QST: Replace subset of DataFrame by another subset of DataFrame
- [x] I have searched the [[pandas] tag](https://stackoverflow.com/questions/tagged/pandas) on StackOverflow for similar questions.

- [ ] I have asked my usage related question on [StackOverflow](https://stackoverflow.com).

---

#### Question about pandas

**Note**: If you'd still like to submit a question, please read [this guide](
https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your question.

```python
# Your code here, if applicable
import numpy as np
import pandas as pd
dic = {'A': [1, 4, 1, 4], 'B': [9, 2, 5, 3], 'C': [0, 0, 5, 3]}
df = pd.DataFrame(dic)
df.index = [0, 0, 1, 1]

df1 = df.copy()
df1[:] = np.nan

df1.loc[0]['A'] = df.loc[0]['A']
```

original `df`:
```
   A  B  C
0  1  9  0
0  4  2  0
1  1  5  5
1  4  3  3
```

I want to replace the subset of the copy with the subset of original `df`:
```
    A   B   C
0 1   NaN NaN
0 4   NaN NaN
1 NaN NaN NaN
1 NaN NaN NaN
```

However, it shows no change at all:
```
    A   B   C
0 NaN NaN NaN
0 NaN NaN NaN
1 NaN NaN NaN
1 NaN NaN NaN
```