I have two dataframes and two lists of dataframes. I want to make a dataframe element "nan" via the list, but the counterpart is also overwritten. How can I modify only the first one? 

```
d
Out[302]: {'col1': [1, 2, 3], 'col2': [4, 5, 6]}

dftemp1 = pd.DataFrame(d)

dftemp2 = dftemp1

dftemp_l = [dftemp1, dftemp2]

dftemp_m = dftemp_l.copy()

dftemp_l[0][:] = np.nan

```

The following is what I don't want. I want it not to change. 

```
dftemp_m[0]
Out[308]: 
   col1  col2
0   NaN   NaN
1   NaN   NaN
2   NaN   NaN

```