Why Should I Choose pyproject.toml over requirements.txt for managing dependencies?

A requirements.txt file lists packages to install. A pyproject.toml file defines a project. That distinction shapes how dependencies get managed, how environments stay in sync, and how projects scale over time.

What requirements.txt does well

The format is plain text, one package per line:

requests>=2.28.0
pandas~=2.0.0

Any tool that speaks pip can consume it. Files can be hosted online and installed via URL (pip install -r https://example.com/requirements.txt). For scripts or one-off environments, that simplicity is hard to beat.

Where requirements.txt falls short

The workflow around requirements.txt demands manual coordination. A developer creates a virtual environment, activates it, installs packages with pip, then remembers to update the file by hand. Each step is a chance for the file and the environment to drift apart.

Beyond that workflow friction, requirements.txt has structural gaps:

• No Python version constraint. Nothing stops installation into an incompatible interpreter.
• No dev/prod separation. Splitting dependencies requires multiple files (requirements-dev.txt, requirements-test.txt) with no standard convention.
• No project metadata. Name, version, authors, and build configuration live elsewhere (setup.py, setup.cfg), spreading a project’s identity across files.

What pyproject.toml changes

PEP 621 standardized project metadata in pyproject.toml. Dependencies, Python version constraints, project metadata, and tool configuration all live in one file. Tools like flit, hatch, pdm, poetry, and uv all support it.

Compare the two workflows side by side. With requirements.txt:

mkdir myproject && cd myproject
python -m venv .venv

source .venv/bin/activate

.venv\Scripts\activate

pip install requests pandas
# now manually add them to requirements.txt

With uv:

uv init myproject && cd myproject
uv add requests pandas

uv init creates the project and its virtual environment. uv add installs packages and records them in pyproject.toml in one step. No activation, no manual bookkeeping. When another developer clones the repo, uv sync reproduces the environment from the lockfile.

One file for the whole project

pyproject.toml replaces the constellation of setup.py, setup.cfg, tox.ini, and requirements files with a single source of truth. The same file works whether the project is an application or a distributable package. Dev dependencies and dependency groups live alongside production dependencies, governed by a standard rather than ad-hoc file naming.

Which to use

Use pyproject.toml for new projects. For existing projects still on requirements.txt, see How to migrate from requirements.txt to pyproject.toml with uv.

Related

• pyproject.toml reference covers the full specification
• requirements.txt reference documents the traditional format
• What are Optional Dependencies and Dependency Groups? explains how pyproject.toml handles dev dependencies
• Create your first Python project walks through starting a new uv project from scratch
• What is a lock file? explains the reproducibility benefits of uv’s lockfile