Pandas groupby - set of different values

问题:

I have this dataframe

x = pd.DataFrame.from_dict({'cat1':['A', 'A', 'A', 'B', 'B', 'C', 'C', 'C'], 'cat2':['X', 'X', 'Y', 'Y', 'Y', 'Y', 'Z', 'Z']})

  cat1 cat2
0    A    X
1    A    X
2    A    Y
3    B    Y
4    B    Y
5    C    Y
6    C    Z
7    C    Z

I want to group by cat1, and then aggregate cat2 as sets of different values, such as

  cat1 cat2
0    A    (X, Y)
1    B    (Y,)
2    C    (Y, Z)

This is part of a bigger dataframe with more columns, each of which has its own aggregation function, so how do I pass this functionality to the aggregation dictionary?

回答1:

Use lambda function with set or unique, also convert output to tuples:

x = pd.DataFrame.from_dict({'cat1':['A', 'A', 'A', 'B', 'B', 'C', 'C', 'C'], 
                            'cat2':['X', 'X', 'Y', 'Y', 'Y', 'Y', 'Z', 'Z'],
                             'col':range(8)})
print (x)
  cat1 cat2  col
0    A    X    0
1    A    X    1
2    A    Y    2
3    B    Y    3
4    B    Y    4
5    C    Y    5
6    C    Z    6
7    C    Z    7

a = x.groupby('cat1').agg({'cat2': lambda x: tuple(set(x)), 'col':'sum'})
print (a)
        cat2  col
cat1             
A     (Y, X)    3
B       (Y,)    7
C     (Y, Z)   18

Or:

a = x.groupby('cat1').agg({'cat2': lambda x: tuple(x.unique()), 'col':'sum'})
print (a)
        cat2  col
cat1             
A     (X, Y)    3
B       (Y,)    7
C     (Y, Z)   18

EDIT:

f = lambda x: tuple(x.unique())
f.__name__ = 'my_name'
a = x.groupby('cat1')['cat2'].agg(['min', 'max', 'nunique', f])
print (a)
     min max  nunique my_name
cat1                         
A      X   Y        2  (X, Y)
B      Y   Y        1    (Y,)
C      Y   Z        2  (Y, Z)

If there is only one lambda function or no problem with column name <lambda>:

a = x.groupby('cat1')['cat2'].agg(['min', 'max', 'nunique', lambda x: tuple(x.unique())])
print (a)
     min max  nunique <lambda>
cat1                          
A      X   Y        2   (X, Y)
B      Y   Y        1     (Y,)
C      Y   Z        2   (Y, Z)

回答2:

x.groupby('cat1')['cat2'].unique().reset_index()

# Returns 
  cat1    cat2
0    A  [X, Y]
1    B     [Y]
2    C  [Y, Z]

This first groups the entire dataframe by 'cat1', selects only the series 'cat2', and reduces each group to the unique set of 'cat2' values. The result puts the 'cat1' values in the index, so reset_index() will pull those values back out as a column if you need it in that format.

回答3:

Groupby and unique gives you unique values

x.groupby('cat1').cat2.unique()

A    [X, Y]
B       [Y]
C    [Y, Z]

If you want to have the output in tuple, try

x.groupby('cat1').cat2.unique().apply(tuple)

A    (X, Y)
B      (Y,)
C    (Y, Z)

回答4:

Or we can filter the dataframe before groupby

x.drop_duplicates().groupby('cat1').cat2.apply(tuple)
Out[777]: 
cat1
A    (X, Y)
B      (Y,)
C    (Y, Z)
Name: cat2, dtype: object

回答5:

x.groupby('cat1').agg(lambda x: set(x))

Output