When does a Python warning turn into an error?

Updated October 2026 · How we answer

Default behavior

Python uses the warnings module to report things like deprecated functions or suspicious code. By default, these messages print to the console and the program keeps running. The message is informative, but it does not stop execution.

You can change that behavior with command-line flags or settings in code that tell Python to treat specific warnings as errors. Test suites often do this so that new deprecations are caught early. Libraries can also raise exceptions directly in later releases, so a warning from one version may become a failure in the next.

Default warnings print and continue. Some setups escalate warnings to errors. Test suites often enforce stricter rules. Deprecated behavior may stop working in later releases.

Checking your current settings

Look at how you run the program and whether the project has a configuration file that changes warning filters. Pinning library versions also keeps behavior predictable while you update.

Run your code with the warnings module set to show every message during development. Seeing everything early is easier than debugging a surprise failure after an upgrade. Reading the changelog for the version you install is another quick way to catch changes that matter. Keep those notes with the project so the next update is easier.

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