Dependency management: subprocess32 needed for Python2.7

Question

I have a library (subx) which depends on subprocess32. The subprocess32 library is a backport for Python2.7 and provides the timeout kwarg.

My library needs the timeout kwarg.

I need subprocess32 only if the target platform is Python2.x.

How should I define the dependency in my project?

I get this error message, if I define a dependency to subprocess32 via "install_requires" (setup.py) and I am inside a python3 virtualenv:

===> pip install -e git+https://github.com/guettli/subx.git#egg=subx
Obtaining subx from git+https://github.com/guettli/subx.git#egg=subx
  Cloning https://github.com/guettli/subx.git to ./src/subx
Collecting subprocess32 (from subx)
  Using cached subprocess32-3.2.7.tar.gz
    Complete output from command python setup.py egg_info:
    This backport is for Python 2.x only.

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-lju3nl1y/subprocess32/

Answer 1

There is a declarative way, but afaik it requires a more or less recent version of setuptools (if I read the release notes correctly, you need at least the 20.2 version). What you will see down below are called environment markers and are specified in PEP 508, read it through for a full list of available markers and a better understanding of the marker syntax if you wish.

For python version, let's take your package as an example: you have subprocess32 dependency that should be installed in python2.X environment. Enhance your dependency like this:

install_requires=[
    'subprocess32; python_version<"3"',
]

Installing the package subx with python2.7 now yields:

Processing ./dist/subx-2017.8.0-py2-none-any.whl
Collecting subprocess32; python_version < "3" (from subx==2017.8.0)
Installing collected packages: subprocess32, subx
Successfully installed subprocess32-3.2.7 subx-2017.8.0

If you install it with python3.X, the output will be:

Processing ./dist/subx-2017.8.0-py3-none-any.whl
Installing collected packages: subx
Successfully installed subx-2017.8.0

Notice that the installation of subprocess32 will be skipped.

---

Another common example is to declare platform-specific dependencies: I have a project that requires auditwheel to be installed on Linux and delocate on MacOS. I declare the dependencies like this:

install_requires=[
    ...
    'auditwheel==1.7.0; "linux" in sys_platform',
    'delocate==0.7.1; "darwin" == sys_platform',
]

Note that this check for Linux is needed if you do not specifically target any major python version because:

$ python2 -c "import sys; print sys.platform"
linux2

but

$ python3 -c "import sys; print sys.platform"
linux

so if for example your package works only with python2.X, you can use the check "linux2" == sys.platform. This will make your dependency installable only with python2.X.

Answer 2

import sys

kw = {}
if sys.version_info[0] == 2:
    kw['install_requires'] = ['subprocess32']

setup(
    …
    **kw
)