def _get_code_cell_str_list(notebook_data_dict):
    """
    Get a list of code cell strings.

    Parameters
    ----------
    notebook_data_dict : dict
        A dictionary of notebook data.

    Returns
    -------
    code_str_list : list of str
        A list of code cell strings.
    """
    code_str_list = []
    has_key = 'cells' in notebook_data_dict
    if not has_key:
        return []
    cells_list = notebook_data_dict['cells']
    for cell_dict in cells_list:
        cell_type = cell_dict['cell_type']
        if cell_type != 'code':
            continue
        source_list = cell_dict['source']
        source = ''.join(source_list)
        code_str_list.append(source)
    return code_str_list
