Pandas distinct count as a DataFrame

Question

Suppose I have a Pandas DataFrame called df with columns a and b and what I want is the number of distinct values of b per each a . I would do:

distcounts = df.groupby('a')['b'].nunique()

which gives the desidered result, but it is as Series object rather than another DataFrame. I'd like a DataFrame instead. In regular SQL, I'd do:

SELECT a, COUNT(DISTINCT(b)) FROM df

and haven't been able to emulate this query in Pandas exactly. How to?

Answer 1

I think you need reset_index :

distcounts = df.groupby('a')['b'].nunique().reset_index()

Sample:

df = pd.DataFrame({'a':[7,8,8],
                   'b':[4,5,6]})

print (df)
   a  b
0  7  4
1  8  5
2  8  6

distcounts = df.groupby('a')['b'].nunique().reset_index()
print (distcounts)
   a  b
0  7  1
1  8  2

Answer 2

使用Groupby.agg另一种替代方法：

df.groupby('a', as_index=False).agg({'b': 'nunique'})