Learn how to use Python's most useful built-in modules — os, sys, math, datetime, random, and json.

Standard Library

Python comes with a huge collection of built-in modules called the standard library. These are installed with Python — no pip needed. They cover file system operations, math, dates, randomness, JSON, and much more.

This file covers the modules you will use in almost every real project.

math — mathematical operations

import math

print(math.pi)          # 3.141592653589793
print(math.e)           # 2.718281828459045

print(math.sqrt(144))   # 12.0 — square root
print(math.pow(2, 10))  # 1024.0 — power
print(math.floor(3.9))  # 3 — round down
print(math.ceil(3.1))   # 4 — round up
print(math.abs(-5))     # use built-in abs() instead
print(math.factorial(5)) # 120 — 5!
print(math.log(100, 10)) # 2.0 — log base 10
print(math.log2(1024))   # 10.0 — log base 2

Trigonometry:

print(math.sin(math.pi / 2))   # 1.0
print(math.cos(0))              # 1.0
print(math.degrees(math.pi))   # 180.0 — radians to degrees
print(math.radians(180))       # 3.14... — degrees to radians

Useful constants and checks:

print(math.inf)              # inf — positive infinity
print(math.isinf(math.inf))  # True
print(math.isnan(float('nan')))  # True — check for not-a-number

random — generating random values

import random

Random numbers:

print(random.random())          # float between 0.0 and 1.0
print(random.uniform(1, 10))    # float between 1 and 10
print(random.randint(1, 100))   # integer between 1 and 100 inclusive
print(random.randrange(0, 100, 5))  # random from 0,5,10,...,95

Random choices from a sequence:

fruits = ["apple", "banana", "mango", "orange"]

print(random.choice(fruits))         # one random item
print(random.choices(fruits, k=3))   # 3 random items with repetition
print(random.sample(fruits, k=3))    # 3 random items without repetition

Shuffling:

cards = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
random.shuffle(cards)
print(cards)   # [7, 2, 9, 1, 5, ...] — shuffled in place

Seeding — reproducible randomness:

random.seed(42)
print(random.randint(1, 100))   # always the same number with seed 42
print(random.randint(1, 100))   # always the same second number

Seeding is useful for testing — same seed always produces the same sequence of random values.

random is not cryptographically secure. Never use it for passwords, tokens, or security-sensitive code. Use Python's secrets module for that instead:

import secrets
print(secrets.randbelow(100))         # cryptographically secure random int
print(secrets.token_hex(16))         # secure random hex string
print(secrets.token_urlsafe(16))     # secure random URL-safe string

datetime — working with dates and times

from datetime import datetime, date, time, timedelta

Today's date and current time:

today = date.today()
print(today)           # 2026-06-02

now = datetime.now()
print(now)             # 2026-06-02 14:35:22.123456

Creating specific dates:

birthday = date(1999, 3, 15)
print(birthday)          # 1999-03-15
print(birthday.year)     # 1999
print(birthday.month)    # 3
print(birthday.day)      # 15

Creating specific datetimes:

meeting = datetime(2026, 6, 15, 9, 30, 0)
print(meeting)   # 2026-06-15 09:30:00

timedelta — differences and arithmetic:

today = date.today()
one_week = timedelta(weeks=1)
ten_days = timedelta(days=10)

print(today + one_week)    # 7 days from now
print(today - ten_days)    # 10 days ago

# difference between two dates
start = date(2026, 1, 1)
end = date(2026, 6, 2)
diff = end - start
print(diff.days)   # 152

Formatting dates — strftime:

now = datetime.now()

print(now.strftime("%Y-%m-%d"))           # 2026-06-02
print(now.strftime("%d/%m/%Y"))           # 02/06/2026
print(now.strftime("%B %d, %Y"))          # June 02, 2026
print(now.strftime("%I:%M %p"))           # 02:35 PM
print(now.strftime("%A, %B %d, %Y"))      # Tuesday, June 02, 2026

Common format codes:

Code	Meaning	Example
`%Y`	4-digit year	2026
`%m`	Month as number	06
`%d`	Day as number	02
`%B`	Full month name	June
`%A`	Full weekday name	Tuesday
`%H`	Hour 24h	14
`%I`	Hour 12h	02
`%M`	Minutes	35
`%p`	AM or PM	PM

Parsing a string into a date — strptime:

date_string = "02/06/2026"
parsed = datetime.strptime(date_string, "%d/%m/%Y")
print(parsed)          # 2026-06-02 00:00:00
print(parsed.year)     # 2026

os — interacting with the operating system

import os

Current directory:

print(os.getcwd())         # /home/ali/projects
os.chdir("/home/ali")      # change directory

Listing files:

files = os.listdir(".")    # list current directory
print(files)               # ['main.py', 'utils.py', 'data.json']

Path operations:

print(os.path.exists("main.py"))       # True or False
print(os.path.isfile("main.py"))       # True
print(os.path.isdir("my_folder"))      # True or False
print(os.path.join("home", "ali", "file.txt"))  # home/ali/file.txt
print(os.path.basename("/home/ali/file.txt"))   # file.txt
print(os.path.dirname("/home/ali/file.txt"))    # /home/ali
print(os.path.splitext("report.pdf"))           # ('report', '.pdf')

Creating and removing directories:

os.mkdir("new_folder")           # create one folder
os.makedirs("a/b/c")             # create nested folders
os.rmdir("new_folder")           # remove empty folder

Environment variables:

home = os.environ.get("HOME", "not set")
print(home)   # /home/ali

# set one for this session
os.environ["MY_VAR"] = "hello"

Running shell commands:

os.system("ls -la")       # runs a shell command

For modern path handling, prefer pathlib over os.path. It is more readable and object-oriented. We cover it in the File Handling section.

sys — system-specific information

import sys

print(sys.version)      # Python version string
print(sys.platform)     # 'linux', 'darwin', 'win32'
print(sys.executable)   # path to the Python interpreter

Command line arguments:

# run as: python3 script.py Ali 22
print(sys.argv)         # ['script.py', 'Ali', '22']
print(sys.argv[0])      # script.py — the script name
print(sys.argv[1])      # Ali — first argument

Exiting the program:

if len(sys.argv) < 2:
    print("Usage: python3 script.py <name>")
    sys.exit(1)    # exit with error code 1

Standard streams:

sys.stdout.write("Hello\n")   # same as print()
sys.stderr.write("Error!\n")  # write to error stream

json — working with JSON data

JSON is the most common format for sending and receiving data on the web. Python's json module makes it easy to work with.

import json

Python dict → JSON string (serializing):

person = {
    "name": "Ali",
    "age": 22,
    "city": "Lahore",
    "skills": ["Python", "JavaScript"],
    "active": True
}

json_string = json.dumps(person)
print(json_string)
# {"name": "Ali", "age": 22, "city": "Lahore", "skills": ["Python", "JavaScript"], "active": true}

# pretty printed
json_pretty = json.dumps(person, indent=4)
print(json_pretty)

Output:

{
    "name": "Ali",
    "age": 22,
    "city": "Lahore",
    "skills": [
        "Python",
        "JavaScript"
    ],
    "active": true
}

JSON string → Python dict (deserializing):

json_string = '{"name": "Ali", "age": 22, "active": true}'

person = json.loads(json_string)
print(person["name"])    # Ali
print(person["active"])  # True — JSON true becomes Python True
print(type(person))      # <class 'dict'>

JSON type mapping:

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

Reading and writing JSON files:

# write to file
with open("data.json", "w") as f:
    json.dump(person, f, indent=4)

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

print(data["name"])   # Ali

json.dump() and json.load() work with files. json.dumps() and json.loads() work with strings. The s stands for string.

A real example — putting it together

A simple program that generates a daily report and saves it as JSON:

import json
import random
from datetime import datetime

def generate_report(team):
    report = {
        "generated_at": datetime.now().strftime("%Y-%m-%d %H:%M:%S"),
        "team": team,
        "members": []
    }

    names = ["Ali", "Sara", "Omar", "Fatima", "Zainab"]

    for name in names:
        report["members"].append({
            "name": name,
            "tasks_completed": random.randint(3, 12),
            "hours_logged": round(random.uniform(5, 9), 1)
        })

    return report

report = generate_report("Backend")

# save to file
with open("report.json", "w") as f:
    json.dump(report, f, indent=4)

# print summary
print(f"Report for: {report['team']} team")
print(f"Generated: {report['generated_at']}")
print(f"Members: {len(report['members'])}")

total_tasks = sum(m["tasks_completed"] for m in report["members"])
print(f"Total tasks completed: {total_tasks}")

Summary

Module	Use for
`math`	Math functions, constants, trigonometry
`random`	Random numbers, choices, shuffling
`secrets`	Cryptographically secure random values
`datetime`	Dates, times, formatting, arithmetic
`os`	File system, paths, environment variables
`sys`	Python info, command line args, exit
`json`	Read and write JSON data

Standard Library

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