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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**
+
+```bash
+pip install requests
+```
+
+2. **Upgrade packages**
+
+```bash
+pip install --upgrade requests
+```
+
+3. **Uninstall packages**
+
+```bash
+pip uninstall requests
+```
+
+4. **List installed packages**
+
+```bash
+pip list
+```
+
+5. **Install from a requirements file**
+
+```bash
+pip install -r requirements.txt
+```
+
+## Where do packages come from?
+
+By default, pip downloads packages from **PyPI** (Python Package Index):
+
+* [https://pypi.org](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:
+
+```python
+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):
+
+```bash
+python3 -m pip install requests
+```
+
+This ensures pip matches the Python version you’re using.
+
+## Virtual environments
+
+`pip` works best with **virtual environments** ([venv](pages/requirement/venv.md)) to avoid breaking system Python:
+
+```bash
+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:
+
+```txt
+flask==3.0.0
+requests>=2.31.0
+gunicorn
+```
+
+## Install dependencies from `requirements.txt`
+
+### Basic usage
+
+```bash
+pip install -r requirements.txt
+```
+
+### Safer (recommended)
+
+```bash
+python3 -m pip install -r requirements.txt
+```
+
+This ensures pip matches your Python version.
+
+## Use with a virtual environment (best practice)
+
+```bash
+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:
+
+```bash
+pip freeze > requirements.txt
+```
+
+⚠️ This captures **everything** installed in the environment.
+
+## Common version formats
+
+```txt
+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
+
+```bash
+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
+
+```txt
+git+https://github.com/pallets/flask.git
+-e .
+```
+
+Then install:
+
+```bash
+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:
+
+```bash
+sudo pip install -r requirements.txt
+```
+
+✅ Good:
+
+```bash
+python3 -m venv venv
+pip install -r requirements.txt
+```
+
+## Typical workflow (TL;DR)
+
+```bash
+python3 -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+python app.py
+```
+