I would like to add a 0 to my index column but while keeping its name. I had a code doing that with an earlier version of pandas (0.18.0), but with pandas 0.22.0 this doesn't work anymore. Here is what I do:

```
df = pd.DataFrame({"a1": [4,6,8,10]}, index = [0.1,0.2,0.3,0.4])
df.index.rename('idx', inplace=True) #now the name of the index is idx, but why is inplace=True needed now?
print("\nafter index rename:\n",df)
df = df.append(pd.DataFrame(index=[0])) # after this, the index name is lost.
print("\nafter append 0:\n",df)
df.index.rename('idx', inplace=True)
print("\nafter index rename2:\n",df)

```

Is there a way of adding a value (a 0 in my case) to the index column without loosing the name of the index column? Of course I can rename afterwards, but that seems like a workaround to me...