Ok I tried a diff approach, Since I didnt knew the conventional way.
What I tried was to create a Sub Dataframe of the original DF which contains only values where Outlet_type = Grocery Store.
Now this sub set only contained values NaN and SMALL for Outlet Size.
I tried to use the fill na simply by assignig all available Nan Values to Small.
It worked without inplace = True.
But now when I add inplace = True its throwing error.
IS this a valid way to fillna, or am I missing something here.
my Code :
Grocery = df.loc[df.Outlet_Type=='Grocery Store']
The Error I am getting :
A value is trying to be set on a copy of a slice from a DataFrame
See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy