Python JSON – Reading, Writing, and Converting JSON Data

Parsing JSON Strings – json.loads()

json.loads() converts a JSON string into Python objects.

import json

json_string = '''{
    "name": "Alice",
    "age": 30,
    "skills": ["Python", "SQL"],
    "active": true
}'''

data = json.loads(json_string)

print(data["name"])    # Alice
print(data["age"])     # 30
print(data["skills"])  # ['Python', 'SQL']
print(type(data))      # <class 'dict'>

Converting to JSON String – json.dumps()

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

import json

python_dict = {
    "name": "Bob",
    "scores": [95, 87, 92],
    "passed": True,
    "grade": None
}

# Basic conversion
json_str = json.dumps(python_dict)
print(json_str)

# Pretty-printed with indentation
pretty = json.dumps(python_dict, indent=2)
print(pretty)

Reading and Writing JSON Files

Use json.load() and json.dump() for file operations.

import json

# Write JSON to file
data = {"users": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}

with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

# Read JSON from file
with open("data.json", "r") as f:
    loaded = json.load(f)

print(loaded["users"][0]["name"])  # Alice

Real-World: Processing API Responses

Most web APIs return JSON. Here is the standard pattern for consuming one.

import json
import urllib.request

# Fetch from a public API
url = "https://api.github.com/users/python"

with urllib.request.urlopen(url) as response:
    data = json.loads(response.read())

print(data["name"])         # Python
print(data["public_repos"]) # number of repos
print(data["followers"])    # follower count

Python ↔ JSON Type Mapping

JSON has a limited set of types. Python converts automatically.

# Python → JSON type mapping:
# dict     → object   {}
# list     → array    []
# str      → string   ""
# int/float→ number   42, 3.14
# True     → true
# False    → false
# None     → null

# Verify:
print(json.dumps({"a": True, "b": None, "c": [1,2]}))

JSON in Python: dumps, loads, and the Type Mismatches

JSON is the lingua franca of APIs. Python's json module converts between JSON text and Python objects — but the two type systems don't line up perfectly, and the gaps cause real bugs.

import json
data = {"name": "Ann", "active": True, "scores": [9, 8]}

text = json.dumps(data, indent=2)   # Python → JSON string (serialize)
back = json.loads(text)             # JSON string → Python (parse)

Three gotchas:

• Dict keys become strings. json.dumps({1: "a"}) gives {"1": "a"} — integer keys round-trip back as strings.
• datetime/set/bytes aren't serializable — dumps raises TypeError. Convert them first (e.g. date.isoformat()) or pass a custom default= function.
• Tuples become lists on the round trip — JSON has no tuple type.

Use json.dump(obj, file)/json.load(file) (no "s") to read/write files directly. For untrusted input, wrap loads in try/except json.JSONDecodeError.

Interview Questions on JSON

• What is JSON and why is it widely used?
• What is the difference between json.loads and json.load?
• What is the difference between json.dumps and json.dump?
• How do Python types map to JSON types?
• How do you pretty-print JSON output?
• How do you serialize a non-standard type like datetime?
• What exception is raised when parsing invalid JSON?

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