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Migrating to Polars 2

What breaks silently between 1.x and 2.0, shown with real runs under both versions

Migrating to Polars 2
Ground Truth Books

Polars 2.0 changed defaults that don't raise errors: lazy queries now stream, and joins no longer keep row order. This short guide shows exactly what breaks when you upgrade from 1.x, with every example run under both 1.44.2 and 2.0.0 and the real output printed side by side.

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About

About

About the Book

You run pip install -U polars on a pipeline that has worked for a year. The tests pass. Then the report comes back with rows in a different order, a join pairs the wrong records, or a nightly CSV import starts failing on a column it used to read.

Polars 2.0.0 shipped on 6 October 2026. Most of its breaking changes are loud: a removed argument raises and names its replacement. The dangerous ones are quiet. The code keeps running, and the result is different.

  • Lazy queries now run on the streaming engine by default.
  • Joins and unpivot no longer come back in input order, and asking for the in-memory engine does not reliably restore it.
  • Integer-versus-float membership checks, signed-plus-unsigned arithmetic, and exploding empty lists now give different answers or raise.
  • Casts that 1.x allowed with a deprecation warning now fail, and some fail only when a row reaches them.
  • CSV reading and schema inference behave differently.

This book walks through those changes area by area and shows each one twice: the same code run under Polars 1.44.2 and under Polars 2.0.0, with the real output from both. No output in the book is invented or retyped. Each output block is a captured run, cited, and the commands are listed so you can reproduce them.

It ends with a migration checklist you can run on your own code:

  1. Pin both versions.
  2. Run your suite on 2.0 with the old engine requested, to surface removals and dtype changes.
  3. Switch to the new default to surface ordering assumptions.
  4. Diff 1.x output against 2.0 output.
  5. Wire the check into CI.

Researched and drafted with AI assistance by Ground Truth Books. Every output shown was produced by real runs, and the text was checked against the official Polars upgrade guide and those captures. "Polars" is used descriptively; this book is not affiliated with or endorsed by Polars Inc.

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About the Author

Ground Truth Books

Ground Truth Books publishes practical technical books on computer science, IT tools, data science, machine learning, software engineering and AI.

The name comes from machine learning, where "ground truth" means the real, verified answers you check a model against. That's how these books are made. They're researched and drafted with AI assistance, and then checked against reality. Examples are run against real captures and data in a lab, and those runs are cited like any other source, so you can see which output came straight from the tool. Every claim is cited to its source, with official documentation, specifications and source code preferred over blog posts.

Each book is built around doing the work. Chapters end with exercises, and an appendix gives worked answers you can reproduce on your own machine, using the same freely available data and captures.

Books are updated when the tools change. If you find an error, please report it: a corrected edition is free for every reader, which is one of the best things about Leanpub.

Contents

Table of Contents

About This Book

Introduction

  1. Who this book is for
  2. What changed in 2.0
  3. How the book is organized
  4. How the examples were produced
  5. Part I: The Engine Change

The Streaming Engine Is Now the Default

  1. What engine=‘auto’ resolves to
  2. Eager operations are unaffected
  3. sink_* already streamed
  4. SQL eager=True now streams
  5. Restoring in-memory execution

Row Order Is No Longer Guaranteed

  1. Joins that lose their order
  2. group_by and unpivot ordering
  3. Requesting order explicitly
  4. A left join under both versions
  5. Part II: What Breaks and How to Fix It

Breaking Changes by Area

  1. The guide’s list, sorted by failure mode
  2. concat and union strictness
  3. explode empty_as_null
  4. Int128 supertype and is_in coercion
  5. Output names and Map dtype
  6. Decimal widening, SQL, and Parquet ENUM
  7. Sequencing the audit

Casts and Operations That Now Fail

  1. Casts that now fail
  2. is_in and list.contains comparisons
  3. Operations that now raise
  4. Five more items from the section
  5. Casting examples under both versions
  6. Finding them in your code

API Reshapes, Removals, and Deprecations

  1. API reshapes
  2. Removed APIs and their new errors
  3. New deprecations in 2.0

CSV Reading and Schema Inference

  1. Your CSV imports now run on the lazy reader
  2. read_csv now dispatches to scan_csv
  3. Arguments read_csv lost
  4. schema_overrides and column order
  5. infer_schema_files and counting from zero
  6. raise_if_empty and file column order
  7. Part III: The Migration

A Migration Checklist

  1. Pinning 1.44.2 and 2.0.0
  2. A dual-version test harness
  3. Diffing 1.x and 2.0 outputs
  4. A CI step that runs the diff

Conclusion

  1. What you can now do
  2. Where to go next

Appendix A: Commands used to produce and verify example outputs

  1. The two pinned environments
  2. The lab data
  3. The probe commands
  4. The migrate_check.py harness
  5. The audit commands

Appendix B: Polars 1.44.2 versus 2.0.0 API change table

  1. Scope
  2. Engine and row order
  3. The same line, two versions
  4. Removed APIs and replaced arguments
  5. Casts no longer supported
  6. Loud changes
  7. Silent changes
  8. Deprecations
  9. New exception types

Glossary

References

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