条件合并数据框

Question

有两个数据帧：

如何将df1与df2合并，以便我们得到以下结果  在X：

Ticker   Type   Date
======   ====   ====
OEX      Index  2018-06-01
XEO      Index  2018-06-01
SPX      Index  2018-06-01
SPX      Index  2018-06-08
SPX      Index  2018-06-22

......等等......

DF1：

    Ticker  Type    Std_Week    EOW
    ======  ====    ========    ===
12  OEX     Index   X           NaN
13  XEO     Index   X           NaN
14  SPX     Index   NaN         X
15  XSP     Index   NaN         X

DF2：

    Std_Week             EOW
    ========             ===
0   2018-06-01 00:00:00  2018-06-01 00:00:00
1   NaN                  2018-06-08 00:00:00
2   NaN                  2018-06-22 00:00:00
3   NaN                  2018-06-29 00:00:00
4   NaN                  2018-07-06 00:00:00