'Find an installed distribution that satisfies or conflicts
with this requirement, and set self.satisfied_by or
self.conflicts_with appropriately.'

def check_if_exists(self):

    if (self.req is None):
        return False
    try:
        self.satisfied_by = pkg_resources.get_distribution(self.req)
    except pkg_resources.DistributionNotFound:
        return False
    except pkg_resources.VersionConflict:
        existing_dist = pkg_resources.get_distribution(self.req.project_name)
        if self.use_user_site:
            if dist_in_usersite(existing_dist):
                self.conflicts_with = existing_dist
            elif (running_under_virtualenv() and dist_in_site_packages(existing_dist)):
                raise InstallationError(('Will    not    install    to    the    user    site    because    it    will    lack    sys.path    precedence    to    %s    in    %s' % (existing_dist.project_name, existing_dist.location)))
        else:
            self.conflicts_with = existing_dist
    return True
