Sameer Qaiser

Posted on Sep 28

Python JSON: Working with Data Files

#python #beginners #programming #json

📌 Quick Info

Topic: Working with JSON in Python
Target Audience: Beginners who know files and dictionaries
Goal: Understand how to save and load structured data

1. Introduction

"I could write text to a file. But what if I needed to save a whole dictionary? A list of users? A config file? That's when I learned about JSON — and now it's how every one of my projects saves data."

2. The Problem (Saving Complex Data to a Text File)

user = {"name": "Ali", "age": 25, "city": "Karachi"}

with open("user.txt", "w") as file:
    file.write(str(user))

What’s in the file:

{'name': 'Ali', 'age': 25, 'city': 'Karachi'}

Problem: It’s ugly. It’s not standard. Other programs can’t read it easily.

3. The Solution (JSON)

import json

user = {"name": "Ali", "age": 25, "city": "Karachi"}

with open("user.json", "w") as file:
    json.dump(user, file)

What’s in the file:

{"name": "Ali", "age": 25, "city": "Karachi"}

Clean. Standard. Readable by almost any language.

4. Reading JSON Back

import json

with open("user.json", "r") as file:
    user = json.load(file)

print(user["name"])   # Ali
print(user["age"])    # 25

The Dictionary comes back exactly as it was saved.

5. How It Works (Line By Line)

import json — Loads Python’s built-in JSON module.

json.dump(data, file) — Writes a Python object to a file as JSON.

json.load(file) — Reads JSON from a file back into a Python object.

json.dumps(data) — Converts a Python object to a JSON string (instead of writing to a file).

json.loads(string) — Parses a JSON string into Python.

6. JSON vs Python: What Maps to What

Python	JSON
dict	Object
List	Array
str	string
int/ float	number
True / False	true / false
None	null

The names are different, but the structure is the same.

7. Real Example (My Practice)

import json

users = [
    {"name": "Ali", "score": 85},
    {"name": "Sara", "score": 92},
    {"name": "Ahmed", "score": 78}
]

with open("scores.json", "w") as file:
    json.dump(users, file, indent=2)

with open("scores.json", "r") as file:
    loaded = json.load(file)

for user in loaded:
    print(f"{user['name']}: {user['score']}")

Output:

Ali: 85
Sara: 92
Ahmed: 78

The indent=2 makes the file human-readable.

8. Common Mistakes Beginners Make

Mistake 1: Forgetting to import json

json.dump(...)

Mistake 2: Confusing dump and dumps

json.dump(): writes to a file
json.dumps(): returns a string

Mistake 3: Trying to save non-JSON types

import json
json.dumps({"date": datetime.now()})

JSON only supports basic types. Convert to strings first.

9. What I learned

-) JSON is the standard for saving structured data
-) json.dump() writes to a file, json.load() reads from it
-) json.dumps() and json.loads() work with strings
-) indent=2 makes files human-readable
-) Not everything can be saved — only dicts, lists, strings, numbers, booleans, and None

10. Conclusion

“JSON solved a problem I didn't know I had. Now every project I build saves its data as JSON — because it's readable, portable, and just works. Once you learn it, you'll never go back to saving dictionaries as text.”

DEV Community