["python","pandas","dataframe","group-by","aggregation"]
Aggregation in Pandas

How can I perform aggregation with Pandas? No DataFrame after aggregation! What happened? How can I aggregate mainly strings columns (to lists, tuples, strings with separator)? How can I aggregate counts? How can I create a new column filled by aggregated values? I've seen these recurring questions asking about various faces of the pandas aggregate functionality. Most of the information regarding aggregation and its various use cases today is fragmented across dozens of badly worded, unsearchable posts. The aim here is to collate some of the more important points for posterity. This Q&A is meant to be the next instalment in a series of helpful user-guides: How can I pivot a dataframe? What are the 'levels', 'keys', and names arguments for in Pandas' concat function? How do I operate on a DataFrame with a Series for every column? Pandas Merging 101 Please note that this post is not meant to be a replacement for the documentation about aggregation and about groupby, so please read that as well!
Question 1 How can I perform aggregation with Pandas? Expanded aggregation documentation. Aggregating functions are the ones that reduce the dimension of the returned objects. It means output Series/DataFrame have less or same rows like original. Some common aggregating functions are tabulated below: Function Description mean() Compute mean of groups sum() Compute sum of group values size() Compute group sizes count() Compute count of group std() Standard deviation of groups var() Compute variance of groups sem() Standard error of the mean of groups describe() Generates descriptive statistics first() Compute first of group values last() Compute last of group values nth() Take nth value, or a subset if n is a list min() Compute min of group values max() Compute max of group values np.random.seed(123) df = pd.DataFrame({'A' : ['foo', 'foo', 'bar', 'foo', 'bar', 'foo'], 'B' : ['one', 'two', 'three','two', 'two', 'one'], 'C' : np.random.randint(5, size=6), 'D' : np.random.randint(5, size=6), 'E' : np.random.randint(5, size=6)}) print (df) A B C D E 0 foo one 2 3 0 1 foo two 4 1 0 2 bar three 2 1 1 3 foo two 1 0 3 4 bar two 3 1 4 5 foo one 2 1 0 Aggregation by filtered columns and Cython implemented functions: df1 = df.groupby(['A', 'B'], as_index=False)['C'].sum() print (df1) A B C 0 bar three 2 1 bar two 3 2 foo one 4 3 foo two 5 An aggregate function is used for all columns without being specified in the groupby function, here the A, B columns: df2 = df.groupby(['A', 'B'], as_index=False).sum() print (df2) A B C D E 0 bar three 2 1 1 1 bar two 3 1 4 2 foo one 4 4 0 3 foo two 5 1 3 You can also specify only some columns used for aggregation in a list after the groupby function: df3 = df.groupby(['A', 'B'], as_index=False)['C','D'].sum() print (df3) A B C D 0 bar three 2 1 1 bar two 3 1 2 foo one 4 4 3 foo two 5 1 Same results by using function DataFrameGroupBy.agg: df1 = df.groupby(['A', 'B'], as_index=False)['C'].agg('sum') print (df1) A B C 0 bar three 2 1 bar two 3 2 foo one 4 3 foo two 5 df2 = df.groupby(['A', 'B'], as_index=False).agg('sum') print (df2) A B C D E 0 bar three 2 1 1 1 bar two 3 1 4 2 foo one 4 4 0 3 foo two 5 1 3 For multiple functions applied for one column use a list of tuples - names of new columns and aggregated functions: df4 = (df.groupby(['A', 'B'])['C'] .agg([('average','mean'),('total','sum')]) .reset_index()) print (df4) A B average total 0 bar three 2.0 2 1 bar two 3.0 3 2 foo one 2.0 4 3 foo two 2.5 5 If want to pass multiple functions is possible pass list of tuples: df5 = (df.groupby(['A', 'B']) .agg([('average','mean'),('total','sum')])) print (df5) C D E average total average total average total A B bar three 2.0 2 1.0 1 1.0 1 two 3.0 3 1.0 1 4.0 4 foo one 2.0 4 2.0 4 0.0 0 two 2.5 5 0.5 1 1.5 3 Then get MultiIndex in columns: print (df5.columns) MultiIndex(levels=[['C', 'D', 'E'], ['average', 'total']], labels=[[0, 0, 1, 1, 2, 2], [0, 1, 0, 1, 0, 1]]) And for converting to columns, flattening MultiIndex use map with join: df5.columns = df5.columns.map('_'.join) df5 = df5.reset_index() print (df5) A B C_average C_total D_average D_total E_average E_total 0 bar three 2.0 2 1.0 1 1.0 1 1 bar two 3.0 3 1.0 1 4.0 4 2 foo one 2.0 4 2.0 4 0.0 0 3 foo two 2.5 5 0.5 1 1.5 3 Another solution is pass list of aggregate functions, then flatten MultiIndex and for another columns names use str.replace: df5 = df.groupby(['A', 'B']).agg(['mean','sum']) df5.columns = (df5.columns.map('_'.join) .str.replace('sum','total') .str.replace('mean','average')) df5 = df5.reset_index() print (df5) A B C_average C_total D_average D_total E_average E_total 0 bar three 2.0 2 1.0 1 1.0 1 1 bar two 3.0 3 1.0 1 4.0 4 2 foo one 2.0 4 2.0 4 0.0 0 3 foo two 2.5 5 0.5 1 1.5 3 If want specified each column with aggregated function separately pass dictionary: df6 = (df.groupby(['A', 'B'], as_index=False) .agg({'C':'sum','D':'mean'}) .rename(columns={'C':'C_total', 'D':'D_average'})) print (df6) A B C_total D_average 0 bar three 2 1.0 1 bar two 3 1.0 2 foo one 4 2.0 3 foo two 5 0.5 You can pass custom function too: def func(x): return x.iat[0] + x.iat[-1] df7 = (df.groupby(['A', 'B'], as_index=False) .agg({'C':'sum','D': func}) .rename(columns={'C':'C_total', 'D':'D_sum_first_and_last'})) print (df7) A B C_total D_sum_first_and_last 0 bar three 2 2 1 bar two 3 2 2 foo one 4 4 3 foo two 5 1 Question 2 No DataFrame after aggregation! What happened? Aggregation by two or more columns: df1 = df.groupby(['A', 'B'])['C'].sum() print (df1) A B bar three 2 two 3 foo one 4 two 5 Name: C, dtype: int32 First check the Index and type of a Pandas object: print (df1.index) MultiIndex(levels=[['bar', 'foo'], ['one', 'three', 'two']], labels=[[0, 0, 1, 1], [1, 2, 0, 2]], names=['A', 'B']) print (type(df1)) <class 'pandas.core.series.Series'> There are two solutions for how to get MultiIndex Series to columns: add parameter as_index=False df1 = df.groupby(['A', 'B'], as_index=False)['C'].sum() print (df1) A B C 0 bar three 2 1 bar two 3 2 foo one 4 3 foo two 5 use Series.reset_index: df1 = df.groupby(['A', 'B'])['C'].sum().reset_index() print (df1) A B C 0 bar three 2 1 bar two 3 2 foo one 4 3 foo two 5 If group by one column: df2 = df.groupby('A')['C'].sum() print (df2) A bar 5 foo 9 Name: C, dtype: int32 ... get Series with Index: print (df2.index) Index(['bar', 'foo'], dtype='object', name='A') print (type(df2)) <class 'pandas.core.series.Series'> And the solution is the same like in the MultiIndex Series: df2 = df.groupby('A', as_index=False)['C'].sum() print (df2) A C 0 bar 5 1 foo 9 df2 = df.groupby('A')['C'].sum().reset_index() print (df2) A C 0 bar 5 1 foo 9 Question 3 How can I aggregate mainly strings columns (to lists, tuples, strings with separator)? df = pd.DataFrame({'A' : ['a', 'c', 'b', 'b', 'a', 'c', 'b'], 'B' : ['one', 'two', 'three','two', 'two', 'one', 'three'], 'C' : ['three', 'one', 'two', 'two', 'three','two', 'one'], 'D' : [1,2,3,2,3,1,2]}) print (df) A B C D 0 a one three 1 1 c two one 2 2 b three two 3 3 b two two 2 4 a two three 3 5 c one two 1 6 b three one 2 Instead of an aggregation function, it is possible to pass list, tuple, set for converting the column: df1 = df.groupby('A')['B'].agg(list).reset_index() print (df1) A B 0 a [one, two] 1 b [three, two, three] 2 c [two, one] An alternative is use GroupBy.apply: df1 = df.groupby('A')['B'].apply(list).reset_index() print (df1) A B 0 a [one, two] 1 b [three, two, three] 2 c [two, one] For converting to strings with a separator, use .join only if it is a string column: df2 = df.groupby('A')['B'].agg(','.join).reset_index() print (df2) A B 0 a one,two 1 b three,two,three 2 c two,one If it is a numeric column, use a lambda function with astype for converting to strings: df3 = (df.groupby('A')['D'] .agg(lambda x: ','.join(x.astype(str))) .reset_index()) print (df3) A D 0 a 1,3 1 b 3,2,2 2 c 2,1 Another solution is converting to strings before groupby: df3 = (df.assign(D = df['D'].astype(str)) .groupby('A')['D'] .agg(','.join).reset_index()) print (df3) A D 0 a 1,3 1 b 3,2,2 2 c 2,1 For converting all columns, don't pass a list of column(s) after groupby. There isn't any column D, because automatic exclusion of 'nuisance' columns. It means all numeric columns are excluded. df4 = df.groupby('A').agg(','.join).reset_index() print (df4) A B C 0 a one,two three,three 1 b three,two,three two,two,one 2 c two,one one,two So it's necessary to convert all columns into strings, and then get all columns: df5 = (df.groupby('A') .agg(lambda x: ','.join(x.astype(str))) .reset_index()) print (df5) A B C D 0 a one,two three,three 1,3 1 b three,two,three two,two,one 3,2,2 2 c two,one one,two 2,1 Question 4 How can I aggregate counts? df = pd.DataFrame({'A' : ['a', 'c', 'b', 'b', 'a', 'c', 'b'], 'B' : ['one', 'two', 'three','two', 'two', 'one', 'three'], 'C' : ['three', np.nan, np.nan, 'two', 'three','two', 'one'], 'D' : [np.nan,2,3,2,3,np.nan,2]}) print (df) A B C D 0 a one three NaN 1 c two NaN 2.0 2 b three NaN 3.0 3 b two two 2.0 4 a two three 3.0 5 c one two NaN 6 b three one 2.0 Function GroupBy.size for size of each group: df1 = df.groupby('A').size().reset_index(name='COUNT') print (df1) A COUNT 0 a 2 1 b 3 2 c 2 Function GroupBy.count excludes missing values: df2 = df.groupby('A')['C'].count().reset_index(name='COUNT') print (df2) A COUNT 0 a 2 1 b 2 2 c 1 This function should be used for multiple columns for counting non-missing values: df3 = df.groupby('A').count().add_suffix('_COUNT').reset_index() print (df3) A B_COUNT C_COUNT D_COUNT 0 a 2 2 1 1 b 3 2 3 2 c 2 1 1 A related function is Series.value_counts. It returns the size of the object containing counts of unique values in descending order, so that the first element is the most frequently-occurring element. It excludes NaNs values by default. df4 = (df['A'].value_counts() .rename_axis('A') .reset_index(name='COUNT')) print (df4) A COUNT 0 b 3 1 a 2 2 c 2 If you want same output like using function groupby + size, add Series.sort_index: df5 = (df['A'].value_counts() .sort_index() .rename_axis('A') .reset_index(name='COUNT')) print (df5) A COUNT 0 a 2 1 b 3 2 c 2 Question 5 How can I create a new column filled by aggregated values? Method GroupBy.transform returns an object that is indexed the same (same size) as the one being grouped. See the Pandas documentation for more information. np.random.seed(123) df = pd.DataFrame({'A' : ['foo', 'foo', 'bar', 'foo', 'bar', 'foo'], 'B' : ['one', 'two', 'three','two', 'two', 'one'], 'C' : np.random.randint(5, size=6), 'D' : np.random.randint(5, size=6)}) print (df) A B C D 0 foo one 2 3 1 foo two 4 1 2 bar three 2 1 3 foo two 1 0 4 bar two 3 1 5 foo one 2 1 df['C1'] = df.groupby('A')['C'].transform('sum') df['C2'] = df.groupby(['A','B'])['C'].transform('sum') df[['C3','D3']] = df.groupby('A')['C','D'].transform('sum') df[['C4','D4']] = df.groupby(['A','B'])['C','D'].transform('sum') print (df) A B C D C1 C2 C3 D3 C4 D4 0 foo one 2 3 9 4 9 5 4 4 1 foo two 4 1 9 5 9 5 5 1 2 bar three 2 1 5 2 5 2 2 1 3 foo two 1 0 9 5 9 5 5 1 4 bar two 3 1 5 3 5 2 3 1 5 foo one 2 1 9 4 9 5 4 4