QST: what are the semantics of the SettingWithCopyWarning?
Hi, on pandas `1.0.5` I get `SettingWithCopyWarning` when I do the following

```python
df['x'] = df['y'] - df['z']
```

1. Is the warning correct in this case? 
2. The suggestion is `Try using .loc[row_indexer,col_indexer] = value instead`

If I do something like

```
df.loc[~df.x.isna() , 'x'] = df['y'] - df['z']
```
I still get the same warning. I am not sure what is the proper way to do it?

Thanks
Prokopis
