Votre premier résumé avec groupby

Often you do not care about single rows, you care about totals per category. groupby bundles rows that share a value so you can summarize each bunch.

You choose a column whose values define the groups. Group by region and every distinct region becomes its own bucket of rows.

A bare groupby is lazy and shows almost nothing. The magic appears once you ask for a summary like a sum or a mean.

Chain size to count how many rows landed in each group, a fast way to see how your data splits across categories.

Add mean after selecting a numeric column to get the average per group, like mean sales for every region.

Swap in sum to total a column per group. This is how you turn daily rows into one number per region or month.

Select one column after grouping so the result stays focused. Without it, pandas tries to aggregate every numeric column at once.

One key and one stat give you a Series indexed by group. The group labels become the index, the stats become the values.

Group results arrive in key order. Chain sort_values to rank groups, like finding which region sells the most.

By default group aggregations like mean quietly ignore NaN, so a few gaps will not break your per-group numbers.

Say the chain out loud: group by region, take sales, compute the mean. Reading it left to right keeps long chains clear.

You need average sales for each region. Which line works?

You can now group rows by a key and summarize with size, sum, or mean, then sort the result to rank groups. Big skill unlocked! 🚀