flake8-trio

A highly opinionated flake8 plugin for Trio-related problems.

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-async and flake8-bugbear.

Installation

pip install flake8-trio

List of warnings

• TRIO100: a with trio.fail_after(...): or with trio.move_on_after(...): context does not contain any await statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.
• TRIO101: yield inside a nursery or cancel scope is only safe when implementing a context manager - otherwise, it breaks exception handling.
• TRIO102: it's unsafe to await inside finally: or except BaseException/trio.Cancelled unless you use a shielded cancel scope with a timeout.
• TRIO103: except BaseException and except trio.Cancelled with a code path that doesn't re-raise.
• TRIO104: Cancelled and BaseException must be re-raised - when a user tries to return or raise a different exception.
• TRIO105: Calling a trio async function without immediately awaiting it.
• TRIO106: trio must be imported with import trio for the linter to work.
• TRIO107: exit or return from async function with no guaranteed checkpoint or exception since function definition.
• TRIO108: 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).
• TRIO109: Async function definition with a timeout parameter - use trio.[fail/move_on]_[after/at] instead
• TRIO110: while <condition>: await trio.sleep() should be replaced by a trio.Event.
• TRIO111: Variable, from context manager opened inside nursery, passed to start[_soon] might be invalidly accesed 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.
• TRIO112: 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.
• TRIO113: using nursery.start_soon in __aenter__ doesn't wait for the task to begin. Consider replacing with nursery.start.
• TRIO114: Startable function (i.e. has a task_status parameter) not in --startable-in-context-manager parameter list, please add it so TRIO113 can catch errors when using it.
• TRIO115: Replace trio.sleep(0) with the more suggestive trio.lowlevel.checkpoint().
• TRIO116: trio.sleep() with >24 hour interval should usually betrio.sleep_forever().

Configuration

You can configure flake8 with command-line options, but we prefer using a config file.

no-checkpoint-warning-decorators

Specify a list of decorators to disable checkpointing checks for, turning off TRIO107 and TRIO108 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:

[flake8]
no-checkpoint-warning-decorators =
  mydecorator,
  mydecoratorpackage.checkpointing_decorators.*,
  ign*,
  *.ignore,

startable-in-context-manager

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:

[flake8]
startable-methods-in-context-manager =
  myfun,
  myfun2,

Changelog

CalVer, YY.month.patch

22.12.1

• TRIO114 will now trigger on the unqualified name, will now only check the first parameter directly, and parameters to function calls inside that.
• TRIO113 now only supports names that are valid identifiers, rather than fnmatch patterns.
• Add TRIO115: Use trio.lowlevel.checkpoint() instead of trio.sleep(0).

22.11.5

• Add TRIO116: trio.sleep() with >24 hour interval should usually be trio.sleep_forever().

22.11.4

• Add TRIO114 Startable function not in --startable-in-context-manager parameter list.

22.11.3

• Add TRIO113, prefer await nursery.start(...) to nursery.start_soon() for compatible functions when opening a context manager

22.11.2

• TRIO105 now also checks that you awaited nursery.start().

22.11.1

• TRIO102 is no longer skipped in (async) context managers, since it's not a missing-checkpoint warning.

22.9.2

• Fix a crash on nontrivial decorator expressions (calls, PEP-614) and document behavior.

22.9.1

• Add --no-checkpoint-warning-decorators option, to disable missing-checkpoint warnings for certain decorated functions.

22.8.8

• Fix false alarm on TRIO107 with checkpointing try and empty finally
• Fix false alarm on TRIO107&108 with infinite loops

22.8.7

• TRIO107+108 now ignores asynccontextmanagers, since both __aenter__ and __aexit__ should checkpoint. async with is also treated as checkpointing on both enter and exit.
• TRIO107 now completely ignores any function whose body consists solely of ellipsis, pass, or string constants.
• TRIO103, 107 and 108 now inspects while conditions and for iterables to avoid false alarms on a couple cases where the loop body is guaranteed to run at least once.

22.8.6

• TRIO103 now correctly handles raises in loops, i.e. raise in else is guaranteed to run unless there's a break in the body.

22.8.5

• Add TRIO111: Variable, from context manager opened inside nursery, passed to start[_soon] might be invalidly accesed 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.
• Add TRIO112: this single-task nursery could be replaced by awaiting the function call directly.

22.8.4

• Fix TRIO108 raising errors on yields in some sync code.
• TRIO109 now skips all decorated functions to avoid false alarms

22.8.3

• TRIO108 now gives multiple error messages; one for each path lacking a guaranteed checkpoint

22.8.2

• Merged TRIO108 into TRIO107
• TRIO108 now handles checkpointing in async iterators

22.8.1

• Added TRIO109: Async definitions should not have a timeout parameter. Use trio.[fail/move_on]_[at/after]
• Added TRIO110: while <condition>: await trio.sleep() should be replaced by a trio.Event.

22.7.6

• Extend TRIO102 to also check inside except BaseException and except trio.Cancelled
• Extend TRIO104 to also check for yield
• Update error messages on TRIO102 and TRIO103

22.7.5

• Add TRIO103: except BaseException or except trio.Cancelled with a code path that doesn't re-raise
• Add TRIO104: "Cancelled and BaseException must be re-raised" if user tries to return or raise a different exception.
• Added TRIO107: Async functions must have at least one checkpoint on every code path, unless an exception is raised
• Added TRIO108: Early return from async function must have at least one checkpoint on every code path before it.

22.7.4

• Added TRIO105 check for not immediately awaiting async trio functions.
• Added TRIO106 check that trio is imported in a form that the plugin can easily parse.

22.7.3

• Added TRIO102 check for unsafe checkpoints inside finally: blocks

22.7.2

• Avoid TRIO100 false-alarms on cancel scopes containing async for or async with.

22.7.1

• Initial release with TRIO100 and TRIO101