如何在pandas数据帧中创建缺少年份和季度的行

import pandas as pd
import numpy as np

years = [2019, 2020, 2021]  # 假设需要的年份范围
quarters = [1, 2, 3, 4]  # 所有可能的季度

df = pd.DataFrame(columns=['Year', 'Quarter', 'Value'])  # 假设数据帧有三列：年份、季度、值

for year in years:
    for quarter in quarters:
        if (year, quarter) not in zip(df['Year'], df['Quarter']):
            df = df.append({'Year': year, 'Quarter': quarter, 'Value': np.nan}, ignore_index=True)

df = df.sort_values(['Year', 'Quarter']).reset_index(drop=True)