Fix installation breakage with pip 19+

[pappasam]

• Added tests for changed code. (not necessary, same interface)
• Updated documentation for changed code. (not necessary, same interface)

This PR fixes bugs installing a poetry package locally with pip >= version 19 installed.

"pip install -e . --no-deps" is replaced with "python setup.py develop --no-deps" to work around pip 19's strict adherence to pip 517. Basically, we can't install in editable mode with pip if a pyproject.toml is present. See: pypa/pip#6331

This commit proposes an alternative approach. Since "pip install -e" implicitly uses setuptools's "develop" mode under the hood, we simply invoke setuptools directly, bypassing pip's correct handling of PEP 517, and adding setuptools to the autogenerated setup.py (necessary to get it to work).

For proof of setuptools's "develop" mode being used under pip's hood: https://github.com/pypa/pip/blob/master/src/pip/_internal/req/req_install.py#L739

Note: like "pip install -e", "python setup.py develop --no-deps" feels like a hack. In the future, we may consider working toward not relying on setuptools / distutils for local installation.

Resolves / deprecates (probably more, there is a lot of complaining about pip version 19):

• Generated setup.py should use setuptools.setup instead of distutils.core.setup #866
• Pip>19 fails to install dependency from different source repository #896
• (I think) Git dependency issues with pip 19 #825
• (not necessary) bryanforbes/botus_receptus@279342a

[pappasam]

@sdispater This also should address #1049

[a-recknagel]

Note: like "pip install -e", "python setup.py develop --no-deps" feels like a hack. In the future, we may consider working toward not relying on setuptools / distutils for local installation.

Editable mode will remain a hack until pep517 is extended with a clean definition of what editable installs are, and how it should be implemented.

Related:

• pfmoore: "I'd be more than happy if the poetry project engaged with the standards process to make sure [editable mode is supported]"
• suggestion to discuss this issue on the packaging mini summit

[frankier]

Looks like another workaround has been released now in 0.12.13 2ed53be

[mitsuhiko]

I don't thin this fix is correct because it does not involve the setuptools shim that pip uses. At the very least it should try to emulate what -e did instead of invoking setup.py develop just like this.

[sdispater]

I just released version 0.12.13 which should fix most of the issues with pip>=19.0 (see commit 2ed53be).

Basically, what it does is:

• for pure python packages it emulates the behavior of -e (create a .egg-info directory, a .egg-link in site-packages and add a line in easye-install.pth in site-packages). This is the bare minimum for it to work and should be enough.
• if the current pip is <19.0, it keeps the current behavior
• if the current pip is >=19.0, at installation time it renames the pyproject.toml file to retrieve the previous behavior and then puts the pyproject.toml file back into place. This is not ideal but this is the simplest way to make it work again.

[mitsuhiko]

if the current pip is >=19.0, at installation time it renames the pyproject.toml file to retrieve the previous behavior and then puts the pyproject.toml file back into place. This is not ideal but this is the simplest way to make it work again.

This will not work if the setup.py file tries to read from the pyproject.toml file.

[sdispater]

@mitsuhiko The fix only addresses the Poetry-generated setup.py which does not use the pyproject.toml file to get its version.

[pappasam]

Ok, I'm going to close this PR since it looks like we're happy with the implemented workaround. Thanks @sdispater !

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