我有以下Pandas DataFrame：
date at weight status buy_ts sell_ts
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0 2010-01-03 00:00:00 1.4286 7 buy 2010-01-04 01:47:00 nan
1 2010-01-03 00:00:00 1.4288 7 buy 2010-01-04 00:00:00 nan
2 2010-01-03 00:00:00 1.4289 7 buy 2010-01-04 00:00:00 nan
3 2010-01-04 00:00:00 1.442 25 buy 2010-01-05 00:00:00 nan
4 2010-01-05 00:00:00 1.4422 15 sell nan 2010-01-06 14:03:00
5 2010-01-05 00:00:00 1.4423 15 sell nan 2010-01-06 14:03:00
6 2010-01-05 00:00:00 1.4424 15 sell nan 2010-01-06 14:03:00
7 2010-01-06 00:00:00 1.4403 18 sell nan 2010-01-07 00:04:00
8 2010-01-06 00:00:00 1.4404 18 sell nan 2010-01-07 00:05:00
9 2010-01-06 00:00:00 1.4405 18 sell nan 2010-01-08 08:54:00
10 2010-01-07 00:00:00 1.4313 26 buy 2010-01-08 00:07:00 nan
11 2010-01-07 00:00:00 1.4314 26 buy 2010-01-08 00:07:00 nan
12 2010-01-07 00:00:00 1.4316 26 sell nan 2010-01-08 00:10:00
buy_ts和sell_ts包含一个Python datetime.datetime对象
我想创建一个名为merged_ts的新列，其中包含buy_ts或sell_ts中的datetime.dateime对象（当一列具有值时，另一列始终为nan，因此不可能同时填充两列。