def find_sheet(book, sheet_id=0):
    """Find a sheet in an Excel book.

    Args:
        book (xlrd.book.Book): initialized Excel book object
        sheet_id (int or str): index or name of sheet to export

    Yields:
        xlrd.sheet.Sheet: individual Excel sheet
    """
    # extract sheet by index or name
    if isinstance(sheet_id, int):
        return book.sheet_by_index(sheet_id)
    elif isinstance(sheet_id, str):
        return book.sheet_by_name(sheet_id)