How To Assign Value To A Pandas Dataframe, When Subset By Complex Index And Boolean Based Conditions?

I would like to replace values in a pandas dataframe, with a complex subsetting pattern. With the .loc accessor, I was only able to subset by chaining multiple conditions, because

Solution 1:

You can do with one .loc chain of loc assignment will be not safe

df.loc[df.index.isin(['2019-01-05','2019-01-09'])&df.a.eq('foo'),'b']=np.nan