Modules and Packages · Lesson 27 of 39

A Tour of the Standard Library

4 min read Updated Nov 7, 2026 Saurabh Samir

On this page

datetime: dates and times
random: chance and shuffling
pathlib: files and folders
re: regular expressions
itertools: loop superpowers
A few more gems
Practice
Summary

Python is famous for being "batteries included": it ships with hundreds of modules for everyday jobs, no installation needed. Knowing what is already there saves you from reinventing the wheel – or installing a package you don't need. Here are the modules you will reach for most.

In this lesson

Dates and times with datetime
Random choices with random
Files and folders with pathlib and os
Text patterns with re, and handy tools from itertools

Standard library highlights

Built in – just import

Dates & timedatetime, time, zoneinfo, calendar
Files & systempathlib, os, shutil, sys
Data formatsjson, csv, sqlite3, zipfile
Textre, string, textwrap
Maths & datamath, random, statistics, decimal
Toolboxcollections, itertools, functools, logging

datetime: dates and times

from datetime import date, datetime, timedelta

exam = date(2026, 12, 15)
today = date(2026, 11, 7)
print("Days left:", (exam - today).days)
print("Revision starts:", exam - timedelta(weeks=4))

meeting = datetime(2026, 11, 7, 15, 30)
print(meeting.strftime("%d %b %Y, %I:%M %p"))
print(datetime.strptime("07/11/2026", "%d/%m/%Y").date())
print(exam.strftime("%A"))

Output

Days left: 38
Revision starts: 2026-11-17
07 Nov 2026, 03:30 PM
2026-11-07
Tuesday

strftime turns a date into text, strptime parses text into a date. In real programs use date.today() or datetime.now() for the current moment, and the zoneinfo module for time zones such as "Asia/Kolkata".

random: chance and shuffling

import random

random.seed(7)        # same "random" results every run – useful for tests
print(random.randint(1, 6))
print(random.choice(["rock", "paper", "scissors"]))
cards = ["A", "K", "Q", "J"]
random.shuffle(cards)
print(cards)
print(random.sample(range(1, 50), 6))

Output

3
rock
['K', 'A', 'Q', 'J']
[5, 35, 7, 24, 38, 4]

Security note

random is predictable by design. For passwords, OTPs or tokens use the secrets module: secrets.token_urlsafe(16) or secrets.randbelow(10**6).

pathlib: files and folders

pathlib treats paths as objects and works the same on Windows, macOS and Linux:

from pathlib import Path

reports = Path("reports")
reports.mkdir(exist_ok=True)
file = reports / "summary.txt"          # join paths with /
file.write_text("Sales up 12%")

print(file.name, file.suffix, file.stem)
print(file.read_text())
print(file.exists(), sorted(p.name for p in reports.glob("*.txt")))

Output

summary.txt .txt summary
Sales up 12%
True ['summary.txt']

The older os and os.path functions do similar jobs and you will see them in existing code; os.environ is still the way to read environment variables such as API keys:

import os
print(os.environ.get("APP_MODE", "development"))

Output

development

re: regular expressions

Regular expressions describe text patterns. They look cryptic at first, but a few patterns go a long way:

import re

text = "Call 98765-43210 or 91234-56789, mail help@learnfrenzy.com"
print(re.findall(r"\d{5}-\d{5}", text))
print(re.search(r"[\w.]+@[\w.]+", text).group())
print(re.sub(r"\d", "#", "OTP 4821"))
print(bool(re.fullmatch(r"[A-Z]{5}\d{4}[A-Z]", "ABCDE1234F")))   # PAN format

Output

['98765-43210', '91234-56789']
help@learnfrenzy.com
OTP ####
True

Pattern	Matches
`\d` / `\w` / `\s`	digit / letter, digit or _ / whitespace
`[A-Z]`	one character from the range
`+` `*` `?` `{5}`	one or more / zero or more / optional / exactly 5
`^` `$`	start / end of the text

itertools: loop superpowers

from itertools import combinations, permutations, product, chain, accumulate

print(list(combinations(["A", "B", "C"], 2)))
print(len(list(permutations("abc"))))
print(list(product([0, 1], repeat=2)))
print(list(chain([1, 2], [3], [4, 5])))
print(list(accumulate([100, 50, 25])))   # running total

Output

[('A', 'B'), ('A', 'C'), ('B', 'C')]
6
[(0, 0), (0, 1), (1, 0), (1, 1)]
[1, 2, 3, 4, 5]
[100, 150, 175]

A few more gems

import statistics, textwrap, calendar

marks = [72, 85, 91, 66, 85]
print(statistics.mean(marks), statistics.median(marks), statistics.mode(marks))
print(textwrap.shorten("Python is a wonderful first programming language", width=30))
print(calendar.isleap(2028))

Output

79.8 85 85
Python is a wonderful [...]
True

Also worth a look later: json and csv (chapter 11), logging for proper log messages, sqlite3 for a built-in database, argparse for command-line options, and unittest for tests.

Practice

Try it yourself

How many days old is someone born on 15 August 2000, on 7 November 2026?
Show solution
```
from datetime import date
print((date(2026, 11, 7) - date(2000, 8, 15)).days)
```
Output
```
9580
```

Extract all hashtags from "Loving #python and #AI at #LearnFrenzy".

Show solution

import re
print(re.findall(r"#\w+", "Loving #python and #AI at #LearnFrenzy"))

Output

['#python', '#AI', '#LearnFrenzy']

Generate a 6-digit OTP safely.

Show solution

import secrets
otp = f"{secrets.randbelow(10**6):06d}"
print(len(otp), otp.isdigit())

Output

6 True

Quick check

Which function turns a date into formatted text?

strftime(). strptime() does the reverse.

Why use secrets instead of random for OTPs?

random is predictable; secrets uses a cryptographically secure source.

How do you join folder and file names with pathlib?

With the / operator: Path("reports") / "summary.txt".

Summary

Check the standard library before installing anything.
datetime for dates, random/secrets for chance and security, pathlib for files.
re finds patterns in text; itertools and statistics save many loops.

Interview practice: Python modules, packages and virtual environments.

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