You can use the following basic syntax to perform a groupby and count with condition in a pandas DataFrame:

df.groupby('var1')['var2'].apply(lambda x: (x=='val').sum()).reset_index(name='count')

This particular syntax groups the rows of the DataFrame based on var1 and then counts the number of rows where var2 is equal to ‘val.’

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

Example: Groupby and Count with Condition 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', 'A', '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    A  Fo      14
4    B  Gu      14
5    B  Gu      11
6    B  Fo      20
7    B  Fo      28

The following code shows how to group the DataFrame by the team variable and count the number of rows where the pos variable is equal to ‘Gu’:

#groupby team and count number of 'pos' equal to 'Gu'
df_count = df.groupby('team')['pos'].apply(lambda x: (x=='Gu').sum()).reset_index(name='count')

#view results
print(df_count)

  team  count
0    A      1
1    B      2

From the output we can see:

• Team A has 1 row where the pos column is equal to ‘Gu’
• Team B has 2 rows where the pos column is equal to ‘Gu’

We can use similar syntax to perform a groupby and count with some numerical condition.

For example, the following code shows how to group by the team variable and count the number of rows where the points variable is greater than 15:

#groupby team and count number of 'points' greater than 15
df_count = df.groupby('team')['points'].apply(lambda x: (x>15).sum()).reset_index(name='count')

#view results
print(df_count)

  team  count
0    A      3
1    B      2

From the output we can see:

• Team A has 3 rows where the points column is greater than 15
• Team B has 2 rows where the points column is greater than 15 

You can use similar syntax to perform a groupby and count with any specific condition you’d like.

Additional Resources

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