groupby() basics in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the groupby() function do in pandas?

It splits the data into groups based on one or more columns, so you can perform operations on each group separately.

beginner

How do you use groupby() to find the average of a column for each group?

First, use df.groupby('column_name') to group the data, then call .mean() on the grouped object to get the average for each group.

beginner

What type of object does groupby() return before applying an aggregation?

It returns a DataFrameGroupBy object, which is like a container holding the groups but not the final result yet.

intermediate

Can you group by multiple columns using groupby()? How?

Yes, by passing a list of column names like df.groupby(['col1', 'col2']). This groups data by unique combinations of those columns.

intermediate

What is the difference between groupby() and filtering data before grouping?

groupby() organizes data into groups for aggregation, while filtering removes rows before grouping. Grouping works on the full data or filtered data depending on when you apply it.

What does df.groupby('Category').sum() do?

A Adds up values in each numeric column for each Category group

B Counts the number of rows in each Category

C Sorts the dataframe by Category

D Filters rows where Category is 'sum'

The sum() function adds numeric values for each group created by groupby('Category').

Which object type does groupby() return before aggregation?

A DataFrame

B DataFrameGroupBy

C Series

D List

groupby() returns a DataFrameGroupBy object that holds grouped data.

How do you group data by two columns 'A' and 'B'?

A df.group_by('A', 'B')

B df.groupby('A', 'B')

C df.groupby('A' & 'B')

D df.groupby(['A', 'B'])

Pass a list of column names to groupby() to group by multiple columns.

Which method gives the average value per group?

A mean()

B sum()

C count()

D max()

mean() calculates the average of numeric columns for each group.

If you want to count rows in each group, which method do you use?

A sum()

B mean()

C count()

D group()

count() counts the number of non-null rows in each group.

Explain how groupby() works in pandas and give a simple example.

Think about how you might group a list of items by category and then find the average price.

Describe how to group data by multiple columns and why this might be useful.

Imagine sorting your music collection by artist and album.