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pip

pip is Python’s package manager. It’s the tool you use to install, upgrade, and remove Python libraries that aren’t included in the standard library.

Think of it like:

• apt on Debian/Ubuntu
• dnf on Fedora
• npm for JavaScript

What does pip do?

With pip, you can:

1. Install packages

pip install requests

2. Upgrade packages

pip install --upgrade requests

3. Uninstall packages

pip uninstall requests

4. List installed packages

pip list

5. Install from a requirements file

pip install -r requirements.txt

Where do packages come from?

By default, pip downloads packages from PyPI (Python Package Index):

• https://pypi.org
• Contains hundreds of thousands of Python libraries
• Example packages: requests, numpy, flask, django, pandas

pip vs Python itself

• Python → the programming language + standard library
• pip → installs extra libraries written by the community

Example:

import math        # standard library (no pip needed)
import requests    # external library (installed via pip)

pip and Python versions

On many systems:

• pip → Python 2 or system default
• pip3 → Python 3

Safer way (recommended):

python3 -m pip install requests

This ensures pip matches the Python version you’re using.

Virtual environments

pip works best with virtual environments (venv) to avoid breaking system Python:

python3 -m venv venv
source venv/bin/activate
pip install flask

What is requirements.txt?

requirements.txt is a list of Python packages (and versions) your project depends on.

Example:

flask==3.0.0
requests>=2.31.0
gunicorn

Install dependencies from requirements.txt

Basic usage

pip install -r requirements.txt

Safer (recommended)

python3 -m pip install -r requirements.txt

This ensures pip matches your Python version.

Use with a virtual environment (best practice)

python3 -m venv venv
source venv/bin/activate

pip install -r requirements.txt

After activation, all packages go into venv/.

Generate requirements.txt from current environment

If you already installed packages:

pip freeze > requirements.txt

⚠️ This captures everything installed in the environment.

Common version formats

flask==3.0.0      # exact version (reproducible builds)
requests>=2.31.0 # minimum version
numpy~=1.26.0    # compatible release
django<5.0       # version upper limit

Install from multiple requirement files

pip install -r base.txt -r dev.txt

Useful for:

• base.txt → production
• dev.txt → development tools (pytest, black, etc.)

Requirements from Git / local path

git+https://github.com/pallets/flask.git
-e .

Then install:

pip install -r requirements.txt

Debian / system Python warning (important)

On Debian-based systems, don’t install globally unless you know what you’re doing:

❌ Bad:

sudo pip install -r requirements.txt

✅ Good:

python3 -m venv venv
pip install -r requirements.txt

Typical workflow (TL;DR)

python3 -m venv venv
source venv/bin/activate
pip install -r requirements.txt
python app.py