Building wheels takes a long time with .tox directories present

[nedbat]

Building the manylinux wheels for coverage.py took 1hour 45minutes. Once I removed all my .tox directories, it took 3 minutes.

The problem is that "pip wheel" copies the entire tree somewhere else first. If we use "python setup.py bdist_wheel", it skips that step. If I do that, then even with .tox directories, building wheels takes only 2 minutes.

Here's an IRC exchange about it:

[13:18:39] nedbat	I have two different ways to build wheels:  "pip wheel . -w wheelhouse" and "python setup.py bdist_wheel".  They both produce the same name wheel, with the same size.  But the first command takes 3min42sec, the second command takes 11sec.
[13:19:11] nedbat	The time for the first one seems to depends on how many files are in the entire tree, including .tox directories... What's the difference between these commands, and is there a reason to prefer the first one?
[13:25:42] dstufft	nedbat: the first command copies the files to a temporary directory, does some gross stuff to ensure you're using setuptools in your setup.py, and then invokes setup.py bdist_wheel
[13:26:13] nedbat	dstufft: any idea why it seems take forever when there are many irrelevant files in the tree?
[13:26:18] Wooble	(doesn't pip wheel also recursively build wheels for all of your deps, and bdist_wheel... not?)
[13:26:31] nedbat	Wooble: the results are the same size (though not the same sha1)
[13:26:33] dstufft	nedbat: presumably because we're copying the entire tree to a temporary directory
[13:26:42] nedbat	dstufft: oh, yikes.
[13:27:50] xafer	(cf https://github.com/pypa/pip/issues/2195)
[13:27:59] dstufft	and yea we also build wheels for all dependencies
[13:28:06] nedbat	dstufft: is there a reason i should use "pip wheel" if the other is working for me?
[13:28:07] xafer	(install/wheel, potato/potato)
[13:28:14] dstufft	but those will be seperate .wl files
[13:28:50] dstufft	nedbat: Not particularly, ``pip wheel`` is typically less used by the maintainer of Foo, and more for consumers of Foo-That-Doesnt-Have-A-Wheel-Already
[13:29:14] nedbat	dstufft: ok, good to know.

[nedbat]

coverage.py has no dependencies, so the "also builds dependent wheels" part doesn't into play for me, but I would expect that people using this repo are building wheels for uploading to PyPI, so they don't want dependent wheels either.

[rmcgibbo]

That stinks. Can you think of any downside of just changing https://github.com/pypa/python-manylinux-demo/blob/master/travis/build-wheels.sh#L10 to use python setup.py bdist_wheel?

[nedbat]

I changed my own version of this script, and it seems fine (and fast!), but I haven't distributed those wheels yet. I found that the wheels made both ways had identical names and sizes, though the sha1 sums were different, maybe just because of timestamps.

[rmcgibbo]

Sounds good. I'd be happy to merge a PR to that effect, if you think it's warranted.

[njsmith]

In the long run we want to move towards pip wheel or similar as the standard because that will support non-setup.py-based build systems. But clearly pip will need some work to optimize it for this use case, and in the mean time you should definitely use bdist_wheel directly if it works better.

[nedbat]

I'm forgetting the details of why I had to change directories, but this is how I changed this script in the coverage.py repo: nedbat/coveragepy@f2517c9

The clean step is needed or one build's files will be packaged into the next build.