熊猫df合并不交换

Question

以下两个数据框：

df_t1 = pd.DataFrame({'Date' : [pd.Timestamp(2017, 1, 1), pd.Timestamp(2017, 1, 2), pd.Timestamp(2017, 1, 3)], 'A': [1, 2, 3]})
df_t1 = df_t1.set_index('Date')

df_t2 = pd.DataFrame({'Date' : [pd.Timestamp(2016, 12, 31), pd.Timestamp(2017, 1, 1), pd.Timestamp(2017, 1, 2), pd.Timestamp(2017, 1, 3), pd.Timestamp(2017, 1, 4)], 'C': [7, 8, 9, 10, 11]})
df_t2 = df_t2.set_index('Date')

可以像这样合并

df_merge = df_t1.merge(df_t2, how='outer', left_on='Date', right_on='Date')

或类似

df_merge2 = df_t2.merge(df_t1, how='outer', left_on='Date', right_on='Date')

我希望两个变体都返回df_merge2，如下所示，为什么不是这样？

            C    A
Date
2016-12-31  7   NaN
2017-01-01  8   1.0
2017-01-02  9   2.0
2017-01-03  10  3.0
2017-01-04  11  NaN