Each Product's Share of Its Category

"What fraction of its category does this product represent" needs a group total put back beside every original row. transform is the tool for that, and not knowing it exists is why people write merges they do not need.

Write category_share(df).

df has columns product, category and revenue, one row per product. Return a new DataFrame with the original columns plus a share column: that product's revenue divided by the total revenue of its category.