pyproject.toml without build-backend should auto-fallback to setup.py

[scop]

Environment

• pip version: 19.1
• Python version:
• OS:

Description

https://github.com/scop/hashpipe/tree/3cbb1283dfe55f7934a86650d2f474240ef2fff1

I have a pyproject.toml file with [tool.black] only. Running tests with tox I get pip 19.1 which errors out: https://travis-ci.org/scop/hashpipe/builds/525585963

ERROR: Error installing 'file:///.../hashpipe': editable mode is not supported for pyproject.toml-style projects. pip is processing this project as pyproject.toml-style because it has a pyproject.toml file. Since the project has a setup.py and the pyproject.toml has no "build-backend" key for the "build_system" value, you may pass --no-use-pep517 to opt out of pyproject.toml-style processing. See PEP 517 for details on pyproject.toml-style projects.

Expected behavior

PEP 517 says:

If the pyproject.toml file is absent, or the build-backend key is missing, the source tree is not using this specification, and tools should fall back to running setup.py.

My case is this one (build-backend key is missing), so per the PEP, pip should automatically fall back to using setup.py and not ask me to opt out of it with --no-use-pep517 (which would be inconvenient and I believe fail on pip versions that don't have that flag).

How to Reproduce

See github.com/scop/hashpipe link above.

Output

See above.

[meejah]

Same situation for a project I contribute to, except we're using pyproject.toml to configure towncrier (only). For now we're limiting pip < 19 for everything, which is obviously not ideal...

[florisla]

This is fixed in #6449 and will probably be released as pip v19.1.1.

A longer discussion is in #6434.

Good catch on the PEP517 excerpt about falling back to setup.py, it's nice to know I'm not violating the PEP by simply using black ;-)

[meejah]

For the record, I tried pip master, and it works fine for my use-case. 👍

[pradyunsg]

Duplicate of #6434.