How to avoid "KeyError: 'None of [2001] are in the columns'" when setting index to pandas DataFrame?

I have a DataFrame called maximoVerano and want to reindex it as I will append other DataFrames with different year indexes. However when I want to use set-index as you can see in the sample example I get the error code:

KeyError: 'None of [2001] are in the columns'

Sample code

>>> maximosVerano
   demanda  demanda31.5  ...  fechaHoraMaxDem31.5   fechaHoraMaxDem63
0   28.037       4.1211  ...  2001-03-16 21:00:00 2001-01-15 22:00:00

[1 rows x 6 columns]
>>> type(maximosVerano)
<class 'pandas.core.frame.DataFrame'>
>>> maximosVerano.index
RangeIndex(start=0, stop=1, step=1)
>>> maximosVerano.set_index([anio], inplace=True)
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
  File "C:\Python39\lib\site-packages\pandas\core\frame.py", line 4727, in set_index
    raise KeyError(f"None of {missing} are in the columns")
KeyError: 'None of [2001] are in the columns'
>>> anio
2001

Question

How can I set the index to the DataFrame and get as index 2001 in this simple case? Be aware that the type for the index is originally a range. Maybe the issue is there but I'm unsure.

Solution 1:

Try maximosVerano.index=[anio]

set_index() is a function for setting a column as the new index of a dataframe. For setting the index, you can use simple assignment

df=pd.DataFrame(index=[0], columns=[1,2,3],data=[[1,2,3]])

df
    1   2   3
0   1   2   3

anio=2001

df.index=[anio]

df
        1   2   3
2001    1   2   3