filter() for group-level filtering in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter() function do in pandas groupby objects?

It keeps or removes entire groups based on a condition applied to each group. It helps to select groups that meet a specific rule.

beginner

How do you use filter() to keep groups with more than 3 rows?

Use df.groupby('column').filter(lambda x: len(x) > 3). This keeps only groups where the number of rows is greater than 3.

beginner

True or False: filter() changes the original DataFrame structure by removing some rows.

True. It returns a new DataFrame with only the rows from groups that passed the filter condition.

beginner

What kind of function do you pass to filter()?

You pass a function that takes a group (DataFrame) and returns True or False. True keeps the group, False removes it.

beginner

Give an example use case for filter() in group-level filtering.

For example, keep only customers who made more than 5 purchases. Group by customer, then filter groups with size > 5.

What does filter() do when used after groupby() in pandas?

A Aggregates group data into summary statistics

B Filters individual rows ignoring groups

C Keeps or removes entire groups based on a condition

D Sorts the groups by size

filter() works on groups and keeps or removes whole groups based on the condition you give.

Which of these is a valid condition function for filter()?

A lambda x: x['value'].mean() > 10

B lambda x: x['value'] + 10

C lambda x: x.sort_values()

D lambda x: x.head()

The function must return True or False. Checking if mean > 10 returns a boolean.

If you want to keep groups with at least 5 rows, what is the correct filter condition?

A lambda x: x.mean()

B lambda x: x.sum() > 5

C lambda x: x['col'] > 5

D lambda x: len(x) >= 5

Length of group >= 5 keeps groups with 5 or more rows.

What is the output type of groupby().filter()?

A GroupBy object

B DataFrame with filtered groups

C Series with group keys

D List of groups

The output is a DataFrame containing only rows from groups that passed the filter.

Can filter() be used to filter groups based on aggregated statistics like sum or mean?

A Yes, by applying a function that returns True or False based on the statistic

B No, it only filters by group size

C No, it only filters by row values

D Yes, but only for numeric columns

You can use any condition that returns True or False, including those based on aggregated values.

Explain how to use filter() to keep only groups where the average of a column is above a threshold.

Think about passing a function that calculates mean and returns True or False.

Describe the difference between filter() and apply() when used with groupby in pandas.

Focus on what each function returns and how they treat groups.