Command-line introspection tools

Source code: Lib/asyncio/tools.py

---

The asyncio module can be invoked as a script via python -m asyncio to inspect the task graph of another running Python process without modifying it or restarting it. The asyncio.tools submodule implements this interface.

The following commands inspect the process identified by PID:

$ python -m asyncio pstree [--retries N] PID
$ python -m asyncio ps [--retries N] PID

The commands read the target process state without executing any code in it. They are only available on supported platforms and may require permission to inspect another process. See the permission-requirements for details.

See also

Call graph introspection

Programmatic APIs for inspecting the async call graph of a task or future in the current process.

The command examples below use this program, which creates a task hierarchy suitable for inspection and prints its process ID:

example.py

import asyncio
import os

async def play(track):
    await asyncio.sleep(3600)
    print(f"🎵 Finished: {track}")

async def album(name, tracks):
    async with asyncio.TaskGroup() as tg:
        for track in tracks:
            tg.create_task(play(track), name=track)

async def main():
    print(f"PID: {os.getpid()}")
    async with asyncio.TaskGroup() as tg:
        tg.create_task(
            album("Sundowning", ["TNDNBTG", "Levitate"]),
            name="Sundowning",
        )
        tg.create_task(
            album("TMBTE", ["DYWTYLM", "Aqua Regia"]),
            name="TMBTE",
        )

asyncio.run(main())

Run the program in one terminal and leave it running:

$ python example.py
PID: 12345

Then pass the printed process ID to the commands from another terminal. Thread IDs, task IDs, file paths, and line numbers vary between runs and source layouts.

Added in version 3.14.

Command-line options

pstree PID

Display task and coroutine relationships as a tree. Each task is shown with its full coroutine stack, nested under the task (if any) that is awaiting it. This subcommand is useful for quickly identifying which branch of a task hierarchy is blocked and where in its coroutine stack execution has paused:

$ python -m asyncio pstree 12345
└── (T) Task-1
    └──  main example.py:12
        └──  TaskGroup.__aexit__ Lib/asyncio/taskgroups.py:75
            └──  TaskGroup._aexit Lib/asyncio/taskgroups.py:124
                ├── (T) Sundowning
                │   └──  album example.py:7
                │       └──  TaskGroup.__aexit__ Lib/asyncio/taskgroups.py:75
                │           └──  TaskGroup._aexit Lib/asyncio/taskgroups.py:124
                │               ├── (T) TNDNBTG
                │               │   └──  play example.py:4
                │               │       └──  sleep Lib/asyncio/tasks.py:702
                │               └── (T) Levitate
                │                   └──  play example.py:4
                │                       └──  sleep Lib/asyncio/tasks.py:702
                └── (T) TMBTE
                    └──  album example.py:7
                        └──  TaskGroup.__aexit__ Lib/asyncio/taskgroups.py:75
                            └──  TaskGroup._aexit Lib/asyncio/taskgroups.py:124
                                ├── (T) DYWTYLM
                                │   └──  play example.py:4
                                │       └──  sleep Lib/asyncio/tasks.py:702
                                └── (T) Aqua Regia
                                    └──  play example.py:4
                                        └──  sleep Lib/asyncio/tasks.py:702

If the await graph contains a cycle, pstree reports an error instead of printing a tree. A cycle in the await graph is unusual and typically indicates a programming error:

$ python -m asyncio pstree 12345
ERROR: await-graph contains cycles - cannot print a tree!

cycle: Task-2 → Task-3 → Task-2

ps PID

Display a flat table of all pending tasks in the process PID. Each row shows the event-loop thread ID, task ID and name, coroutine stack, and the awaiting task’s stack, name, and ID, if any.

This subcommand prints all tasks regardless of whether the await graph contains cycles:

$ python -m asyncio ps 12345
tid        task id              task name            coroutine stack                                    awaiter chain                                      awaiter name    awaiter id
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
18445801   0x10a456060          Task-1               TaskGroup._aexit -> TaskGroup.__aexit__ -> main                                                                       0x0
18445801   0x10a439f60          Sundowning           TaskGroup._aexit -> TaskGroup.__aexit__ -> album   TaskGroup._aexit -> TaskGroup.__aexit__ -> main    Task-1          0x10a456060
18445801   0x10a439d70          TMBTE                TaskGroup._aexit -> TaskGroup.__aexit__ -> album   TaskGroup._aexit -> TaskGroup.__aexit__ -> main    Task-1          0x10a456060
18445801   0x10a2a3a80          TNDNBTG              sleep -> play                                      TaskGroup._aexit -> TaskGroup.__aexit__ -> album   Sundowning      0x10a439f60
18445801   0x10a2a38a0          Levitate             sleep -> play                                      TaskGroup._aexit -> TaskGroup.__aexit__ -> album   Sundowning      0x10a439f60
18445801   0x10a2d7150          DYWTYLM              sleep -> play                                      TaskGroup._aexit -> TaskGroup.__aexit__ -> album   TMBTE           0x10a439d70
18445801   0x10a6bdaa0          Aqua Regia           sleep -> play                                      TaskGroup._aexit -> TaskGroup.__aexit__ -> album   TMBTE           0x10a439d70

--retries N

Retry failed attempts to inspect the target process up to N times. This can help when the target process changes while its state is being read.

Added in version 3.15.