Parameters
    ----------
    plugins_directory : str
        Path to MicroDrop user plugins directory.

    Returns
    -------
    list
        List of package strings corresponding to installed plugin versions.

def freeze(plugins_directory):
    '''
    Parameters
    ----------
    plugins_directory : str
        Path to MicroDrop user plugins directory.

    Returns
    -------
    list
        List of package strings corresponding to installed plugin versions.
    '''
    # Check existing version (if any).
    package_versions = []
    for plugin_path_i in plugins_directory.dirs():
        try:
            plugin_metadata = yaml.load(plugin_path_i
                                        .joinpath('properties.yml').bytes())
            if plugin_path_i.name != plugin_metadata['package_name']:
                continue
            package_versions.append((plugin_metadata['package_name'],
                                     plugin_metadata['version']))
        except:
            continue
    return ['%s==%s' % v for v in package_versions]