Warning when doing pandas operation - A value is trying to be set on a copy of a slice from a DataFr-CodePudding

I am learning pandas and were trying to do following.

I think this is not duplicate question which is why I am posting it here.

I want to add column Average Speed by taking mean of that particular animal available in the dataframe. I could do it, may be not correct way. But at the end I am getting a warning.

d = {'Animal': ['Parrot','Falcon','Parrot','Falcon'], 'MaxSpeed' : [56,360,58,380 ]}
adf = pd.DataFrame(d)

grp_spd = adf.groupby(by=['Animal']).mean()

adf.insert(column='Average Speed',loc=2, value="")
for x,y in adf.iterrows():
    print(x)
    print(y.MaxSpeed)
    print(grp_spd.loc[y.Animal].MaxSpeed )
    adf['Average Speed'][x] = grp_spd.loc[y.Animal].MaxSpeed
    #adf.insert(2, 'Average Speed', grp_spd.loc[y.Animal].MaxSpeed)
adf

I am getting following warning message

A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy adf['Average Speed'][x] = grp_spd.loc[y.Animal].MaxSpeed

Can someone tell me how to get rid of this warning and what is the right way to do it.

CodePudding user response：

The reason is when adf['Average Speed'][x] = value is executed, it is not guaranteed if a view is accessed or a reference to the object. Setting value on a view has a risk that as the view object gets cleared, the changes will be lost. You can read more details here.

You can modify your source code as:

d = {'Animal': ['Parrot','Falcon','Parrot','Falcon'], 'MaxSpeed' : [56,360,58,380 ]}
adf = pd.DataFrame(d)
adf["Average Speed"] = adf.groupby("Animal")["MaxSpeed"].transform("mean")

   Animal  MaxSpeed  Average Speed
0  Parrot        56           57.0
1  Falcon       360          370.0
2  Parrot        58           57.0
3  Falcon       380          370.0