When to install compatibility packages #194

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The legacy package 'vo' was found.
To install astropy's compatibility layer instead, uninstall
'vo' and then reinstall astropy.
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found_legacy_module = False
try:
    location = imp.find_module(old_package)
except ImportError:
    pass
else:
    # We want ImportError to raise here, because that means it was
    # found, but something else went wrong.

    # We could import the module here to determine if its "real"
    # or just a legacy alias.  However, importing the legacy alias
    # may cause importing of code within the astropy source tree,
    # which may require 2to3 to have been run.  It's safer to just
    # open the file and search for a string.
    filename = os.path.join(location[1], '__init__.py')
    if os.path.exists(filename):
        with open(filename, 'U') as fd:
            if '_is_astropy_legacy_alias' in fd.read():
                found_legacy_module = True

shim_dir = os.path.join(get_legacy_alias_dir(), old_package)

if found_legacy_module:
    warn('-' * 60)
    warn("The legacy package '{0}' was found.".format(old_package))
    warn("To install astropy's compatibility layer instead, uninstall")
    warn("'{0}' and then reinstall astropy.".format(old_package))
    warn('-' * 60)

    if os.path.isdir(shim_dir):
        shutil.rmtree(shim_dir)
    return (None, None)

if '_is_astropy_legacy_alias' not in fd.read():