Standard Library

Python comes with "batteries included" - a rich collection of modules that cover common programming tasks. The standard library includes over 200 modules for everything from text processing to networking. You do not need to install anything - just import and use.

Common Standard Library Modules

The datetime module provides classes for working with dates and times. You can get the current date and time, perform arithmetic on dates, format dates as strings, and parse strings into dates. It is essential for any application dealing with timestamps.

Getting Current Date and Time

from datetime import datetime, date, time, timedelta

# Current date and time
now = datetime.now()
print(now)  # 2026-01-21 14:30:45.123456

# Just the date
today = date.today()
print(today)  # 2026-01-21

# Access individual components
print(f"Year: {now.year}")    # 2026
print(f"Month: {now.month}")  # 1
print(f"Day: {now.day}")      # 21

Creating Specific Dates

from datetime import datetime, date

# Create a specific date
birthday = date(1995, 6, 15)
print(birthday)  # 1995-06-15

# Create a specific datetime
event = datetime(2026, 12, 31, 23, 59, 59)
print(event)  # 2026-12-31 23:59:59

# Parse a date string
date_str = "2026-01-21"
parsed = datetime.strptime(date_str, "%Y-%m-%d")
print(parsed)  # 2026-01-21 00:00:00

Date Arithmetic with timedelta

from datetime import datetime, timedelta

now = datetime.now()

# Add days to a date
one_week_later = now + timedelta(days=7)
print(f"One week from now: {one_week_later}")

# Calculate difference between dates
deadline = datetime(2026, 2, 1)
remaining = deadline - now
print(f"Days until deadline: {remaining.days}")

# Complex timedelta
duration = timedelta(hours=5, minutes=30)
meeting_end = now + duration

Formatting Dates as Strings

from datetime import datetime

now = datetime.now()

# Format with strftime
print(now.strftime("%Y-%m-%d"))        # 2026-01-21
print(now.strftime("%B %d, %Y"))       # January 21, 2026
print(now.strftime("%I:%M %p"))        # 02:30 PM
print(now.strftime("%A"))              # Wednesday

# ISO format (standard)
print(now.isoformat())  # 2026-01-21T14:30:45.123456

from datetime import datetime, date

def age_in_days(birth_str):
    birth = datetime.strptime(birth_str, "%Y-%m-%d").date()
    today = date.today()
    return (today - birth).days

# Test
print(age_in_days("2000-06-15"))  # Days since June 15, 2000

from datetime import datetime

def format_today():
    return datetime.now().strftime("%A, %B %d, %Y")

# Test
print(format_today())  # Wednesday, January 21, 2026

from datetime import date

def days_until_birthday(birth_month, birth_day):
    today = date.today()
    this_year_bday = date(today.year, birth_month, birth_day)
    
    if this_year_bday < today:
        next_bday = date(today.year + 1, birth_month, birth_day)
    else:
        next_bday = this_year_bday
    
    return (next_bday - today).days

print(days_until_birthday(6, 15))  # Days until June 15

The random module generates pseudo-random numbers for games, simulations, testing, and any situation where you need unpredictable values. For cryptographic purposes, use the secrets module instead.

Basic Random Functions

import random

# Random float between 0 and 1
print(random.random())  # 0.7234...

# Random integer in range (inclusive)
print(random.randint(1, 6))  # Dice roll: 1-6

# Random choice from a sequence
colors = ["red", "green", "blue"]
print(random.choice(colors))  # e.g., "green"

# Random sample (multiple unique choices)
winners = random.sample(range(1, 100), 3)
print(winners)  # e.g., [42, 17, 88]

Shuffling and More

import random

# Shuffle a list in place
deck = list(range(1, 53))
random.shuffle(deck)
print(deck[:5])  # First 5 cards

# Random float in range
temp = random.uniform(20.0, 30.0)
print(f"Temperature: {temp:.1f}")

# Reproducible results with seed
random.seed(42)
print(random.randint(1, 100))  # Always 82
random.seed(42)
print(random.randint(1, 100))  # Always 82 again

The os module provides functions for interacting with the operating system. The newer pathlib module offers an object-oriented approach to file paths. Both are essential for file system operations.

os Module Basics

import os

# Current working directory
print(os.getcwd())  # /home/user/project

# List directory contents
files = os.listdir(".")
print(files)  # ['main.py', 'data', 'README.md']

# Environment variables
home = os.environ.get("HOME", "Not set")
print(f"Home: {home}")

# Check if path exists
print(os.path.exists("main.py"))  # True
print(os.path.isdir("data"))      # True

pathlib - Modern Path Handling

from pathlib import Path

# Create path objects
current = Path.cwd()
home = Path.home()

# Build paths with / operator
data_file = current / "data" / "users.json"
print(data_file)  # /home/user/project/data/users.json

# Path properties
print(data_file.name)      # users.json
print(data_file.suffix)    # .json
print(data_file.parent)    # /home/user/project/data
print(data_file.exists())  # True or False

import random

def roll_dice(n):
    total = sum(random.randint(1, 6) for _ in range(n))
    return total

# Test - roll 3 dice
print(roll_dice(3))  # e.g., 11

import random
import string

def generate_password(length):
    chars = string.ascii_letters + string.digits
    return ''.join(random.choice(chars) for _ in range(length))

print(generate_password(12))  # e.g., "aB3kL9mNpQ2x"

from pathlib import Path

def find_python_files(directory):
    path = Path(directory)
    return list(path.glob("*.py"))

# Test
py_files = find_python_files(".")
for f in py_files:
    print(f.name)

from pathlib import Path

def directory_size_kb(directory):
    path = Path(directory)
    total = sum(f.stat().st_size for f in path.iterdir() if f.is_file())
    return total / 1024

# Test
size = directory_size_kb(".")
print(f"Directory size: {size:.2f} KB")

The collections module provides specialized container datatypes beyond the built-in list, dict, set, and tuple. These include Counter for counting, defaultdict for default values, and namedtuple for lightweight objects.

Counter - Count Occurrences

from collections import Counter

# Count items in a list
fruits = ["apple", "banana", "apple", "cherry", "banana", "apple"]
counts = Counter(fruits)
print(counts)  # Counter({'apple': 3, 'banana': 2, 'cherry': 1})

# Most common items
print(counts.most_common(2))  # [('apple', 3), ('banana', 2)]

# Count characters in a string
letter_counts = Counter("mississippi")
print(letter_counts)  # Counter({'i': 4, 's': 4, 'p': 2, 'm': 1})

defaultdict - Default Values

from collections import defaultdict

# Regular dict raises KeyError for missing keys
# defaultdict provides a default value

# Group items by category
words = ["apple", "ant", "ball", "banana", "cat"]
by_letter = defaultdict(list)

for word in words:
    by_letter[word[0]].append(word)

print(dict(by_letter))
# {'a': ['apple', 'ant'], 'b': ['ball', 'banana'], 'c': ['cat']}

namedtuple - Lightweight Classes

from collections import namedtuple

# Create a Point class with x, y attributes
Point = namedtuple("Point", ["x", "y"])

p1 = Point(10, 20)
p2 = Point(x=5, y=15)

print(p1.x, p1.y)  # 10 20
print(p2)          # Point(x=5, y=15)

# Useful for structured data
Person = namedtuple("Person", ["name", "age", "city"])
user = Person("Alice", 30, "NYC")

from collections import Counter

def most_common_word(words):
    counts = Counter(words)
    return counts.most_common(1)[0][0]

words = ["the", "cat", "sat", "on", "the", "mat", "the"]
print(most_common_word(words))  # "the"

from collections import defaultdict

def group_by_length(words):
    groups = defaultdict(list)
    for word in words:
        groups[len(word)].append(word)
    return dict(groups)

words = ["a", "to", "tea", "ted", "ten", "i", "in"]
print(group_by_length(words))
# {1: ['a', 'i'], 2: ['to', 'in'], 3: ['tea', 'ted', 'ten']}

from collections import namedtuple

Student = namedtuple("Student", ["name", "grades"])

def average_grade(student):
    return sum(student.grades) / len(student.grades)

alice = Student("Alice", [90, 85, 92, 88])
print(f"{alice.name}: {average_grade(alice):.1f}")  # Alice: 88.8