sdist cannot be installed on Python 2

[jaraco]

Attempting to install pep517 from an sdist on Python 2.7 results in an error:

~ $ python2.7 -m pip install --no-binary=:all: pep517                                                                                                                          
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Collecting pep517
  Using cached https://files.pythonhosted.org/packages/79/27/1e6a8ba83dab2b243e1020e340552c129b0076fbb5518e0c2a9c44c7d056/pep517-0.5.0.tar.gz
  Installing build dependencies ... error
  Complete output from command /usr/local/opt/python@2/bin/python2.7 /usr/local/lib/python2.7/site-packages/pip install --ignore-installed --no-user --prefix /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-build-env-sq1HqN/overlay --no-warn-script-location --no-binary :all: --only-binary :none: -i https://pypi.org/simple -- flit:
  DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
  Collecting flit
    Could not find a version that satisfies the requirement flit (from versions: )
  No matching distribution found for flit
  
  ----------------------------------------
Command "/usr/local/opt/python@2/bin/python2.7 /usr/local/lib/python2.7/site-packages/pip install --ignore-installed --no-user --prefix /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-build-env-sq1HqN/overlay --no-warn-script-location --no-binary :all: --only-binary :none: -i https://pypi.org/simple -- flit" failed with error code 1 in None

[duckinator]

I think what's happening is that Pip tries to install+run Flit due to pep517's pyproject.toml.

However, if I'm understanding Flit's README correctly, you need Python 3 to run Flit, but Flit can distribute Python 2 modules.

If that's the case, I think that means you can never install Flit-produced sdists on Python 2?

[duckinator]

Flit's pyproject.toml seems to confirm what I suspected:

requires-python=">=3"

[takluyver]

Yes, having a bit of trouble with that. Before PEP 517, it was fine: flit would generate a setup.py which worked with setuptools for Python 2 or 3. Now that PEP 517 is in effect, tools need to install flit first.

I'm considering making the core parts of flit Python 2 compatible to allow that. I'm just dragging my feet because it's late 2019, and it feels backwards to be porting code back to Python 2.

[takluyver]

It can now be installed from source with pip install --no-binary=pep517 pep517.

There's some bootstrapping problem I'm still investigating which prevents it from working with --no-binary=:all: (which means building flit_core from source too) at the moment.

[takluyver]

The bootstrapping problem is pypa/pip#6599 - I've just made a PR for that.