Use python_requires to check the python version so py2.py3 wheels are not built and pip will work correctly.

[bryder]

Problem

• The wheels being built for ipywidgets claim it is python 2 compatible. eg ipywidgets-7.6.3-py2.py3-none-any.whl
• Modern versions of pip do not understand that ipywidgets is not python 2 compatible. setup.py is not run when downloading a wheel.

So it installs it without error on python 2.

I believe the py2.py3 wheels are being built because the python_requires argument is not being passed to setup but I do not know that for sure.

Proposed Solution

Add a python_requires argument to the setup call to solve it.

EG Add this to setup_args in setup.py

python_requires = '>=3.5',

Then you can remove this

#-----------------------------------------------------------------------------
# Minimal Python version sanity check
#-----------------------------------------------------------------------------

import sys

v = sys.version_info
if v[:2] < (3, 5):
    error = "ERROR: %s requires Python version 3.5 or above." % name
    print(error, file=sys.stderr)
    sys.exit(1)

This issue from another project has a very good description of the problem and how to solve it

PLOS/allofplos#20

[jasongrout]

As far as I know, ipywidgets 7.x does support python 2 (this is in the 7.x branch in the repo). The upcoming 8.0 release will drop support for python 2, and indeed I agree it should use python_requires. We'd definitely welcome a pull request updating setup.py to use python_requires!