Reset the categories of categorical index in Pandas

Question

I have a dataframe with a column being categorical.

I remove all the rows having one the categories.

How can I make sure the resulting dataframe has only those categories that exist and does not keep the deleted categories in its index?

Answer 1

df = pd.DataFrame({'color':np.random.choice(['Blue','Green','Brown','Red'], 50)})

df.color = df.color.astype('category')

df.color.head()

Output:

0     Blue
1    Green
2     Blue
3    Green
4    Brown
Name: color, dtype: category
Categories (4, object): [Blue, Brown, Green, Red]

Remove Brown from dataframe and category.

df = df.query('color != "Brown"')

df.color = df.color.cat.remove_categories('Brown')

df.color.head()

Output:

0     Blue
1    Green
2     Blue
3    Green
7      Red
Name: color, dtype: category
Categories (3, object): [Blue, Green, Red]