Python — JSON

JSON ↔ Python dictionary

JSON is the internet's data format; Python speaks it natively via the json module:

import json

data = '{"name": "Ada", "age": 36, "langs": ["py", "js"]}'
user = json.loads(data)        # parse string → dict

user["name"]                   # "Ada"
user["langs"][0]               # "py"

Python	JSON
dict	object
list	array
str	string
int / float	number
True / False	true / false
None	null

dumps — the reverse direction

user = {"name": "Ada", "admin": False}

json.dumps(user)
# '{"name": "Ada", "admin": false}'

# pretty-printed for humans:
print(json.dumps(user, indent=2))
# {
#   "name": "Ada",
#   "admin": false
# }

Handy flags: indent=2 · sort_keys=True · separators=(",", ":") for compact.

Files: load & dump (no s!)

# write
with open("config.json", "w") as f:
    json.dump(settings, f, indent=2)

# read
with open("config.json") as f:
    settings = json.load(f)

Memory aid: the s versions work with strings; without s — files.

Talking to APIs (the real-world use)

import urllib.request  # or requests after pip install

with urllib.request.urlopen("https://api.github.com/users/octocat") as r:
    user = json.loads(r.read())

user["name"], user["public_repos"]

Handling bad JSON gracefully

try:
    data = json.loads(user_input)
except json.JSONDecodeError as e:
    print(f"Invalid JSON at line {e.lineno}: {e.msg}")
    data = {}

Non-string keys gotcha

JSON keys are ALWAYS strings:

json.dumps({1: "a"})       # '{"1": "a"}'  — int key became text!
json.loads('{"1": "a"}')   # {"1": "a"}    — still a STRING key

Round-tripping dicts with non-string keys silently mutates them.

Custom objects need help

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

json.dumps(User("Ada"))
# TypeError: Object of type User is not JSON serializable

json.dumps(User("Ada"), default=lambda o: o.__dict__)   # ✓
User(**json.loads('{"name":"Ada"}'))                     # back to object

Mini Practice

1. Round-trip a nested dict through dumps/loads.
2. Pretty-print with indent=2 + sorted keys.
3. Save/load a settings dict to a real file.
4. Fetch any public API; extract three fields.
5. Catch JSONDecodeError from deliberately broken input.

Next: RegEx →