Adding and removing categories in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a category in pandas and why use it?

A category in pandas is a special data type for variables with a fixed set of possible values. It saves memory and speeds up operations when you have repeated values like colors or types.

beginner

How do you add a new category to a pandas Categorical column?

Use the method cat.add_categories() on the column. For example, df['col'] = df['col'].cat.add_categories(['new_cat']) adds 'new_cat' as a new category.

beginner

How can you remove a category from a pandas Categorical column?

Use cat.remove_categories() method. For example, df['col'] = df['col'].cat.remove_categories(['old_cat']) removes 'old_cat' from categories.

intermediate

What happens if you remove a category that is currently used in the data?

The values in that category become NaN (missing) because the category no longer exists in the list of allowed categories.

intermediate

How do you reset categories to only those currently used in the data?

Use cat.remove_unused_categories() to drop categories not used in the data, cleaning up the category list.

Which pandas method adds a new category to a Categorical column?

A cat.append_categories()

B cat.insert_categories()

C cat.add_categories()

D cat.new_category()

The correct method to add categories is cat.add_categories().

What happens to data values in a category that is removed using cat.remove_categories()?

A They are duplicated

B They become NaN (missing)

C They are converted to strings

D They stay unchanged

Removing a category causes its values to become NaN because the category no longer exists.

How do you remove categories that are not used in the data?

A cat.remove_unused_categories()

B cat.drop_unused()

C cat.clean_categories()

D cat.trim_categories()

The method cat.remove_unused_categories() removes categories not present in the data.

Why use categories in pandas?

A To sort numbers faster

B To store images

C To create plots automatically

D To save memory and speed up operations

Categories save memory and speed up operations for repeated values.

Which of these is NOT a valid way to modify categories in pandas?

A cat.update_categories()

B cat.remove_categories()

C cat.add_categories()

D cat.remove_unused_categories()

There is no cat.update_categories() method in pandas.

Explain how to add and remove categories in a pandas Categorical column. Include what happens to data when categories are removed.

Think about how categories control allowed values and what happens if you remove one.

Describe why and how you would clean up unused categories in a pandas Categorical column.

Consider categories that no longer appear in the data.