understanding level =0 and group_keys

Question

I am using pandas groupby and finding size for ex:

dd=df.groupby(['value','year','team']).size()

and it giving me output as:

value  year  team
0      2000  B       2
1      2000  A       2
       2001  A       1
2      2001  B       1
3      2001  A       2

my question is what is level =0 and group_keys (given below )which is applying on grouped dataframe dd.

ddf3=dd.groupby(level=0,group_keys=False).apply(function).reset_index()

is (level=0) be 'value' column in grouped dataframe dd.

Please help me.

Answer 1

df.groupby(level=0)

It specifies the first index of the Dataframe. When you have multiple indices and you need to groupby only one index of those multiple indices of the dataframe we use it.

It means:

• level 0 -> First Index
• level 1 -> Second Index
• etc..

Answer 2

The level in groupby() is used when you have multiple indices and you want to use only one index of the DataFrame. For example:

df = pd.DataFrame([{'values':0,'year':2000,'team':'A'}, 
                   {'values':1,'year':2000,'team':'B'}, 
                   {'values':2,'year':2001,'team':'B'}
                  ])
df = df.groupby(['values','year','team']).size()
df

Output:

values  year  team
0       2000  A       1
1       2000  B       1
2       2001  B       1

df = df.groupby(level=1).size()
df

Output:

year
2000    2
2001    1