How best to solve a cyclic dependency during package installation

[dfremont]

I'm using Tox to test a package let's call A (for clarity), which has an optional dependency on package B, which in turn depends on A. One of my Tox environments installs A using an extra that also pulls in B. While pip install A[extra] works just fine, as pip knows how to deal with the cyclic dependency, Tox's 2-step method of first installing A's dependencies and then using a second pip command for A itself doesn't always work. In particular, the following can happen:

1. During the install_package_deps stage, Tox uses pip to install B, and pip starts searching PyPI for a compatible version of A to install (as it doesn't know that Tox will separately install A later).
2. If the dependencies of the version of A I'm developing have changed, there may be no compatible versions on PyPI already, so the installation fails.

I'm currently working around this issue by using a Tox plugin to add an additional installation stage which installs A (with the --no-deps option) before the install_package_deps stage, so that during that stage pip's resolver will know the newer version of A is available:

Plugin code

import tox.plugin

@tox.plugin.impl
def tox_on_install(tox_env, arguments, section, of_type):
    if section == "RunToxEnv" and of_type == "package":
        assert len(arguments) == 1
        package = arguments[0]
        path = package.path
        install_args = ["--force-reinstall", "--no-deps", str(path)]
        tox_env.installer._execute_installer(install_args, "package_early")

This approach is fragile (I'm using some undocumented Tox APIs), and still has the issue that A and B are installed in separate pip commands, so dependency conflicts could be missed.

My questions are:

1. Why doesn't Tox use a single pip command to install both the built package and its dependencies? That would solve the cyclic dependency problem.
2. Assuming there's a good reason for the current behavior, is there a more robust way for me to work around it? I went the plugin route after not finding any configuration options that did what I needed, but I might have missed something.

Thanks in advance!

[gaborbernat]

The plugin route is supported, just under documented 👍

[dfremont]

Thanks, Bernát. Is there any chance of an option for Tox to install the built package and its dependencies in a single pip call? I imagine there's some reason why those are done separately right now.

[gaborbernat]

Likely will not happen. Just not use deps?