依赖管理：Python2.7需要subprocess32

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
)