You can use the following basic syntax to group rows by year in a pandas DataFrame:
df.groupby(df.your_date_column.dt.year)['values_column'].sum()
This particular formula groups the rows by date in your_date_column and calculates the sum of values for the values_column in the DataFrame.
Note that the dt.year() function extracts the year from a date column in pandas.
The following example shows how to use this syntax in practice.
Example: How to Group by Year in Pandas
Suppose we have the following pandas DataFrame that shows the sales made by some company on various dates:
import pandas as pd
#create DataFrame
df = pd.DataFrame({'date': pd.date_range(start='1/1/2020', freq='3m', periods=10),
'sales': [6, 8, 9, 11, 13, 8, 8, 15, 22, 9],
'returns': [0, 3, 2, 2, 1, 3, 2, 4, 1, 5]})
#view DataFrame
print(df)
date sales returns
0 2020-01-31 6 0
1 2020-04-30 8 3
2 2020-07-31 9 2
3 2020-10-31 11 2
4 2021-01-31 13 1
5 2021-04-30 8 3
6 2021-07-31 8 2
7 2021-10-31 15 4
8 2022-01-31 22 1
9 2022-04-30 9 5
Related: How to Create a Date Range in Pandas
We can use the following syntax to calculate the sum of sales grouped by year:
#calculate sum of sales grouped by year
df.groupby(df.date.dt.year)['sales'].sum()
date
2020 34
2021 44
2022 31
Name: sales, dtype: int64
Here’s how to interpret the output:
- The total sales made during 2020 was 34.
- The total sales made during 2021 was 44.
- The total sales made during 2022 was 31.
We can use similar syntax to calculate the max of the sales values grouped by year:
#calculate max of sales grouped by year
df.groupby(df.date.dt.year)['sales'].max()
date
2020 11
2021 15
2022 22
Name: sales, dtype: int64
We can use similar syntax to calculate any value we’d like grouped by the year value of a date column.
Note: You can find the complete documentation for the GroupBy operation in pandas here.
Additional Resources
The following tutorials explain how to perform other common operations in pandas:
Pandas: How to Calculate Cumulative Sum by Group
Pandas: How to Count Unique Values by Group
Pandas: How to Calculate Correlation By Group
