Build issue: PyWavelets does not install from sdist

[zvezdan]

PyWavelets fails to install from sdist with standard Python tooling because it relies on presence of numpy in its setup.py. Thus, running pip install --no-binary :all: ... or python setup.py sdist fails with a clean Python installation (no external packages). Since the package is published on PyPI (https://pypi.python.org/pypi/PyWavelets), it needs to be able to build from source distribution (tar.gz file) self-sufficiently.

The fact that the package imports numpy in setup.py indicates that the package needs numpy in setup_requires. This creates a chicken-and-egg kind of problem that is not easy to hack around, but it may be possible. Please take a look at scipy and what they are doing in setup.py: https://github.com/scipy/scipy/blob/master/setup.py. This may help with fixing this issue for PyWavelets.

[rgommers]

Thanks for the report @zvezdan. This should be resolved by gh-321, which adds support for PEP 518.

[zvezdan]

I don't see how that fixes the problem.
The offending import is still there: https://github.com/PyWavelets/pywt/blob/master/setup.py#L12
That fails on the first attempt of pip install --no-binary :all: ... if numpy is not already pre-installed.

[rgommers]

With a recent enough pip, what happens is:

1. pip parses pyproject.toml to find setup dependencies
2. pip installs those dependencies
3. pip then builds pywavelets by executing setup.py, producing a wheel
4. that wheel then gets installed

It's possible that you need a development version of pip, I haven't kept up with its release schedule over the last couple of months.

Note that pip has never supported setup_requires, and what scipy does works but is a bit hacky. PEP 518, while very new, is the official recommended way to get things to work the way they should.