# URL Fields

The `schema`, `ui_schema`, and `form_data` properties for any
[`forms`](../schema.rst#any-form) describe independently constrained documents, which
doesn't work well with most JSON schema tools.

For a complex form, it can make sense to keep documents in separate files, generate them
on the fly, or rely on a form user's browser to fetch them at run-time.

## Remote URLs

Instead of local files, the `schema`, `ui_schema`, and `form_data` options may be given
as URLs starting with `http://` or `https://`. These will be passed unchanged to the
underlying form.

This is useful for reusing assets hosted somewhere else, or which use complex relative
`$refs`. However, if the linked files move, become unavailable, or otherwise cannot be
reached due to browser limitations, a form will not be rendered.

## Python

The `py:` prefix denotes an importable python module, which must be available at
CLI/Sphinx run time, either as an installed python package on `sys.path`, or by hacking
the `PYTHONPATH` environment variable.

These may return either a concrete value or callable which returns:

- a JSON-compatible python dictionary which will be encoded as a normalized JSON file
- a [URL](./remote.md#remote-urls) string