def get_storage_blob(bucket, source_folder_name, source_file_name):
    """
    Fetches and returns the single source file blob from the buck on
    Google Cloud Storage
    """
    source_path = source_file_name
    if source_folder_name != '':
        source_path = f'{source_folder_name}/{source_file_name}'
    blob = bucket.get_blob(source_path)
    try:
        blob.exists()
        return blob
    except Exception as e:
        print(f'File {source_path} does not exist')
        raise(e)