Working with JSON files in Python - Step-by-Step Execution

Concept Flow - Working with JSON files

Open JSON file in read mode

↓

Read file content as string

↓

Parse string to Python object (dict/list)

↓

Use or modify Python object

↓

Open JSON file in write mode

↓

Convert Python object to JSON string

↓

Write JSON string to file

↓

Close file

This flow shows how Python reads JSON data from a file into Python objects, modifies them, then writes back as JSON.

Execution Sample

Python

import json

with open('data.json', 'r') as f:
    data = json.load(f)

print(data['name'])

This code reads JSON from 'data.json' file and prints the value of the 'name' key.

Execution Table

Step	Action	Code Line	Result/Value
1	Open 'data.json' file in read mode	with open('data.json', 'r') as f:	File object 'f' opened
2	Read JSON content and parse to Python dict	data = json.load(f)	{'name': 'Alice', 'age': 30}
3	Access 'name' key in dict	print(data['name'])	Alice
4	File 'data.json' automatically closed	End of with block	File closed

💡 File is closed after reading; program prints 'Alice' and ends.

Variable Tracker

Variable	Start	After json.load	Final
f	None	File object opened	File closed
data	None	{'name': 'Alice', 'age': 30}	{'name': 'Alice', 'age': 30}

Key Moments - 3 Insights

Why do we use 'with open(...) as f' instead of just open()?

Using 'with' ensures the file is properly closed after reading, even if errors occur, as shown in step 4 of the execution_table.

What type is 'data' after json.load(f)?

'data' becomes a Python dictionary representing the JSON object, as seen in step 2 where data = {'name': 'Alice', 'age': 30}.

What happens if the JSON file is empty or malformed?

json.load(f) will raise an error and stop execution; this is why proper JSON format is required before loading.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'data' after step 2?

A None

B File object

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

D 'Alice'

💡 Hint

Check the 'Result/Value' column in row with Step 2 in execution_table.

At which step is the file closed?

A Step 4

B Step 2

C Step 1

D Step 3

💡 Hint

Look at the 'Action' column in execution_table for file closing.

If the JSON file contained a list instead of a dict, what would 'data' be after json.load(f)?

A A string

B A Python list

C A file object

D An integer

💡 Hint

json.load converts JSON arrays to Python lists, similar to how it converts JSON objects to dicts.

Concept Snapshot

Working with JSON files in Python:
- Use 'import json' module
- Open file with 'with open(filename, mode) as f'
- Read JSON with 'json.load(f)' to get Python dict/list
- Modify Python object as needed
- Write back with 'json.dump(obj, f)'
- 'with' block auto-closes file
- JSON maps to Python types (object->dict, array->list)

Full Transcript

This lesson shows how Python reads and writes JSON files. First, the program opens a JSON file in read mode using a 'with' statement, which safely manages the file. Then it reads the file content and parses it into a Python dictionary using json.load. The program accesses data from this dictionary, for example printing the value of the 'name' key. After the 'with' block ends, the file is automatically closed. This process allows Python programs to work with JSON data easily by converting it to familiar Python objects. Writing JSON back to a file uses a similar process with json.dump. Understanding these steps helps beginners handle JSON files safely and effectively.

Practice

1.

What is the main purpose of using JSON files in Python programs?

easy

A. To create graphical user interfaces

B. To execute Python code faster

C. To save and load data in a simple text format

D. To compile Python programs into executables

Solution

1. Step 1: Understand JSON file usage

   JSON files store data in a text format that is easy to read and share.

2. Step 2: Identify the correct purpose

   Saving and loading data simply matches the main use of JSON files in Python.

3. Final Answer:

   To save and load data in a simple text format -> Option C

4. Quick Check:

   JSON files = data storage [OK]

Hint: JSON files store data simply as text [OK]

Common Mistakes:

• Thinking JSON speeds up code execution
• Confusing JSON with GUI creation
• Assuming JSON compiles code

2.

Which of the following is the correct way to open a JSON file named data.json for reading in Python?

easy

A. open('data.json', 'r')

B. open('data.json', 'w')

C. open('data.json', 'a')

D. open('data.json', 'x')

Solution

1. Step 1: Understand file modes

   'r' mode opens a file for reading, 'w' for writing, 'a' for appending, 'x' for creating new file.

2. Step 2: Choose mode for reading JSON

   To read JSON data, the file must be opened in 'r' mode.

3. Final Answer:

   open('data.json', 'r') -> Option A

4. Quick Check:

   Read mode = 'r' [OK]

Hint: Use 'r' mode to read files [OK]

Common Mistakes:

• Using 'w' which overwrites the file
• Using 'a' which appends data
• Using 'x' which fails if file exists

3.

What will be the output of the following Python code?

import json

with open('data.json', 'w') as f:
    json.dump({'name': 'Alice', 'age': 30}, f)

with open('data.json', 'r') as f:
    data = json.load(f)
print(data['age'])

medium

A. Alice

B. 30

C. {'name': 'Alice', 'age': 30}

D. Error: file not found

Solution

1. Step 1: Write dictionary to JSON file

   The code writes {'name': 'Alice', 'age': 30} to 'data.json' using json.dump.

2. Step 2: Read JSON file and access 'age'

   json.load reads the file back as a dictionary, then data['age'] accesses the value 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   data['age'] = 30 [OK]

Hint: json.load returns Python dict from JSON file [OK]

Common Mistakes:

• Expecting string output instead of integer
• Confusing json.dump and json.load usage
• Assuming file read fails without writing first

4.

Identify the error in this code snippet that tries to read JSON data from a file:

import json

f = open('data.json', 'r')
data = json.load(f)
f.close()
print(data)

medium

A. Missing import statement for json

B. File not opened in write mode

C. File not closed properly

D. No error, code works correctly

Solution

1. Step 1: Check import and file open

   The code imports json and opens the file in 'r' mode correctly.

2. Step 2: Verify json.load and file close

   json.load reads JSON data properly, and file is closed with f.close().

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   Code reads JSON file correctly [OK]

Hint: json.load needs file opened in 'r' mode [OK]

Common Mistakes:

• Forgetting to import json
• Opening file in wrong mode
• Not closing the file after reading

5.

You want to save a Python dictionary {'a': 1, 'b': 2, 'c': 3} to a JSON file and then read it back. Which code snippet correctly does this and prints the value of key 'b'?

hard

A. import json
with open('file.json', 'w') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.load(f)
print(data['b'])

B. import json
with open('file.json', 'r') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'w') as f:
data = json.load(f)
print(data['b'])

C. import json
f = open('file.json', 'w')
json.load({'a': 1, 'b': 2, 'c': 3}, f)
f.close()
f = open('file.json', 'r')
data = json.dump(f)
f.close()
print(data['b'])

D. import json
with open('file.json', 'w') as f:
json.load({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.dump(f)
print(data['b'])

Solution

1. Step 1: Write dictionary to JSON file correctly

   json.dump writes Python dict to file opened in 'w' mode.

   import json
   with open('file.json', 'w') as f:
       json.dump({'a': 1, 'b': 2, 'c': 3}, f)
   with open('file.json', 'r') as f:
       data = json.load(f)
   print(data['b'])

   does this correctly.

2. Step 2: Read JSON file and access key 'b'

   json.load reads JSON back from file opened in 'r' mode, then data['b'] prints 2.

3. Final Answer:

   Code in Option A correctly saves and reads JSON, printing 2 -> Option A

4. Quick Check:

   json.dump to write, json.load to read [OK]

Hint: Use json.dump to write, json.load to read JSON files [OK]

Common Mistakes:

• Using json.load to write data
• Using json.dump to read data
• Opening files in wrong modes