Modules & packages.
Stop writing from scratch.

# 1. import module — access as module.name
import math
print(math.sqrt(16)) # 4.0
print(math.pi) # 3.141592653589793

# 2. import with alias — useful for long names
import datetime as dt
now = dt.datetime.now()
print(now.year) # 2026

# 3. from module import specific name
from math import sqrt, pi
print(sqrt(25)) # 5.0
print(pi) # 3.141592653589793

# 4. from module import name with alias
from random import randint as rnd
print(rnd(1, 10)) # random int between 1 and 10

# AVOID: from module import *
# This dumps everything into your namespace and makes code hard to trace.

# os — filesystem paths, environment
import os
print(os.getcwd()) # current directory
print(os.listdir(".")) # files in current dir

# pathlib — modern path handling (preferred over os.path)
from pathlib import Path
p = Path("data") / "file.txt"
print(p.exists())

# json — read/write JSON
import json
data = json.dumps({"a": 1}) # '{"a": 1}'

# datetime — dates and times
from datetime import datetime, timedelta
print(datetime.now())
print(datetime.now() + timedelta(days=7)) # a week from now

# random — random numbers and choices
import random
print(random.choice(["a", "b", "c"]))
print(random.randint(1, 100))

# re — regular expressions
import re
emails = re.findall(r"\S+@\S+", "contact a@b.com or c@d.org")

# time — sleep, timestamps
import time
print(time.time()) # unix timestamp
# time.sleep(1) # wait 1 second

# collections — Counter, defaultdict, namedtuple
from collections import Counter
counts = Counter(["a", "b", "a"])
print(counts) # Counter({'a': 2, 'b': 1})

# sys — system info, argv, exit
import sys
print(sys.version)
# sys.exit(1) # exit with error code

# Install a package
pip install requests

# Install a specific version
pip install requests==2.31.0

# Upgrade to latest
pip install --upgrade requests

# Uninstall
pip uninstall requests

# See what's installed
pip list

# See info about one package
pip show requests

# 1. Create the project folder
mkdir my-project
cd my-project

# 2. Create a virtual environment called "venv"
python3 -m venv venv
# Windows: python -m venv venv

# 3. Activate it
source venv/bin/activate
# Windows Command Prompt: venv\Scripts\activate
# Windows PowerShell: .\venv\Scripts\Activate.ps1

# You'll see (venv) appear at the start of your prompt.

# 4. Now pip install goes into THIS project, not the system
pip install requests

# 5. When you're done, deactivate
deactivate

# To resume work later, activate again
source venv/bin/activate

# After installing everything your project needs, freeze it
pip freeze > requirements.txt

# requirements.txt now looks like this:
requests2.31.0
beautifulsoup44.12.2
lxml==4.9.3

# Someone else (or future-you on a new machine) runs:
pip install -r requirements.txt
# Exactly those versions are installed. Same behavior guaranteed.

my-project/
├── venv/ (created by python -m venv)
├── helpers.py (our helper module)
├── main.py (the entry point)
└── requirements.txt (pinned dependencies)

# helpers.py — reusable functions

def count_words(text):
"""Return a dict of {word: count} for the text."""
counts = {}
for word in text.lower().split():
counts[word] = counts.get(word, 0) + 1
return counts

def top_words(counts, n=3):
"""Return the top N (word, count) pairs."""
return sorted(counts.items(), key=lambda kv: kv[1], reverse=True)[:n]

# main.py — the entry point
from helpers import count_words, top_words

text = "the cat sat the cat ran the cat slept"
counts = count_words(text)
for word, count in top_words(counts):
print(f"{word}: {count}")

# Output:
# the: 3
# cat: 3
# sat: 1

cd my-project
python3 -m venv venv
source venv/bin/activate
python main.py
the: 3
cat: 3
sat: 1