# Managing dependencies with Poetry

This is a quick cheat sheet for developers on how to use `poetry`.

# Background

Synapse uses a variety of third-party Python packages to function as a homeserver. Some of these are direct dependencies, listed in `pyproject.toml` under the `[tool.poetry.dependencies]` section. The rest are transitive dependencies (the things that our direct dependencies themselves depend on, and so on recursively.)

We maintain a locked list of all our dependencies (transitive included) so that we can track exactly which version of each dependency appears in a given release. See here for discussion of why we wanted this for Synapse. We chose to use `poetry` to manage this locked list; see this comment for the reasoning.

The locked dependencies get included in our "self-contained" releases: namely, our docker images and our debian packages. We also use the locked dependencies in development and our continuous integration.

Separately, our "broad" dependencies—the version ranges specified in `pyproject.toml`—are included as metadata in our "sdists" and "wheels" uploaded to PyPI. Installing from PyPI or from the Synapse source tree directly will not use the locked dependencies; instead, they'll pull in the latest version of each package available at install time.

## Example dependency

An example may help. We have a broad dependency on `phonenumbers`, as declared in this snippet from pyproject.toml as of Synapse 1.57:

    [tool.poetry.dependencies]
    # ...
    phonenumbers = ">=8.2.0"

In our lockfile this is pinned to version 8.12.44, even though newer versions are available.

    [[package]]
    name = "phonenumbers"
    version = "8.12.44"
    description = "Python version of Google's common library for parsing, formatting, storing and validating international phone numbers."
    category = "main"
    optional = false
    python-versions = "*"

The lockfile also includes a cryptographic checksum of the sdists and wheels provided for this version of `phonenumbers`.

    [metadata.files]
    # ...
    phonenumbers = [
        {file = "phonenumbers-8.12.44-py2.py3-none-any.whl", hash = "sha256:cc1299cf37b309ecab6214297663ab86cb3d64ae37fd5b88e904fe7983a874a6"},
        {file = "phonenumbers-8.12.44.tar.gz", hash = "sha256:26cfd0257d1704fe2f88caff2caabb70d16a877b1e65b6aae51f9fbbe10aa8ce"},
    ]

We can see this pinned version inside the docker image for that release:

    $ docker pull matrixdotorg/synapse:v1.57.0
    ...
    $ docker run --entrypoint pip matrixdotorg/synapse:v1.57.0 show phonenumbers
    Name: phonenumbers
    Version: 8.12.44
    Summary: Python version of Google's common library for parsing, formatting, storing and validating international phone numbers.
    Home-page: https://github.com/daviddrysdale/python-phonenumbers
    Author: David Drysdale
    Author-email: dmd@lurklurk.org
    License: Apache License 2.0
    Location: /usr/local/lib/python3.9/site-packages
    Requires:
    Required-by: matrix-synapse

Whereas the wheel metadata just contains the broad dependencies:

    $ cd /tmp
    $ wget https://files.pythonhosted.org/packages/ca/5e/d722d572cc5b3092402b783d6b7185901b444427633bd8a6b00ea0dd41b7/matrix_synapse-1.57.0rc1-py3-none-any.whl
    ...
    $ unzip -c matrix_synapse-1.57.0rc1-py3-none-any.whl matrix_synapse-1.57.0rc1.dist-info/METADATA | grep phonenumbers
    Requires-Dist: phonenumbers (>=8.2.0)

# Tooling recommendation: direnv

`direnv` is a tool for activating environments in your shell inside a given directory. Its support for poetry is unofficial (a community wiki recipe only), but works solidly in our experience. We thoroughly recommend it for daily use. To use it:

1. Install `direnv` - it's likely packaged for your system already.
2. Teach direnv about poetry. The shell config here needs to be added to `~/.config/direnv/direnvrc` (or more generally `$XDG_CONFIG_HOME/direnv/direnvrc`).
3. Mark the synapse checkout as a poetry project: `echo layout poetry > .envrc`.
4. Convince yourself that you trust this `.envrc` configuration and project. Then formally confirm this to `direnv` by running `direnv allow`.

Then whenever you navigate to the synapse checkout, you should be able to run e.g. `mypy` instead of `poetry run mypy`; `python` instead of `poetry run python`; and your shell commands will automatically run in the context of poetry's venv, without having to run `poetry shell` beforehand.

# How do I...

## ...reset my venv to the locked environment?

    poetry install --extras all --remove-untracked

## ...run a command in the `poetry` virtualenv?

Use `poetry run cmd args` when you need the python virtualenv context. To avoid typing `poetry run` all the time, you can run `poetry shell` to start a new shell in the poetry virtualenv context. Within `poetry shell`, `python`, `pip`, `mypy`, `trial`, etc. are all run inside the project virtualenv and isolated from the rest o the system.

Roughly speaking, the translation from a traditional virtualenv is:

- `env/bin/activate` -> `poetry shell`, and
- `deactivate` -> close the terminal (Ctrl-D, `exit`, etc.)

See also the direnv recommendation above, which makes `poetry run` and `poetry shell` unnecessary.

## ...inspect the `poetry` virtualenv?

Some suggestions:

    # Current env only
    poetry env info
    # All envs: this allows you to have e.g. a poetry managed venv for Python 3.7,
    # and another for Python 3.10.
    poetry env list --full-path
    poetry run pip list

Note that `poetry show` describes the abstract lock file rather than your on-disk environment. With that said, `poetry show --tree` can sometimes be useful.

## ...add a new dependency?

Either:

- manually update `pyproject.toml`; then `poetry lock --no-update`; or else
- `poetry add packagename`. See `poetry add --help`; note the `--dev`, `--extras` and `--optional` flags in particular.
  - NB: this specifies the new package with a version given by a "caret bound". This won't get forced to its lowest version in the old deps CI job: see this TODO.

Include the updated `pyproject.toml` and `poetry.lock` files in your commit.

## ...remove a dependency?

This is not done often and is untested, but

    poetry remove packagename

ought to do the trick. Alternatively, manually update `pyproject.toml` and `poetry lock --no-update`. Include the updated `pyproject.toml` and poetry.lock` files in your commit.

## ...update the version range for an existing dependency?

Best done by manually editing `pyproject.toml`, then `poetry lock --no-update`. Include the updated `pyproject.toml` and `poetry.lock` in your commit.

## ...update a dependency in the locked environment?

Use

    poetry update packagename

to use the latest version of `packagename` in the locked environment, without affecting the broad dependencies listed in the wheel.

There doesn't seem to be a way to do this whilst locking a specific version of `packagename`. We can workaround this (crudely) as follows:

    poetry add packagename==1.2.3
    # This should update pyproject.lock.

    # Now undo the changes to pyproject.toml. For example
    # git restore pyproject.toml

    # Get poetry to recompute the content-hash of pyproject.toml without changing
    # the locked package versions.
    poetry lock --no-update

Either way, include the updated `poetry.lock` file in your commit.

## ...export a `requirements.txt` file?

    poetry export --extras all

Be wary of bugs in `poetry export` and `pip install -r requirements.txt`.

Note: `poetry export` will be made a plugin in Poetry 1.2. Additional config may be required.

## ...build a test wheel?

I usually use

    poetry run pip install build && poetry run python -m build

because `build` is a standardish tool which doesn't require poetry. (It's what we use in CI too). However, you could try `poetry build` too.