You can use the following basic syntax to add a ‘count’ column to a pandas DataFrame:

df['var1_count'] = df.groupby('var1')['var1'].transform('count')

This particular syntax adds a column called var1_count to the DataFrame that contains the count of values in the column called var1.

The following example shows how to use this syntax in practice.

Example: Add Count Column in Pandas

Suppose we have the following pandas DataFrame that contains information about various basketball players:

import pandas as pd

#create DataFrame
df = pd.DataFrame({'team': ['A', 'A', 'A', 'B', 'B', 'B', 'B', 'B'],
                   'pos': ['Gu', 'Fo', 'Fo', 'Fo', 'Gu', 'Gu', 'Fo', 'Fo'],
                   'points': [18, 22, 19, 14, 14, 11, 20, 28]})

#view DataFrame
print(df)

  team pos  points
0    A  Gu      18
1    A  Fo      22
2    A  Fo      19
3    B  Fo      14
4    B  Gu      14
5    B  Gu      11
6    B  Fo      20
7    B  Fo      28

We can use the following code to add a column called team_count that contains the count of each team:

#add column that shows total count of each team
df['team_count'] = df.groupby('team')['team'].transform('count')

#view updated DataFrame
print(df)

  team pos  points  team_count
0    A  Gu      18           3
1    A  Fo      22           3
2    A  Fo      19           3
3    B  Fo      14           5
4    B  Gu      14           5
5    B  Gu      11           5
6    B  Fo      20           5
7    B  Fo      28           5

There are 3 rows with a team value of A and 5 rows with a team value of B.

Thus:

• For each row where the team is equal to A, the value in the team_count column is 3.
• For each row where the team is equal to B, the value in the team_count column is 5.

You can also add a ‘count’ column that groups by multiple variables.

For example, the following code shows how to add a ‘count’ column that groups by the team and pos variables:

#add column that shows total count of each team and position
df['team_pos_count'] = df.groupby(['team', 'pos')['team'].transform('count')

#view updated DataFrame
print(df)

  team pos  points  team_pos_count
0    A  Gu      18               1
1    A  Fo      22               2
2    A  Fo      19               2
3    B  Fo      14               3
4    B  Gu      14               2
5    B  Gu      11               2
6    B  Fo      20               3
7    B  Fo      28               3

From the output we can see:

• There is 1 row that contains A in the team column and Gu in the pos column.
• There are 2 rows that contain A in the team column and Fo in the pos column.
• There are 3 rows that contain B in the team column and Fo in the pos column.
• There are 2 rows that contain B in the team column and Gu in the pos column.

Additional Resources

The following tutorials explain how to perform other common tasks in pandas: