Altcademy - a Forbes magazine logo Best Coding Bootcamp 2023

How to add a column in Pandas

Understanding Pandas DataFrames

Before we dive into the process of adding a column, let's quickly understand what we're working with. In Pandas, a DataFrame is like a table you might find in a spreadsheet. It's made up of rows and columns, where each column can be thought of as a list of entries, much like a column in an Excel spreadsheet.

Adding a Column: The Basics

Imagine you have a DataFrame that's like a recipe card. Each row is a different recipe, and each column contains information about that recipe, like the name, the prep time, and the main ingredient. Now, let's say you want to add a new piece of information: the number of calories. This is like sticking a post-it note onto your recipe card in a new column.

Here's how you can do this in Pandas:

import pandas as pd

# Let's create a simple DataFrame to work with
df = pd.DataFrame({
    'Recipe Name': ['Pancakes', 'Spaghetti', 'Chocolate Cake'],
    'Prep Time': [10, 20, 45],
    'Main Ingredient': ['Flour', 'Pasta', 'Chocolate']

# Now, let's add a new column for calories
df['Calories'] = [300, 400, 700]

# Let's see what our DataFrame looks like now

After running this code, you'll see that your DataFrame now has a new column labeled "Calories" with the values you provided.

Using Assignment to Add Columns

The method we just used is called assignment. It's like telling Pandas, "Hey, here's a new list of values. Please add it as a new column to my DataFrame." When you use the equals sign (=), you're assigning the list of calories to a new column in the DataFrame.

Adding Calculated Columns

Sometimes, you might want to add a column that's calculated from other columns. For example, if you want to add a column that shows the prep time in hours instead of minutes, you'd divide the 'Prep Time' column by 60.

Here's how you can do that:

# Adding a new column 'Prep Time in Hours'
df['Prep Time in Hours'] = df['Prep Time'] / 60


Now, your DataFrame has a new column that's calculated from the 'Prep Time' column.

Inserting Columns at Specific Positions

What if you want to control where the new column appears? For instance, you might want the 'Calories' column to be the second column, not the last. Pandas has a function called insert() for this purpose.

# Inserting the 'Calories' column as the second column
df.insert(1, 'Calories', [300, 400, 700])


The insert() function takes three arguments: the position where you want the new column, the name of the new column, and the list of values.

Adding Columns with assign()

There's another way to add columns in a more "functional programming" style, which is using the assign() method. This is like saying, "Please give me a new DataFrame that's just like the old one but with this additional information."

# Using 'assign()' to add a 'Serves' column
df = df.assign(Serves=[2, 4, 8])


The assign() method is non-destructive, meaning it doesn't change the original DataFrame unless you explicitly save the result back into the original variable, like we did here.

Dealing with Missing Data

When adding a new column, you might not have all the data you need. In this case, Pandas allows you to fill in the gaps with a placeholder called NaN (Not a Number). This is like saying, "I don't have this piece of information right now, so I'll just leave a blank space here."

# Adding a 'Dietary Notes' column with missing values
df['Dietary Notes'] = pd.Series(['Vegetarian', None, 'Gluten-Free'])


In the 'Dietary Notes' column, we don't have information for the Spaghetti recipe, so it's filled with None, which Pandas converts to NaN.

Using Functions to Populate Columns

You can also use functions to populate a new column. Let's say you want to add a column that says whether the prep time is "Quick" (less than 30 minutes) or "Long" (30 minutes or more). You can use a function to determine this.

# Define a function to categorize prep time
def categorize_prep_time(minutes):
    if minutes < 30:
        return 'Quick'
        return 'Long'

# Apply the function to the 'Prep Time' column
df['Prep Time Category'] = df['Prep Time'].apply(categorize_prep_time)


Here, apply() is used to run the categorize_prep_time function on each value in the 'Prep Time' column.

Intuition and Analogies

Adding a column in Pandas can be thought of as adding a new feature to your favorite gadget. It's a way to enhance the information you're working with and make it more useful.

It's like planting a new tree in your garden. You choose the right spot (position in the DataFrame), plant the tree (add the column), and watch it grow (populate it with data).


Adding a column to a DataFrame in Pandas is a fundamental skill that can open up many possibilities for data analysis. Whether you're sticking a post-it note onto a recipe card, planting a new tree in the garden, or enhancing your gadget with a new feature, the ability to add new information to your data is powerful.

Remember, when you're adding columns, you're not just inserting numbers or text; you're providing new lenses through which to view and understand your data. With the tools and techniques we've discussed, you can now confidently add dimensions to your DataFrames, enrich your analyses, and perhaps, uncover insights that were not visible before.

Happy Data Wrangling!