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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_catalog(xsd_schemas=None, xmlcatalog_dir=None, xmlcatalog_file=None):
 """Generating an XML catalog for use in resolving schemas Creates the XML Catalog directory if it doesn't already exist. Uses :meth:`download_schema` to save local copies of schemas, adding a comment indicating the date downloaded by eulxml. Generates a new catalog.xml file, with entries for all schemas that downloaded successfully. If no schemas downloaded, the catalog is not generated. .. Note:: Currently this method overwites any existing schema and catalog files, without checking if they are present or need to be updated. """

    # if requests isn't available, warn and bail out
    if requests is None:
        sys.stderr.write(req_requests_msg)
        return

    logger.debug("Generating a new XML catalog")
    if xsd_schemas is None:
        xsd_schemas = XSD_SCHEMAS

    if xmlcatalog_file is None:
        xmlcatalog_file = XMLCATALOG_FILE

    if xmlcatalog_dir is None:
        xmlcatalog_dir = XMLCATALOG_DIR
    # if the catalog dir doesn't exist, create it
    if not os.path.isdir(xmlcatalog_dir):
        os.mkdir(xmlcatalog_dir)

    # new xml catalog to be populated with saved schemas
    catalog = Catalog()

    # comment string to be added to locally-saved schemas
    comment = 'Downloaded by eulxml %s on %s' % \
        (__version__, date.today().isoformat())

    for schema_uri in xsd_schemas:
        filename = os.path.basename(schema_uri)
        schema_path = os.path.join(xmlcatalog_dir, filename)
        saved = download_schema(schema_uri, schema_path, comment)
        if saved:
            # if download succeeded, add to our catalog.
            # - name is the schema identifier (uri)
            # - uri is the local path to load
            # NOTE: using path relative to catalog file
            catalog.uri_list.append(Uri(name=schema_uri, uri=filename))

    # if we have any uris in our catalog, write it out
    if catalog.uri_list:
        with open(xmlcatalog_file, 'wb') as xml_catalog:
            catalog.serializeDocument(xml_catalog, pretty=True)
    return catalog