dumps(dict(map(str, data.items())))

B json.dumps(data)

C json.dumps({str(k): v for k, v in data.items()})

D json.dumps(data, ensure_ascii=False)

💡 Hint

JSON keys must be strings. Python dict keys can be other types.

Option C converts integer keys to strings before serializing, producing valid JSON. Option C raises TypeError. Option C is invalid syntax. Option C does not fix non-string keys.

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()