7. Modules & packages¶

Beginner · 7 min read

A module is any .py file; a package is a folder of modules. Imports let you split code across files and use other people's libraries.

7.1 Importing¶

import math                          # import a whole module
from pathlib import Path             # import one name from a module
from datetime import datetime as dt  # rename on import
import json, os                      # several at once (fine for quick scripts)

print(math.sqrt(16))                 # → 4.0
print(Path("notes/rag.md").suffix)   # → .md
print(dt(2026, 1, 1).year)           # → 2026

7.2 The standard library¶

Python ships with "batteries included". The modules you'll use most:

Module	Use it for
`pathlib`	file paths that work on Windows and macOS
`json`	reading/writing JSON (API payloads, configs)
`os`	environment variables, the operating system
`datetime`	dates and times
`re`	regular expressions (pattern matching in text)
`collections`	`Counter`, `defaultdict`, `deque`
`logging`	proper logs instead of `print`

from collections import Counter

words = "rag agents rag llm rag".split()
print(Counter(words).most_common(1))   # → [('rag', 3)]

7.3 Your own modules¶

text_utils.py

"""Small helpers for cleaning text."""

def clean(text):
    """Collapse whitespace and trim the ends."""
    return " ".join(text.split())

main.py

from text_utils import clean          # same folder → import by file name

print(clean("  hello    world "))     # → hello world

A folder becomes a package when you import from it: from rag.retrieval import search (an empty __init__.py inside the folder makes it explicit).

7.4 `if name == "main":`¶

text_utils.py

def clean(text):
    return " ".join(text.split())

if __name__ == "__main__":            # True only when you run this file directly…
    print(clean("  quick   test "))   # …not when another file imports it

7.5 Installing packages with pip¶

pip install openai                    # install the latest version
pip install "openai>=1.40,<2"         # install within a version range
pip list                              # what's installed
pip freeze > requirements.txt         # save exact versions for your project

Always install inside a virtual environment.

A faster alternative: uv

uv creates the environment, installs packages and locks versions in one tool — see Manage a GenAI project with uv.

7.6 Secrets in environment variables¶

import os

# Read the key from the environment; never write keys in your code.
api_key = os.getenv("OPENAI_API_KEY", "")   # "" if it isn't set
print("key set:", bool(api_key))

With the python-dotenv package you can keep keys in a .env file (added to .gitignore) and call load_dotenv() at start-up.

Why it matters for GenAI

Every LLM project is "glue" between packages — an SDK, a vector store, a web framework. Knowing how imports, pip and environment variables work saves hours of setup errors.

Practice¶

Use Counter to find the 2 most common words in a paragraph of your choice.
Move a helper function into its own module and import it from main.py.