How to Build a Scalable Python Monorepo with UV: From Setup to Dependency Management

Previously I introduced UV , a Python package manager that does more than just install packages. Curious about its capabilities, I used UV to design a monorepo.

Repository Structure

When creating a monorepo I usually include three key projects:

API project : handles various APIs such as FastAPI REST API, gRPC, GraphQL, etc.

Core project : provides the core functionality of the monorepo.

Common library project : a shared library containing utilities, constants, and other common code.

This layout ensures scalability, better code organization, and improved dependency management.

Three Nested Projects

py-api

py-basecamp

py-zcommonlib

py-api and py-basecamp are independent and cannot import each other. Shared functionality should be placed in py-zcommonlib to keep the projects isolated.

Creating the Three Projects with UV

Run the following commands:

<code>uv init py-zcommonlib --lib
uv init py-api
uv init py-basecamp</code>

The --lib flag creates a src directory for Python modules; without it, UV defaults to --app , producing a flat structure.

Note: the flat structure is not wrong, just a different approach.

Dependency Management

Each project has its own pyproject.toml , allowing independent and clear dependency control.

Adding a Common Module in py-zcommonlib

Create datetime_lib.py with the following code:

<code>from datetime import datetime, timezone

def get_utc_timestamp():
    """Return the current UTC timestamp in ISO 8601 format.
    Example: '2025-02-15T12:34:56Z'"""
    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")

if __name__ == "__main__":
    print(get_utc_timestamp())</code>

Run it with UV:

<code>uv run src/py_zcommonlib/datetime_lib.py</code>

Using the Module in py-basecamp

Update main.py to import the function:

<code>from py_zcommonlib.datetime_lib import get_utc_timestamp

def main():
    print(get_utc_timestamp())

if __name__ == "__main__":
    main()</code>

The script fails because py-zcommonlib is not yet listed as a dependency of py-basecamp .

Updating py-basecamp pyproject.toml

Add py-zcommonlib as an editable dependency using a relative path:

<code>[project]
name = "py-basecamp"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.9"
dependencies = ["py-zcommonlib"]

[tool.uv.sources]
py_zcommonlib = { path="../py-zcommonlib", editable=true }</code>

Run uv sync in the py-basecamp directory to install the dependency, then uv run main.py to test.

Understanding [tool.uv.sources]

The tool.uv.sources table extends the standard dependency list, allowing alternative sources such as editable installs and relative paths, which are not supported by project.dependencies .

Using the Module in py-api

In the FastAPI project, import and expose the timestamp endpoint:

<code>from fastapi import FastAPI
from py_zcommonlib.datetime_lib import get_utc_timestamp

app = FastAPI()

@app.get("/")
async def utc_timestamp():
    return get_utc_timestamp()</code>

Corresponding pyproject.toml for py-api :

<code>[project]
name = "py-api"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.9"
dependencies = [
    "fastapi>=0.115.11",
    "uvicorn>=0.34.0",
    "py-zcommonlib",
]

[tool.uv.sources]
py_zcommonlib = { path="../py-zcommonlib", editable=true }</code>