The error “ValueError: cannot setitem on a categorical with a new category, set the categories first” occurs when trying to assign a new category to a categorical variable without specifying the categories beforehand. This generally happens when using pandas Categorical data type.
The Categorical data type in pandas allows efficient storage and processing of data with a limited number of unique values. It is useful when dealing with data that has a small number of distinct categories or levels.
To resolve this error, you need to set the categories of the categorical variable explicitly before assigning a new category. Here’s an example to illustrate this:
import pandas as pd
# Create a DataFrame with a categorical column
df = pd.DataFrame({'fruit': ['apple', 'banana', 'apple', 'orange']})
df['fruit'] = df['fruit'].astype('category')
# Try to assign a new category without setting the categories first
df.loc[3, 'fruit'] = 'grape' # Throws ValueError
# Set the categories explicitly
df['fruit'] = df['fruit'].cat.set_categories(['apple', 'banana', 'orange', 'grape'])
# Now assign a new category
df.loc[3, 'fruit'] = 'grape' # No error
# Resulting DataFrame
print(df)
In this example, we first create a DataFrame with a categorical column “fruit”. We convert the column to a categorical data type using the “astype” method. Then, we attempt to assign a new category “grape” to the fourth row without setting the categories first, which raises the ValueError.
To resolve the error, we use the method “cat.set_categories” to explicitly set the categories of the “fruit” column. We pass the updated list of categories [‘apple’, ‘banana’, ‘orange’, ‘grape’] to include the new category. After setting the categories, we can successfully assign the “grape” category to the fourth row without any errors.
It is important to note that when using categorical data type, assigning a value that is not in the specified categories will raise an error unless you include the new category explicitly.