Python Packaging: pyproject.toml build backends under the hood

<section data-background="pizza-boxes.jpg" />

##  ![Fedora Loves Python](flp.svg)

---
```pycon
$ sudo dnf install python3.13
$ python3.13 -m venv venv
$ . venv/bin/activate
(venv) $ python
Python 3.13.0a4 (main, Feb 16 2024, 00:00:00) [GCC 13.2.1 20231205 (Red Hat 13.2.1-6)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
```

---
# pizza.py

```python []
"""
This is a 🍕 module.
"""
```
---
# pyproject.toml

```toml [1-8|1-3|5-8]
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[project]
name = "pizza"
version = "2024"
description = "🍕"
```

---
```pycon [1-4|5-6]
>>> import tomllib
>>> with open('pyproject.toml', 'rb') as f:
...     pyproject = tomllib.load(f)
... 
>>> pyproject['build-system']['requires']
['hatchling']
>>> pyproject['build-system']['build-backend']
'hatchling.build'
```

---
```bash
(venv) $ pip install hatchling
```

---
```pycon [7-8]
>>> import tomllib
>>> with open('pyproject.toml', 'rb') as f:
...     pyproject = tomllib.load(f)
... 
>>> pyproject['build-system']['requires']
['hatchling']
>>> pyproject['build-system']['build-backend']
'hatchling.build'
```

---
```pycon []
>>> requires = ['setuptools >= 40.8']
>>> backend_name = 'setuptools.build_meta:__legacy__'
```

---
```pycon [2-3]
>>> import importlib
>>> backend_name = pyproject['build-system']['build-backend']
>>> backend = importlib.import_module(backend_name)
>>> backend
<module 'hatchling.build' from '.../site-packages/hatchling/build.py'>
```

---
```pycon [3-6|7-13]
>>> import importlib, sys
>>> backend_name = pyproject['build-system']['build-backend']
>>> backend_path = pyproject['build-system'].get('backend-path')
>>> if backend_path:
...     sys.path = backend_path + sys.path
... 
>>> module_name, _, object_name = backend_name.partition(':')
>>> backend_module = importlib.import_module(module_name)
>>> if object_name:
...     backend = getattr(backend_module, object_name)
... else:
...     backend = backend_module
... 
>>> backend
<module 'hatchling.build' from '.../site-packages/hatchling/build.py'>
```

---
```pycon [1-4|5-6]
>>> hasattr(backend, 'get_requires_for_build_wheel')
True
>>> backend.get_requires_for_build_wheel()
[]
>>> backend.build_wheel('.')
'pizza-2024-py2.py3-none-any.whl'
```

---
```pycon [1-4|5-6]
>>> hasattr(backend, 'get_requires_for_build_sdist')
True
>>> backend.get_requires_for_build_sdist()
[]
>>> backend.build_sdist('.')
'pizza-2024.tar.gz'
```

---
```pycon [1-4|5-6]
>>> hasattr(backend, 'get_requires_for_build_editable')
True
>>> backend.get_requires_for_build_editable()
[]
>>> backend.build_editable('.')
'pizza-2024-py2.py3-none-any.whl'
```

---
 * **PEP 517**: A build-system independent format for source trees
 * **PEP 518**: Specifying Minimum Build System Requirements for Python Projects
 * **PEP 660**: Editable installs for pyproject.toml based builds (wheel based)

---

<section data-background="cheese.jpg" />

---
# pyproject.toml

```toml [5-8]
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[project]
name = "pizza"
version = "2024"
description = "🍕"
```

---
# pizza-2024.dist-info/METADATA

```email []
Metadata-Version: 2.1
Name: pizza
Version: 2024
Summary: 🍕
```

---
# pizza.py

```python []
"""
This is a 🍕 module.
"""
```

---
![distutils](distutils-meme.png)

---
 * **PEP 566**+: Metadata for Python Software Packages
 * **PEP 621**: Storing project metadata in pyproject.toml
 * github.com/**ofek/extensionlib**

---

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