The Project Standard: pyproject.toml

Last updated on 2026-02-11 | Edit this page

Overview

Questions

How do I turn my folder of code into an installable library?
What is pyproject.toml and why is it the standard?
How does uv simplify project management?
Why use the src layout?

Objectives

Use uv init to generate a standard pyproject.toml (PEP 621).
Organize code using the src layout to prevent import errors.
Manage dependencies and lockfiles using uv add.
Run code in an isolated environment using uv run.

The Installation Problem

In the previous episode, we hit a wall: our chemlib package only worked when the interpreter starts from the project folder. To fix this, we need to Install the package into our Python environment.

As far as Python is concerned, an “installation” involves placing the files somewhere the interpreter will find them. One of the simplest ways involves setting the PYTHONPATH terminal variable.

The packaging gradient, from the Hashemi PyBay’17 and Goswami PyCon 2020 presentation

Callout

The Packaging Timeline

An annotated timeline of tooling:

2003: PEP 301 defines PyPI
2004: setuptools declares dependencies
2005: packages are hosted on PyPI
2007: virtualenv is released to support multiple Python versions
2008: pip is released for better dependency management
2012: multi-language distribution discussions from PEP 425 and PEP 427 ¹
2013: PEP 427 standardizes the wheel format, replacing eggs
2016: PEP 518 introduces pyproject.toml to specify build dependencies ²
2017: PEP 517 separates the build frontend (pip) from the backend (flit, hatch, poetry)
2020: PEP 621 standardizes project metadata in pyproject.toml, removing the need for setup.py configuration
2022: PEP 668 marks system Python environments as “externally managed” to prevent accidental breakage
2021: PEP 665 attempts (and fails) to standardize lockfiles. ³
2024: PEP 723 enables inline script metadata. ⁴
2024: PEP 735 introduces dependency groups (e.g., separating test or lint dependencies) without requiring a package build.
2025: PEP 751 formalizes the pylock.toml file.

Enter `uv`

Sometime in the early 2020s Python projects began adopting a Rust core. Starting with ruff and moving up to uv and pixi in the past few years, these tools are often able cache aggressively, and provide saner resolution of versions and other requirements for packaging.

We will use uv, a convenient, modern Python package manager, which also doubles as a frontend, replacing pip with uv pip and a backend for pure Python distributions.

Initializing a Project

Let’s turn our chemlib folder into a proper project. We will use uv init to generate the configuration.

BASH

# First, ensure we are in the project root
cd project_folder

# Initialize a library project
uv init --lib --name chemlib

This creates a `pyproject.toml` file. Let’s inspect it.

SH

[project]
name = "chemlib"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
authors = [
    { name = "Rohit Goswami", email = "rohit.goswami@epfl.ch" }
]
requires-python = ">=3.12"
dependencies = []

[build-system]
requires = ["uv_build>=0.9.26,<0.10.0"]
build-backend = "uv_build"

Breakdown

[project]: This table is standardized by PEP 621. It defines what your package is (name, version, dependencies).
[build-system]: This defines how to build it, with an appropriate build backend. uv defaults to uv_build⁵.

The Python Packaging User Guide provides a complete description of the fields in the pyproject.toml.

The `src` Layout

uv init --lib automatically sets up the src layout for us ⁶. Your folder structure should now look like this:

project_folder/
├── pyproject.toml
├── src/
│   └── chemlib/
│       ├── __init__.py
│       └── py.typed

Comparison of Flat Layout vs Src Layout folder structures

Why the `src` directory?

Testing against the installed package: With a flat layout (package in root), running pytest often imports the local folder instead of the installed package. This hides installation bugs (like missing data files).
Cleaner root: Your root directory defines the project (config, docs, scripts), while src holds the product (the source code).

Managing Dependencies

In the first episode, we saw how numpy caused crashes when not part of the environment. Let’s add numpy to our project properly.

BASH

uv add numpy
Using CPython 3.11.14
Creating virtual environment at: .venv
Resolved 2 packages in 120ms
      Built chemlib @ file:///home/goswami/blah
Prepared 2 packages in 414ms
Installed 2 packages in 20ms
 + chemlib==0.1.0 (from file:///home/goswami/blah)
 + numpy==2.4.2

This performs two critical actions:

It adds "numpy" to the dependencies list in pyproject.toml.
It creates a uv.lock file.

The Lockfile: This file records the exact version of numpy (e.g., 2.1.0) and every underlying dependency installed. This guarantees that your teammates (and your future self) get the exact same environment.

Running Code with implicit virtual environments

You might notice that uv didn’t ask you to activate a virtual environment. It manages one for you automatically.

To run code in this project’s environment, we use uv run.

BASH

# Run a quick check
uv run python -c "import chemlib; print(chemlib.__file__)"

.../project_folder/.venv/lib/python3.12/site-packages/chemlib/__init__.py

Notice the path! Python is loading chemlib from .venv/lib/.../site-packages. This means uv has performed an Editable Install.

We can edit src/chemlib/geometry.py.
The changes appear immediately in the installed package.
But Python treats it as a properly installed library.

Challenge

Challenge: Update the Geometry Module

Now that numpy is installed, modify src/chemlib/geometry.py to use it. Remember to expose the functionality within __init__.py as in the previous lesson.

Import numpy.
Change center_of_mass to accept a list of positions and return the mean position using np.mean.

Show me the solution

PYTHON

# src/chemlib/geometry.py
import numpy as np

def center_of_mass(atoms):
    print("Calculating Center of Mass with NumPy...")
    # Assume atoms is a list of [x, y, z] coordinates
    data = np.array(atoms)
    # Calculate mean along axis 0 (rows)
    com = np.mean(data, axis=0)
    return com

Test it using uv run:

BASH

uv run python -c "import chemlib; print(chemlib.center_of_mass([[0,0,0], [2,2,2]]))"

Output:

Calculating Center of Mass with NumPy...
[1. 1. 1.]

Dependency Resolution and Conflicts

A robust package manager must handle Constraint Satisfaction Problems. You might require Library A, which relies on Library C (v1.0), while simultaneously requiring Library B, which relies on Library C (v2.0).

If these version requirements do not overlap, a conflict arises. uv detects these impossible states before modifying the environment.

Let us artificially construct a conflict using pydantic, a data validation library often used alongside scientific tools.

Discussion

Challenge: Inducing a Conflict

We will attempt to install incompatible versions of pydantic and pydantic-settings ⁷.

Request an older version of pydantic (<2.0).
Request a newer version of pydantic-settings (>=2.0), which technically depends on Pydantic 2.0+.

BASH

uv add "pydantic<2" "pydantic-settings>=2"

The output should resemble:

× No solution found when resolving dependencies:
╰─▶ Because only the following versions of pydantic-settings are available:
        pydantic-settings<=2.0.0
        ...
    and pydantic-settings==2.0.0 depends on pydantic>=2.0b3, we can conclude that
    pydantic-settings>=2.0.0,<2.0.1 depends on pydantic>=2.0b3.
    And because pydantic-settings>=2.0.1,<=2.0.3 depends on pydantic>=2.0.1, we can conclude that
    pydantic-settings>=2.0.0,<2.1.0 depends on pydantic>=2.0b3.
    And because pydantic-settings>=2.1.0,<=2.2.1 depends on pydantic>=2.3.0 and pydantic>=2.7.0, we
    can conclude that pydantic-settings>=2.0.0 depends on pydantic>=2.0b3.
    And because your project depends on pydantic<2 and pydantic-settings>=2, we can conclude that
    your project's requirements are unsatisfiable.
help: If you want to add the package regardless of the failed resolution, provide the `--frozen` flag
      to skip locking and syncing.

Graph showing two dependencies requiring incompatible versions of pydantic

This failure protects the development environment. uv refuses to install a broken state.

Abstract vs. Concrete Dependencies

We now resolve the conflict by allowing the solver to select the latest compatible versions (removing the manual version pins).

BASH

uv add pydantic pydantic-settings

This brings us to a critical distinction in Python packaging:

Abstract Dependencies (pyproject.toml): These define the minimum requirements for the project. For a library like chemlib, we prefer loose constraints (e.g., metatrain>=0.1.0) to maximize compatibility with other packages.
Concrete Dependencies (uv.lock): This file records the exact resolution (e.g., metatrain==0.1.5, torch==2.1.0) used in development. It ensures reproducibility.

The lockfile guarantees that all developers operate on an identical atomic substrate, eliminating the “works on my machine” class of defects.

Key Points

pyproject.toml is the standard recipe for Python projects (PEP 621).
uv add manages dependencies and ensures reproducibility via uv.lock.
uv run executes code in an isolated, editable environment without manual activation.
Isolation: uv enforces a clean environment, preventing accidental usage of unlisted packages.
Manifest vs. Lock: pyproject.toml declares what we need; uv.lock records exactly what we installed.

conda arrives here↩︎
This solves the “chicken and egg” problem of needing tools to install tools↩︎
A universal lockfile standard remains elusive; tools like pdm and poetry start providing specific implementations.↩︎
Allows single-file scripts to declare their own dependencies.↩︎
For compiled code, this will need to be switched out with meson-python, setuptools, or scikit-build to handle C++/Fortran code.↩︎
the flat layout has some drawbacks related to testing, though the Hitchhiker’s guide disagrees ↩︎
adapted from the uv PyCon 2025 tutorial ↩︎

The Project Standard: pyproject.toml

Overview

Questions

Objectives

The Installation Problem

The Packaging Timeline

Enter uv

Initializing a Project

BASH

SH

Breakdown

The src Layout

Why the src directory?

Managing Dependencies

BASH

Running Code with implicit virtual environments

BASH

Challenge: Update the Geometry Module

Show me the solution

PYTHON

BASH

Dependency Resolution and Conflicts

Challenge: Inducing a Conflict

BASH

Abstract vs. Concrete Dependencies

BASH

Enter `uv`

The `src` Layout

Why the `src` directory?