WIP towards make PyCall use Conda if system python is not found

[dhoegh]

This is a WIP towards figuring out how Conda can be used as a replacement of system python and it is parallel work to JuliaLang/IJulia.jl#357.
I still haven't figured out the API for how PyPlot/PyCall/IJulia should interact with Conda and if they should require Conda or if the user should add Conda.

[stevengj]

The difficulty with this is that many people using PyCall will be Python users who want to call Python packages that they've installed by their familiar way in Python. If PyCall uses Conda's Python, they will be surprised because it won't find any of the packages they've installed with their regular Python.

[dhoegh]

Yes that is the reasoning about not requiring Conda, but if Conda is installed then it prefers the python distribution in Conda. This means the changes will not affect people having a system wide python and do not have Conda installed.

[dhoegh]

I have updated the PR. The miniconda python distribution in Conda only takes up 123 MB when nothing is installed which acceptable. I have added a use_conda constant for the purpose of automatically installation of python dependencies when Conda is used. This is shown in JuliaPy/PyPlot.jl#170.

[stevengj]

Probably Conda.add("numpy") should be done in Pkg.build, since it is generally a good idea to have NumPy with PyCall, to support zero-copy arrays.

[stevengj]

Would be good to have Travis testing too. Maybe just add Python 2.6 to the Travis tests, if that is allowed by Travis. With the version check I suggested above, that will force Pkg.build to use Conda.

[dhoegh]

I have addressed your comments. Maybe you could cancel the old appveyor build to reduce the load on appveyor.

[stevengj]

cancelled the build.

[stevengj]

Travis is failing with could not spawn /home/travis/.julia/v0.3/Conda/deps/usr/python -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('VERSION'))": no such file or directory (ENOENT)

Probably, python is in Conda.BINDIR on non-Windows systems.

(I don't understand why they couldn't just use the same paths on Unix and Windows.)

[dhoegh]

I have made a PR to Conda.jl to add PYTHONDIR. I will comeback tomorrow and finish.

[Luthaf]

(I don't understand why they couldn't just use the same paths on Unix and Windows.)

Because they reproduce the way Python is packaged on Windows. But I have no idea why it was done so for Python in the first place.

[tkelman]

endless frustration caused by someone deciding to be different on different platforms decades ago

[dhoegh]

Everything is passing now.

[stevengj]

Thanks!