setuptools "at least one of these" dependency specification

Question

In some cases, there are various modules which each implement a common API (in my case, the old pure-python elementtree, cElementTree, lxml.etree, and the built-in xml.etree). I can write the module using ElementTree to try each of these options, and take the first one that exists according to my own preference order -- but I can't find any information on specifying that only one of these must be installed in setup.py. I want to write something that looks like this:

setup(
    ...,
    install_requires="""
        elementtree | cElementTree | lxml
    """,
    ...
)

Is this, or something like it, possible?

Answer 1

I don't think so, but, if you're using a reasonably recent Python, elementtree being part of the standard Python libraries, why do you worry it might be absent? (I do understand this would be a problem for other cases of several possible implementations of an API, I just wonder if you really need it for your specific use case).

Answer 2

You might want to try doing something like this in setup.py:

install_requires = ["elementree"]

try:
    import cElementTree
    install_requires = []
except ImportError:
    try:
        import lxml
        install_requires = []
    except ImportError:
        # etc for remaining equivalent modules

setup(
    install_requires = install_requires,
    # rest of setup parameters
)

This will basically install elementree package as dependency if none of the equivalent are installed.