Working with Polars: UDFs, Returning, and Concatenating DataFrames

What will you learn?

In this comprehensive guide, you will delve into the world of User Defined Functions (UDFs) in Polars. Specifically, you will master the art of returning and concatenating DataFrames. By the end of this tutorial, you will be equipped to elevate your data manipulation skills in Polars by creating custom functions and efficiently merging their results for in-depth analysis.

Introduction to Problem and Solution

Are you seeking to enhance your data manipulation capabilities in Polars? Look no further! This tutorial focuses on harnessing the power of User Defined Functions (UDFs) within the Polars library. By exploring how to process data flexibly using UDFs that return DataFrames and effectively concatenate these outputs, you will unlock a new realm of possibilities for advanced data analysis.

Understanding UDFs and DataFrame Operations in Polars

Polars stands out as a high-speed data manipulation library for Python enthusiasts looking to streamline their data analysis tasks. With its expressive syntax and robust features, Polars empowers users to leverage User Defined Functions (UDFs) for applying custom operations on datasets. However, when dealing with UDFs that yield DataFrames, the challenge lies in concatenating these disparate parts back into a cohesive whole.

To address this challenge, we will walk through a systematic approach where we define a UDF to process segments of our dataset and then seamlessly combine these processed parts into a unified DataFrame. This methodology proves invaluable when handling extensive datasets or executing intricate transformations that benefit from a segmented processing approach.


import polars as pl

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

# Define a simple UDF that doubles each value in the DataFrame
def double_values(df: pl.DataFrame) -> pl.DataFrame:
    return df * 2

# Apply the UDF and concatenate results if needed
result_df = double_values(df)


# Copyright PHD


The code snippet above exemplifies the fundamental usage of UDFs within Polars:

  • Defining Our Own Function: The double_values function takes a pl.DataFrame as input and returns another DataFrame where each value has been doubled.
  • Applying Our Function: By invoking double_values with our initial DataFrame df, we demonstrate how custom operations can be seamlessly integrated into our data processing workflow.
  • Viewing Results: Printing out result_df showcases the transformed dataset obtained through our UDF.

This pattern not only illustrates the utilization of UDFs but also underscores Polars’ adaptability in efficiently handling both row-wise and columnar operations.

  1. How do I install Polars?

  2. To install Polars, simply run:

  3. pip install polars
  4. # Copyright PHD
  5. Can I use lambda functions as UDFs in Polars?

  6. Yes! Lambda functions are suitable for short one-off operations within Polars.

  7. How does parallel execution work with Polars?

  8. Polars is optimized for speed and automatically leverages parallel execution across all available CPU cores for enhanced performance.

  9. What types of operations can I perform inside my own defined function?

  10. You can perform various operations within your custom function ranging from basic arithmetic calculations to complex logic checks or even utilizing built-in functions from Pandas/Polars.

  11. Can I return different types from my user-defined function based on conditions?

  12. While it’s possible to return different types based on conditions, ensure consistency in returned structure/type across all paths for seamless concatenation or subsequent processing steps.


Mastering User Defined Functions (UDFs) opens up endless opportunities for tailored data manipulation tasks within Polars. By grasping how to craft these functions effectively and seamlessly merge their outputs, you gain a powerful edge in tackling intricate analytical challenges with ease. Embrace the full capabilities offered by this dynamic library to enhance your productivity significantly throughout your data analysis workflows.

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