dfgroupbykeystransform

The `dfgroupbykeystransform` function is a method commonly found in data manipulation libraries, particularly those built for working with DataFrames. Its primary purpose is to perform a transformation operation on groups of data within a DataFrame. The function works by first grouping the DataFrame based on one or more specified "keys," which are columns that define the distinct categories or segments of the data. Once the DataFrame is grouped, a transformation function is applied independently to each of these groups.

Unlike aggregation functions, which reduce each group to a single value, transformation functions return a result that has the same index and size as the original group. This means that for each row in a group, the transformation function calculates a new value that is then placed back into the corresponding row of the resulting DataFrame. This is particularly useful for operations such as calculating a percentage of a group total, filling missing values based on group statistics, or applying a custom normalization within each group. The `dfgroupbykeystransform` function simplifies these group-wise calculations, making them efficient and easy to implement.