To calculate the sum of sales for each salesman in a month before their training date, we need to group by "salesman" and "transaction_month", then apply the aggregation function `sum` to the 'sales' column.

Creating a Seaborn Heatmap with Nested Rows: Advanced Customization Techniques

Merging Rows in a Pandas DataFrame Based on a Date Range

Applying a Function on a Column of a DataFrame Depending on the Value of Another Column and Then GroupBy Using NumPy's `where` Function and Pandas' `groupby` Method

Customizing String Retrieval in Pandas MultiIndex DataFrames for Advanced Analysis

Transforming and Applying Functions with Complex Operations in Pandas: A Step-by-Step Guide

Iterating Through Pandas Dataframe Dict and Outputting The Same Row From All of Them

Returning Multiple Rows of Data from a Pandas DataFrame Using Vectorized Operations

Grouping by One Column and Summing Elements of Another Column in Pandas with Pivot Tables and Crosstabulations

Plotting Nested Lists in a Dictionary: A Step-by-Step Guide