Converting to categorical in Pandas

Introduction

We convert data to categorical to save memory and make analysis faster. It also helps group similar values together.

When you have a column with a few repeated text values, like colors or categories.

When you want to reduce the size of a large dataset with repeated labels.

When preparing data for machine learning models that work better with categories.

When you want to order categories, like small, medium, large.

When you want to improve performance of filtering or grouping operations.

Syntax

Pandas

df['column'] = df['column'].astype('category')

Use astype('category') to convert a column to categorical type.

You can also specify the order of categories using pd.Categorical.

Examples

Convert the 'color' column to categorical type.

Pandas

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

Create an ordered categorical column 'size' with specific order.

Pandas

df['size'] = pd.Categorical(df['size'], categories=['small', 'medium', 'large'], ordered=True)

Convert 'status' to categorical and print its categories.

Pandas

df['status'] = df['status'].astype('category')
print(df['status'].cat.categories)

Sample Program

This code creates a small table with colors and sizes. It changes 'color' to a category type and 'size' to an ordered category. Then it prints info about the data, the table itself, and the categories for each column.

Pandas

import pandas as pd

# Create a sample DataFrame
data = {'color': ['red', 'blue', 'green', 'blue', 'red', 'green', 'red'],
        'size': ['small', 'large', 'medium', 'medium', 'small', 'large', 'small']}
df = pd.DataFrame(data)

# Convert 'color' to categorical
df['color'] = df['color'].astype('category')

# Convert 'size' to ordered categorical
df['size'] = pd.Categorical(df['size'], categories=['small', 'medium', 'large'], ordered=True)

# Show DataFrame info to see memory usage and types
print(df.info())

# Show the DataFrame
print(df)

# Show categories of 'color'
print('Color categories:', df['color'].cat.categories)

# Show categories of 'size'
print('Size categories:', df['size'].cat.categories)

Important Notes

Categorical data uses less memory than strings.

Ordered categories allow comparisons like "small < medium".

Be careful: converting back to string requires astype(str).

Summary

Convert repeated text columns to categorical to save memory and speed up analysis.

Use astype('category') for simple conversion.

Use pd.Categorical to set category order.

Practice

1. What is the main benefit of converting a text column to a categorical type in pandas?

easy

A. It removes duplicate rows from the DataFrame.

B. It changes the text to numbers automatically.

C. It saves memory and speeds up analysis.

D. It sorts the DataFrame by that column.

Solution

1. Step 1: Understand what categorical type does

   Converting text to categorical stores repeated values efficiently, saving memory.

2. Step 2: Recognize impact on analysis speed

   Operations on categorical data are faster because pandas uses codes internally.

3. Final Answer:

   It saves memory and speeds up analysis. -> Option C

4. Quick Check:

   Converting to categorical = Save memory and speed [OK]

Hint: Categorical = less memory + faster analysis [OK]

Common Mistakes:

• Thinking it changes text to numbers visibly
• Assuming it sorts data automatically
• Confusing with removing duplicates

2. Which of the following is the correct syntax to convert a pandas DataFrame column df['color'] to categorical?

easy

A. df['color'] = df['color'].astype('category')

B. df['color'] = pd.to_categorical(df['color'])

C. df['color'] = df['color'].convert('category')

D. df['color'] = df['color'].category()

Solution

1. Step 1: Recall pandas method for type conversion

   The correct method is astype('category') to convert column type.

2. Step 2: Check each option

   df['color'] = df['color'].astype('category') uses astype('category'), which is correct. Others are invalid pandas syntax.

3. Final Answer:

   df['color'] = df['color'].astype('category') -> Option A

4. Quick Check:

   Use astype('category') to convert [OK]

Hint: Use astype('category') to convert columns [OK]

Common Mistakes:

• Using non-existent methods like convert() or category()
• Trying pd.to_categorical which is not a pandas function
• Missing quotes around 'category'

3. Given the code:

import pandas as pd
df = pd.DataFrame({'size': ['S', 'M', 'L', 'M', 'S']})
df['size'] = pd.Categorical(df['size'], categories=['S', 'M', 'L'], ordered=True)
print(df['size'].dtype)

What will be the output?

medium

A. category

B. object

C. string

D. int64

Solution

1. Step 1: Understand pd.Categorical usage

   Using pd.Categorical converts the column to categorical dtype with order.

2. Step 2: Check dtype after conversion

   The dtype of the column becomes category, not object or string.

3. Final Answer:

   category -> Option A

4. Quick Check:

   pd.Categorical sets dtype to category [OK]

Hint: pd.Categorical sets dtype to category [OK]

Common Mistakes:

• Expecting dtype to remain object
• Confusing category with string dtype
• Thinking dtype becomes int64

4. What is wrong with this code snippet?

import pandas as pd
df = pd.DataFrame({'grade': ['A', 'B', 'C', 'B', 'A']})
df['grade'] = pd.Categorical(df['grade'], categories=['A', 'B', 'C'])
df['grade'] = df['grade'].astype('category', ordered=True)

medium

A. DataFrame column cannot be converted twice.

B. astype() does not accept ordered=True argument.

C. pd.Categorical requires categories to be sorted alphabetically.

D. Missing import statement for pandas.

Solution

1. Step 1: Check astype() method parameters

   The astype() method does not accept an ordered parameter.

2. Step 2: Identify correct way to set ordered

   Ordering must be set when creating the categorical with pd.Categorical(..., ordered=True), not via astype.

3. Final Answer:

   astype() does not accept ordered=True argument. -> Option B

4. Quick Check:

   ordered=True only in pd.Categorical, not astype [OK]

Hint: ordered=True only works in pd.Categorical, not astype [OK]

Common Mistakes:

• Trying to set ordered in astype()
• Assuming categories must be sorted alphabetically
• Thinking multiple conversions cause error

5. You have a DataFrame df with a column 'status' containing repeated text values: 'low', 'medium', 'high'. You want to convert it to a categorical type with the order low < medium < high, and then get the counts of each category in order. Which code snippet correctly does this?

hard

A. df['status'] = df['status'].astype('category', ordered=True)
print(df['status'].value_counts(sort=False))

B. df['status'] = df['status'].astype('category')
print(df['status'].value_counts())

C. df['status'] = pd.Categorical(df['status'])
print(df['status'].value_counts(sort=True))

D. df['status'] = pd.Categorical(df['status'], categories=['low', 'medium', 'high'], ordered=True)
print(df['status'].value_counts(sort=False))

Solution

1. Step 1: Convert with ordered categories

   Use pd.Categorical with categories=['low', 'medium', 'high'] and ordered=True to set order.

2. Step 2: Get counts without sorting

   Use value_counts(sort=False) to get counts in category order.

3. Final Answer:

   df['status'] = pd.Categorical(df['status'], categories=['low', 'medium', 'high'], ordered=True) print(df['status'].value_counts(sort=False)) -> Option D

4. Quick Check:

   pd.Categorical + ordered + value_counts(sort=False) = correct [OK]

Hint: Use pd.Categorical with ordered and value_counts(sort=False) [OK]

Common Mistakes:

• Using astype('category') without order
• Not setting ordered=True for category order
• Using value_counts() without sort=False loses order