Micropackage to redirect users to flake8-async.

pip install flake8-trio==22.11.3


pre-commit.ci status Checked with pyright


A highly opinionated flake8 plugin for problems related to Trio, AnyIO, or asyncio.

This can include anything from outright bugs, to pointless/dead code, to likely performance issues, to minor points of idiom that might signal a misunderstanding.

It may well be too noisy for anyone with different opinions, that's OK.

Pairs well with flake8-bugbear.

Some checks are incorporated into ruff.

This plugin was previously known as flake8-trio, and there was a separate small plugin known as flake8-async for asyncio. But this plugin was a superset of the checks in flake8-async, and support for anyio was added, so it's now named flake8-async to more properly convey its usage. At the same time all error codes were renamed from TRIOxxx to ASYNCxxx, as was previously used by the old flake8-async.


pip install flake8-async

List of warnings

  • ASYNC100: A with [trio/anyio].fail_after(...): or with [trio/anyio].move_on_after(...): context does not contain any await statements. This makes it pointless, as the timeout can only be triggered by a checkpoint. This check also allows yield statements, since checkpoints can happen in the caller we yield to.
  • ASYNC101: yield inside a trio/anyio nursery or cancel scope is only safe when implementing a context manager - otherwise, it breaks exception handling.
  • ASYNC102: It's unsafe to await inside finally: or except BaseException/trio.Cancelled/anyio.get_cancelled_exc_class()/asyncio.exceptions.CancelledError unless you use a shielded cancel scope with a timeout. This is currently not able to detect asyncio shields.
  • ASYNC103: except BaseException/trio.Cancelled/anyio.get_cancelled_exc_class()/asyncio.exceptions.CancelledError, or a bare except: with a code path that doesn't re-raise. If you don't want to re-raise BaseException, add a separate handler for trio.Cancelled/anyio.get_cancelled_exc_class()/asyncio.exceptions.CancelledError before.
  • ASYNC104: trio.Cancelled/anyio.get_cancelled_exc_class()/asyncio.exceptions.CancelledError/BaseException must be re-raised. The same as ASYNC103, except specifically triggered on return or a different exception being raised.
  • ASYNC105: Calling a trio async function without immediately awaiting it. This is only supported with trio functions, but you can get similar functionality with a type-checker.
  • ASYNC106: trio/anyio/asyncio must be imported with import trio/import anyio/import asyncio for the linter to work.
  • ASYNC109: Async function definition with a timeout parameter - use [trio/anyio].[fail/move_on]_[after/at] instead.
  • ASYNC110: while <condition>: await [trio/anyio].sleep() should be replaced by a [trio/anyio].Event.
  • ASYNC111: Variable, from context manager opened inside nursery, passed to start[_soon] might be invalidly accessed while in use, due to context manager closing before the nursery. This is usually a bug, and nurseries should generally be the inner-most context manager.
  • ASYNC112: Nursery body with only a call to nursery.start[_soon] and not passing itself as a parameter can be replaced with a regular function call.
  • ASYNC113: Using nursery.start_soon in __aenter__ doesn't wait for the task to begin. Consider replacing with nursery.start.
  • ASYNC114: Startable function (i.e. has a task_status keyword parameter) not in --startable-in-context-manager parameter list, please add it so ASYNC113 can catch errors when using it.
  • ASYNC115: Replace [trio/anyio].sleep(0) with the more suggestive [trio/anyio].lowlevel.checkpoint().
  • ASYNC116: [trio/anyio].sleep() with >24 hour interval should usually be [trio/anyio].sleep_forever().
  • ASYNC118: Don't assign the value of anyio.get_cancelled_exc_class() to a variable, since that breaks linter checks and multi-backend programs.

Warnings for blocking sync calls in async functions

Note: 22X, 23X and 24X has not had asyncio-specific suggestions written.

  • ASYNC200: User-configured error for blocking sync calls in async functions. Does nothing by default, see async200-blocking-calls for how to configure it.
  • ASYNC210: Sync HTTP call in async function, use httpx.AsyncClient. This and the other ASYNC21x checks look for usage of urllib3 and httpx.Client, and recommend using httpx.AsyncClient as that's the largest http client supporting anyio/trio.
  • ASYNC211: Likely sync HTTP call in async function, use httpx.AsyncClient. Looks for urllib3 method calls on pool objects, but only matching on the method signature and not the object.
  • ASYNC212: Blocking sync HTTP call on httpx object, use httpx.AsyncClient.
  • ASYNC220: Sync process call in async function, use await nursery.start([trio/anyio].run_process, ...). asyncio users can use asyncio.create_subprocess_[exec/shell].
  • ASYNC221: Sync process call in async function, use await [trio/anyio].run_process(...). asyncio users can use asyncio.create_subprocess_[exec/shell].
  • ASYNC222: Sync os.* call in async function, wrap in await [trio/anyio].to_thread.run_sync(). asyncio users can use asyncio.loop.run_in_executor.
  • ASYNC230: Sync IO call in async function, use [trio/anyio].open_file(...). asyncio users need to use a library such as aiofiles, or switch to anyio.
  • ASYNC231: Sync IO call in async function, use [trio/anyio].wrap_file(...). asyncio users need to use a library such as aiofiles, or switch to anyio.
  • ASYNC232: Blocking sync call on file object, wrap the file object in [trio/anyio].wrap_file() to get an async file object.
  • ASYNC240: Avoid using os.path in async functions, prefer using [trio/anyio].Path objects. asyncio users should consider aiopath or anyio.
  • ASYNC250: Builtin input() should not be called from async function. Wrap in [trio/anyio].to_thread.run_sync() or asyncio.loop.run_in_executor().
  • ASYNC251: time.sleep(...) should not be called from async function. Use [trio/anyio/asyncio].sleep(...).

Warnings disabled by default

  • ASYNC900: Async generator without @asynccontextmanager not allowed. You might want to enable this on a codebase since async generators are inherently unsafe and cleanup logic might not be performed. See #211 and https://discuss.python.org/t/using-exceptiongroup-at-anthropic-experience-report/20888/6 for discussion.
  • ASYNC910: Exit or return from async function with no guaranteed checkpoint or exception since function definition. You might want to enable this on a codebase to make it easier to reason about checkpoints, and make the logic of ASYNC911 correct.
  • ASYNC911: Exit, yield or return from async iterable with no guaranteed checkpoint since possible function entry (yield or function definition) Checkpoints are await, async for, and async with (on one of enter/exit).

Removed Warnings

  • TRIOxxx: All error codes are now renamed ASYNCxxx
  • TRIO107: Renamed to TRIO910
  • TRIO108: Renamed to TRIO911
  • TRIO117: Don't raise or catch trio.[NonBase]MultiError, prefer [exceptiongroup.]BaseExceptionGroup. MultiError was removed in trio==0.24.0.


install and run through flake8

pip install flake8 flake8-async
flake8 .

install and run with pre-commit

If you use pre-commit, you can use it with flake8-async by adding the following to your .pre-commit-config.yaml:

minimum_pre_commit_version: '2.9.0'
- repo: https://github.com/python-trio/flake8-async
  rev: 23.2.5
    - id: flake8-async
      # args: [--enable=ASYNC, --disable=ASYNC9, --autofix=ASYNC]

This is often considerably faster for large projects, because pre-commit can avoid running flake8-async on unchanged files.

Afterwards, run

pip install pre-commit flake8-async
pre-commit run .

install and run as standalone

If inside a git repository, running without arguments will run it against all *.py files in the repository.

pip install flake8-async

with autofixes

flake8-async --autofix=ASYNC

specifying source files

flake8-async my_python_file.py
flake8-async **/*.py


You can configure flake8 with command-line options, but we prefer using a config file. The file needs to start with a section marker [flake8] and the following options are then parsed using flake8's config parser, and can be used just like any other flake8 options. Note that it's not currently possible to use a configuration file when running flake8-async standalone.


Comma-separated list of error codes to enable, similar to flake8 --select but is additionally more performant as it will disable non-enabled visitors from running instead of just silencing their errors.


Comma-separated list of error codes to disable, similar to flake8 --ignore but is additionally more performant as it will disable non-enabled visitors from running instead of just silencing their errors.


Comma-separated list of error-codes to enable autofixing for if implemented. Requires running as a standalone program. Pass --autofix=ASYNC to enable all autofixes.


Whether to also print an error message for autofixed errors.


Change the default library to be anyio instead of trio. If trio is imported it will assume both are available and print suggestions with [anyio/trio].


Comma-separated list of decorators to disable checkpointing checks for, turning off ASYNC910 and ASYNC911 warnings for functions decorated with any decorator matching any in the list. Matching is done with fnmatch. Defaults to disabling for asynccontextmanager.

Decorators-to-match must be identifiers or dotted names only (not PEP-614 expressions), and will match against the name only - e.g. foo.bar matches foo.bar, foo.bar(), and foo.bar(args, here), etc.

For example:

no-checkpoint-warning-decorators =


Comma-separated list of methods which should be used with .start() when opening a context manager, in addition to the default trio.run_process, trio.serve_tcp, trio.serve_ssl_over_tcp, and trio.serve_listeners. Names must be valid identifiers as per str.isidentifier(). For example:

startable-in-context-manager =


Comma-separated list of pairs of values separated by -> (optional whitespace stripped), where the first is a pattern for a call that should raise an error if found inside an async function, and the second is what should be suggested to use instead. It uses fnmatch as per no-checkpoint-warning-decorators for matching. The part after -> is not used by the checker other than when printing the error, so you could add extra info there if you want.

The format of the error message is User-configured blocking sync call {0} in async function, consider replacing with {1}., where {0} is the pattern the call matches and {1} is the suggested replacement.


async200-blocking-calls =
  my_blocking_call -> async.alternative,
  module.block_call -> other_function_to_use,
  common_error_call -> alternative(). But sometimes you should use other_function(). Ask joe if you're unsure which one,
  dangerous_module.* -> corresponding function in safe_module,
  *.dangerous_call -> .safe_call()

Specified patterns must not have parentheses, and will only match when the pattern is the name of a call, so given the above configuration

async def my_function():
    my_blocking_call()  # this would raise an error
    x = my_blocking_call(a, b, c)  # as would this
    y = my_blocking_call  # but not this
    y()  # or this
    [my_blocking_call][0]()  # nor this

    def my_blocking_call():  # it's also safe to use the name in other contexts