Python: Standard Library Basics (Part 1)

Last updated: 2026-09-22

One of Python's greatest features is "Batteries Included" — a full suite of standard libraries comes with every Python installation. This lesson covers four of the most commonly used modules: random, time, sys, and os. Even learning just the most popular functions from each will help you solve many real-world problems.

Related Built-in Functions：exit()

1. random — Random Numbers

import random

# Random float [0, 1)
print(random.random())              # 0.3745...

# Random integer [a, b] (inclusive)
print(random.randint(1, 10))        # Random integer between 1 and 10

# Random choice
fruits = ["apple", "banana", "orange", "grape"]
print(random.choice(fruits))        # Pick one randomly

# Random sample (no replacement)
print(random.sample(fruits, 2))     # Pick 2 randomly

# Shuffle order
cards = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
random.shuffle(cards)
print(cards)                        # Order is shuffled

▶ Example: Lottery Program (Difficulty ⭐)

Output:

# Executed successfully

2. time — Time Handling

import time

# Current timestamp (seconds since 1970-01-01)
print(time.time())                  # 1758588800.123

# Pause execution (seconds)
print("Start")
time.sleep(1)                       # Pause for 1 second
print("1 second later")

# Format current time
print(time.strftime("%Y-%m-%d %H:%M:%S"))       # 2026-06-23 16:00:00

▶ Example: Timer (Difficulty ⭐)

Output:

\r⏰ Time

3. sys — System Interaction

import sys

# Python version
print(sys.version)                  # 3.13.0 ...

# Command-line arguments (when running python script.py a b c)
print(sys.argv)                     # ['script.py', 'a', 'b', 'c']

# Exit the program
# sys.exit(0)                       # Normal exit

# Standard input
# line = sys.stdin.readline()       # Read one line of input

▶ Example: Command-Line Calculator (Difficulty ⭐⭐)

Output:

Usage: python calculator.py num1 operator num2
Example: python calculator.py 3 + 5
Error: cannot divide by zero

4. os — Operating System

import os

# Current working directory
print(os.getcwd())                  # D:\project

# Change directory
# os.chdir("/tmp")

# List directory contents
for item in os.listdir("."):
    print(item)

# Create directory
# os.mkdir("new_folder")

# Check paths
print(os.path.exists("test.txt"))   # True
print(os.path.isfile("test.txt"))   # True
print(os.path.isdir("data"))        # True

# Join paths
path = os.path.join("data", "files", "test.txt")
print(path)                         # data/files/test.txt

▶ Example: Batch File Renaming (Difficulty ⭐⭐)

Output:

Batch rename example (display only, not executed):
Iterate all .txt files, rename to prefix_001.txt format

5. Common Use Cases

• random: Lottery, CAPTCHA, random test data generation, random game events.
• time: Delayed execution, performance timing, timers, scheduled tasks.
• sys: Read command-line arguments, control program exit, get Python environment info.
• os: File management, directory operations, path handling, batch renaming.

Related Built-in Functions：exit()

❓ FAQ

📖 Summary

• random: randint(), choice(), sample(), shuffle() — random selection and generation
• time: time() timestamp, sleep() pause, strftime() formatting
• sys: argv command-line arguments, exit() program exit
• os: File path checks, directory operations, file renaming
• These four modules are the "starter pack" for Python's standard library, used frequently in daily development

📝 Exercises

1. Basic (Difficulty ⭐): Write a program that generates 10 random integers between 1 and 100, outputs them and their average.

2. Intermediate (Difficulty ⭐⭐): Write a password generator function generate_password(length=8) that creates random passwords containing uppercase, lowercase, and digits. Hint: Use string.ascii_letters + string.digits as the character pool; use random.choice() to pick characters.

3. Challenge (Difficulty ⭐⭐⭐): Write a "folder statistics tool." Given a directory path (passed via sys.argv), count the number of files and total size for each file type (extension). Hint: Use os.listdir() to iterate, os.path.splitext() for extension, os.path.getsize() for file size.

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