为什么Pandas df达不到预期?

我编写了以下简单代码来说明我遇到的问题:

import pandas as pd

symbols = {'TEST1', 'TEST2', 'TEST3'}

i = 0

for sym in symbols:
    if i == 0:
        cum_df = pd.DataFrame([sym])
        i = 1
    else:
        cum_df.append(pd.DataFrame([sym]), ignore_index=True)

cum_df

I was expecting cum_df to look like this:

+---+-------+
|   |   0   |
+---+-------+
| 0 | TEST1 |
| 1 | TEST2 |
| 2 | TEST3 |
+---+-------+

但是它看起来像这样:

+---+-------+
|   |   0   |
+---+-------+
| 0 | TEST3 |
+---+-------+

我要去哪里错了?