# 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
```