File I/O & DataLesson 2 of 2

JSON & Regex

3 min

JSON and regular expressions are built into JavaScript as globals. In Python they are just as built in, but they live in standard library modules you import: json and re. (CSV is the one where JS usually needs an npm package; Python ships a csv module.)

The json module maps directly: JSON.parse() becomes json.loads() and JSON.stringify() becomes json.dumps(). The s at the end stands for string. For files there is a matching pair: json.load(file) reads from a file object and json.dump(obj, file) writes to one, with no intermediate string step.

JSON types map naturally: objects become dicts, arrays become lists, null becomes None, true/false become True/False. One difference: json.dumps() raises a TypeError for values it does not know, such as datetime, Decimal, or your own classes, where JSON.stringify would silently produce {} or call toJSON().

Python's regex module (re) uses a function-based API instead of regex literals. re.search() finds the first match anywhere, re.findall() returns all matches, re.sub() replaces (all matches by default), and re.fullmatch() checks the whole string. The trap: re.match() only matches at the START of the string, which confuses JS developers who expect it to search everywhere like str.match().

Quick reference

JavaScript	Note
JSON.parse(str)	json.loads(str)	`s` = from string
JSON.stringify(obj, null, 2)	json.dumps(obj, indent=2)	`s` = to string
fs + JSON.parse	json.load(f) / json.dump(obj, f)	Read/write file objects
str.match(/p/g)	re.findall(r"p", s)	All matches as a list
str.match(/p/)	re.search(r"p", s)	search = anywhere
str.replace(/p/g, r)	re.sub(r"p", r, s)	Replaces all by default
/p/.test(str)	bool(re.search(r"p", s))	No `.test()` method

Side by side

JavaScript

// JavaScript
const obj = JSON.parse(jsonString);
const str = JSON.stringify(obj, null, 2);

const match = text.match(/\d+/);
const all = text.match(/\d+/g);
const replaced = text.replace(/old/g, "new");
/^test/.test(text);

Python

# Python JSON (standard library)
import json
obj = json.loads(json_string)       # parse
text = json.dumps(obj, indent=2)    # stringify

with open("data.json", encoding="utf-8") as f:
    data = json.load(f)              # read file
with open("out.json", "w", encoding="utf-8") as f:
    json.dump(data, f, indent=2)     # write file

# Regex (standard library)
import re
match = re.search(r"\d+", text)     # first match
if match:
    print(match.group())
all_nums = re.findall(r"\d+", text) # all matches
replaced = re.sub(r"old", "new", text)
valid = bool(re.match(r"test", s))  # only at the start!

Exercise

Real Python, running in your browser

Write Python code that:

1. Parse this string into data: '{"users": [{"name": "Alice", "age": 30}]}' 2. Store the first user's name in first_name 3. Convert data back to formatted JSON (indent=2) in formatted 4. Find all emails in text = "Contact alice@dev.com or bob@test.org" and store them in emails 5. Store text with every email replaced by "[REDACTED]" in redacted

solution.py

Ctrl + Enter runCtrl + Shift + Enter checkTab indentEsc then Tab to leave the editorCtrl + / comment