Patrick Gerken: Don't catch import errors, use pkg_resources

After debugging a funny problem in tests which came from Circular Imports that got eaten, let me show you how you can test, if some package is available, and how you can test, if that package is new enough:

So, this is a common snippet:

try:

    import Products.LinguaPlone

except ImportError:

    HAS_LINGUA_PLONE = False

else:

    HAS_LINGUA_PLONE = True

That might trigger circular import exceptions. It will not always fail, depending

on what gets imported first. The circular import gets caught by the "except ImportError:" line. So you will not see an exception here, but in some other funny place. 

Good luck debugging that.

Another thing that I see sometimes is a check to implement backward compatibility

try:

    from foo import new_thing

except ImportError:

    OLD_FOO = True

else:

    OLD_FOO = False

if OLD_FOO:

    def new_thing(something):

    return something + 'cool'

This can sometimes trigger ImportErrors and it does not show its intention like it could.

pkg_resources has an API which can help you to clarify things:

import pkg_resources

try:

    pkg_resources.get_distribution('Products.LinguaPlone>=4.0.0')

except pkg_resources.DistributionNotFound:

    HAS_LINGUA_PLONE = False

    HAS_CURRENT_LINGUA_PLONE = False

except pkg_resources.VersionConflict:

    HAS_LINGUA_PLONE = True

    HAS_CURRENT_LINGUA_PLONE = True

else:

    HAS_LINGUA_PLONE = True

    HAS_CURRENT_LINGUA_PLONE = True

Now, that is a bit more verbose, but

  We don't try to import things and can't trigger circular exceptions

  You can see in the code which version is needed for the workaround

If someone sees a try: import, they most probably do not know for which version it was necessary. On the other hand, if they see the version requirement in plain code, they might have an idea that this is a test for a VERY outdated version of a dependency and might decide to delete it, since the package does not support the old version in other parts of the code anyway.