擅长:python、mysql、java
<p>我不明白你为什么说这是错的,这一行产生了你想要的:</p>
<pre><code>df['t_A'] = 17500-df.groupby('Date_1')["C_B"].cumsum()
</code></pre>
<p>结果如下:</p>
<pre><code>Date_1 Date_2 C_B t_A
0 01/09/2019 02/08/2019 7 17493
1 01/09/2019 03/08/2019 1189 16304
2 01/09/2019 04/08/2019 8 16296
3 01/09/2019 04/08/2019 1650 14646
4 01/09/2019 05/08/2019 9 14637
5 01/09/2019 05/08/2019 1805 12832
6 01/09/2019 06/08/2019 63 12769
7 01/09/2019 06/08/2019 2 12767
8 01/09/2019 07/08/2019 1985 10782
9 02/09/2019 01/09/2019 886 16614
10 02/09/2019 01/09/2019 1980 14634
11 02/09/2019 03/08/2019 3 14631
12 02/09/2019 03/08/2019 4 14627
13 02/09/2019 04/08/2019 138 14489
14 02/09/2019 04/08/2019 8 14481
15 02/09/2019 05/08/2019 259 14222
16 03/09/2019 01/09/2019 829 16671
17 03/09/2019 01/09/2019 509 16162
18 03/09/2019 02/09/2019 1884 14278
19 03/09/2019 04/08/2019 6 14272
20 03/09/2019 04/08/2019 2 14270
21 03/09/2019 05/08/2019 4 14266
22 03/09/2019 05/08/2019 161 14105
</code></pre>