The update_function_configuration currently returns an empty dict no matter what but certain lambda libraries (ie, Zappa) require a realistic return value... which is simply the current config after being updated.

https://github.com/localstack/localstack/blob/9cba1010dfcbf2b7672df4d7da46b7e8ab287f64/localstack/services/awslambda/lambda_api.py#L872

https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/lambda.html#Lambda.Client.update_function_configuration

There might also be some other magic enriching that result but my quick read-through looks like it's an empty dict.



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zip_file_content = bytes_io.getvalue()
        except Exception as e:
            raise ClientError('Unable to fetch Lambda archive from S3: %s' % e, 404)
    elif 'ZipFile' in function_code:
        zip_file_content = function_code['ZipFile']
        zip_file_content = base64.b64decode(zip_file_content)
    else:
        raise ClientError('No valid Lambda archive specified.')
    return zip_file_content
def get_java_handler(zip_file_content, handler, main_file):
    """Creates a Java handler from an uploaded ZIP or JAR.
    :type zip_file_content: bytes
    :param zip_file_content: ZIP file bytes.
    :type handler: str
    :param handler: The lambda handler path.
    :type main_file: str
    :param main_file: Filepath to the uploaded ZIP or JAR file.
    :returns: function or flask.Response
    """
    if not is_jar_archive(zip_file_content):
        with zipfile.ZipFile(BytesIO(zip_file_content)) as zip_ref:
            jar_entries = [e for e in zip_ref.infolist() if e.filename.endswith('.jar')]
            if len(jar_entries) != 1:
                raise ClientError('Expected exactly one *.jar entry in zip file, found %s' % len(jar_entries))
            zip_file_content = zip_ref.read(jar_entries[0].filename)
            LOG.info('Found jar file %s with %s bytes in Lambda zip archive' %
                     (jar_entries[0].filename, len(zip_file_content)))
            main_file = new_tmp_file()
            save_file(main_file, zip_file_content)
    if is_jar_archive(zip_file_content):