I need to change the type of few columns in a pandas dataframe. Can't do so using iloc

I guess it is a bug in 1.0.5. I tested on my 1.0.5. I have the same issue as yours. The .loc also has the same issue, so I guess pandas devs break something in iloc/loc. You need to update to latest pandas or use a workaround. If you need a workaround, using assignment as follows

df1[df1.columns[0:27]] = df1.iloc[:, 0:27].astype('int')

I tested it. Above way overcomes this bug. It will turn first 27 columns to dtype int32