Introduce installation order or a "priority install" feature.

[Demetrio92]

Problem
I have a requirements.txt which just won't install automatically because package X depends on cython. Both of them are in the requirements.txt, but there is no way to tell pip to install cython first (even though cython is placed above the package X). So the line with cython in the file is simply useless. Everyone trying to set up the dev enviroment will figure out they need it.

Proposition 1
Install packages in the order of requirements.txt.
Example:

cython
cartopy

would install without a fail, but

cartopy
cython

would fail because cartopy depends on cython.

Gain: automatic installation saving millions of developers over the world the hassle of reading the traceback only to remember that pip still does not have this feature.

Alternative Solutions

• Alternative 1 (workaround on the user side): use multiple requirements.txt. E.g.
  requirements_INSTALL_ME_FIRST.txt with cython in it, and then requirements.txt with the rest.
  At least those who read won't be caught by a surprise pip install -r requirements.txt fails.

• Alternative 2 (implement the workaround on the pip side): add a property "priority", e.g.

cython, priority

or

cython==2.1, priority

would mean pip puts it into a separate temporary requirements-priority.txt and installs it first.

Additional context
#3480
#3481
#2362

https://stackoverflow.com/questions/5394356/how-to-specify-install-order-for-python-pip

[benoit-pierre]

Alternative 3: the cartopy package switch to PEP 518 to declare it's build time dependency on Cython. I see that it's setup.py is doing some weird things too, instead of using setup_requires, which I'm pretty sure does not work with recent versions of setuptools.

[Demetrio92]

@benoit-pierre I will forward it to them, thanks a lot!
And sorry for my misunderstanding of the general paradigm. Indeed, I think dependencies should be on the packages, not on requirements.txt

[pfmoore]

Agreed. This problem has been addressed by PEP 518 (which has only recently been added to pip, so it's fine that cartopy hasn't yet got round to updating to use the capability). There's no point in any other workaround for this problem in pip, when we already have a well-designed (hopefully!) standard solution.

[pfmoore]

And sorry for my misunderstanding of the general paradigm

@Demetrio92 No problem - it's fairly new functionality, so no surprise if people aren't familiar with it yet.

[benoit-pierre]

Their work-around should be updated to:

Distribution().fetch_build_eggs(['Cython>=0.15.1', 'numpy>=1.10'])

[Demetrio92]

@benoit-pierre
@pfmoore
Thanks both of you!

[sashgorokhov]

Their work-around should be updated to:

Distribution().fetch_build_eggs(['Cython>=0.15.1', 'numpy>=1.10'])

you just saved my life!

[lock]

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