reentrant

Code is reentrant when one copy of it can be interrupted partway through, called again from the start, and complete that second call correctly before the first one continues. This works because each call keeps its own working data, so two calls that overlap in time never overwrite each other’s values.

Reentrancy matters whenever the same code can be entered again before an earlier call finishes. A recursive function calls itself, a signal handler can interrupt a function and call it again, and two threads can run the same function at once.

To stay correct in these situations, reentrant code keeps its data in local variables and parameters instead of global or static storage. It also never rewrites its own instructions and calls only other reentrant code.

Python Thread Safety: Using a Lock and Other Techniques

Tutorial

Python Thread Safety: Using a Lock and Other Techniques

In this tutorial, you'll learn about the issues that can occur when your code is run in a multithreaded environment. Then you'll explore the various synchronization primitives available in Python's threading module, such as locks, which help you make your code safe.

intermediate python stdlib

For additional information on related topics, take a look at the following resources:

By Martin Breuss • Updated July 10, 2026 • Reviewed by Leodanis Pozo Ramos
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