How to Add a New Column to an Existing Polars DataFrame

What will you learn?

In this comprehensive guide, you will master the art of adding new columns to an existing Polars DataFrame. This skill is crucial for effective data manipulation and analysis in Python.

Introduction to the Problem and Solution

When working with data in Python, utilizing data frames for storage and manipulation is a common practice. Polars, a high-performance DataFrames library written in Rust, offers exceptional speed. One frequent operation involves adding new columns based on calculations or static values to enhance datasets or prepare them for analysis.

The solution lies in leveraging Polars’ functionalities to either directly assign a new column or use methods that enable conditional additions or computed columns based on existing data. We will delve into both simple assignments of static values and dynamic computations derived from other column values.


import polars as pl

# Sample DataFrame
df = pl.DataFrame({
  "A": [1, 2, 3],
  "B": [4, 5, 6]

# Adding a new static column 'C'
df = df.with_column(pl.lit(7).alias("C"))

# Adding a dynamic column 'D', which is A + B
df = df.with_column((pl.col("A") + pl.col("B")).alias("D"))


# Copyright PHD


  • Importing Polars Library: Begin by importing the polars module as pl to access its functions.

  • Creating a Sample DataFrame: Generate a basic DataFrame df with two columns (“A” and “B”) for demonstration purposes.

  • Adding Static Column: Introduce a new static column named “C” with all values set to 7 using the .with_column() method along with pl.lit(7).alias(“C”).

  • Adding Dynamic Column: Incorporate a dynamic column (“D”) by calculating the sum of columns “A” and “B” using .with_column(). Specify (pl.col(“A”) + pl.col(“B”)).alias(“D”) to compute each row’s value in “D”.

    1. Can I add multiple columns at once?

      • Yes, you can chain multiple .with_column() calls or pass several expressions inside one call separated by commas.
    2. How do I create conditional columns?

      • Utilize the .with_columns() method alongside conditions constructed with logical operators within expressions like (pl.col(‘column_name’) > some_value).then(value_if_true).otherwise(value_if_false).
    3. Is it possible to remove columns?

      • To remove columns from your DataFrame, utilize the .drop() method specifying the name(s) of the column(s) you wish to drop.
    4. Can I modify an existing column instead of adding a new one?

      • Modify an existing column by reassigning it with .with_column(pl.col(‘existing_column’).some_operation().alias(‘existing_column’)).
    5. How does adding large numbers of rows affect performance?

      • While Polars is optimized for performance, operations involving extensive datasets may still take more time due to resource limitations.

Mastering the addition of new columns�whether through direct assignment or computation based on other dataset elements�in Polars is fundamental for efficient dataset manipulation. This knowledge empowers you with flexibility and capability when preparing your data for analysis or visualization tasks.

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