# Module discovery

## First-party modules

First-party modules are Python files that are part of your project source code.

By default, ty searches for first-party modules in the project's root directory or the `src`
directory, if present.

If your project uses a different layout, configure the project's
[`environment.root`](./reference/configuration.md#root) in your `pyproject.toml` or `ty.toml`. For example,
if your project's code is in an `app/` directory:

```text
example-pkg
├── README.md
├── pyproject.toml
└── app
    └── example_pkg
        └── __init__.py
```

then set [`environment.root`](./reference/configuration.md#root) in your `pyproject.toml` to `["./app"]`:

```toml title="pyproject.toml"
[tool.ty.environment]
root = ["./app"]
```

Note that a `./python` folder is automatically added to the project `root` if it exists,
and is not itself a package (i.e. does not contain an `__init__.py` file or an
`__init__.pyi` file).

## Third-party modules

Third-party modules are Python packages that are not part of your project or the standard library.
These are usually declared as dependencies in a `pyproject.toml` or `requirements.txt` file
and installed using a package manager like uv or pip. Examples of popular third-party
modules are `requests`, `numpy` and `django`.

ty searches for third-party modules in the configured [Python environment](#python-environment).

## Python environment

The Python environment is used for discovery of third-party modules.

By default, ty will attempt to discover a virtual environment.

First, ty checks for an active virtual environment using the `VIRTUAL_ENV` environment variable. If
not set, ty will search for a `.venv` directory in the project root or working directory. ty only
supports discovery of virtual environments at this time.

!!! note

    When using project management tools, such as uv or Poetry, the `run` command usually automatically
    activates the virtual environment and will be detected by ty.

The Python environment may be explicitly configured using the
[`environment.python`](./reference/configuration.md#python) setting or
[`--python`](./reference/cli.md#ty-check--python) flag.

When setting the environment explicitly, non-virtual environments can be provided.