Relevent issues:

#6375
#6314
#6370

Environment

• pip version: 19.1
• Python version: 3.6
• OS: Linux

Description

pip install fails with a requirements file that references a setup.py file, in a project that has a pyproject.toml file that has no build tooling section.

My project setup:

setup.py  # Contains the actual project/package requirements
pyproject.toml  # Contains only 2 lines of black configuration
requirements/base.txt  # Contains only `-e .`
requirements/testing.txt  # Contains `-r base.txt` and then other stuff like pytest etc.
src/mypackage/...  # Actual project/package code

This setup allows us to:

• Specify the project requirements exactly once (in the setup.py)
• Use a command like pip install -r requirements/testing.txt to install both the project requirements along with other stuff needed to run the tests.
• Build the package with python setup.py sdist

The suggested workaround in #6370 (comment) doesn't seem to work for me:

pip install --no-use-pep517 -q --upgrade --requirement /..../src/requirements/development.txt
ERROR: Error installing 'file:///.... (from -r /..../src/requirements/_base.txt (line 1))': 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.

pip install -r requirements/testing.txt fails

Expected behavior

Historically (in <= 19.0.3) pip would install the project and testing/dev requirements (into the current virtualenv).

How to Reproduce

Set up a project as per above, try to install the dev/testing requirements with pip install -r foo.txt.

Output

As per description.