python-debugpy

Add breakpoint() at the line you want to inspect and run the script normally to land in an interactive pdb prompt. From there use n to step, p to print expressions, w for the stack, and interact for a full Python REPL in the current scope.

Use debugpy with python -m debugpy --listen 127.0.0.1:5678 --pid <pid> to inject into a running process, then connect a DAP client. Alternatively, insert remote_pdb.set_trace() in the code and connect with nc 127.0.0.1 4444 for a plain pdb prompt.

debugpy speaks the Debug Adapter Protocol and integrates with VS Code, Cursor, and Zed, making it thread-aware and IDE-friendly. remote-pdb exposes a plain pdb prompt over a TCP socket, which is simpler for terminal-only or agent-driven debugging.

pdb does not function under pytest-xdist because tests run in worker processes without an interactive terminal, so the prompt never appears and the test hangs. Disable parallelism with -p no:xdist or -n 0 when using --pdb or --trace.

Check whether PYTHONBREAKPOINT=0 is set, which disables all breakpoint() calls. Other common causes are running under pytest-xdist, or a debugpy session where execution finished before the client attached because wait_for_client() was omitted.

pdb only debugs the current thread, so multithreaded programs need debugpy's thread-aware DAP or per-thread settrace. In coroutines, pdb works but awaiting inside the prompt requires Python 3.13+ or workarounds like asyncio.ensure_future on older versions.