Thanks, Dima and Andrew, for your suggestions.

Re: loop.slow_callback_duration, that's interesting. I didn't know
about that. However, it doesn't seem like that alone would work to
catch many operations that are normally fast, but strictly speaking
blocking. For example, it seems like simple disk I/O operations
wouldn't be caught even with slow_callback_duration set to a small
value. Dima suggests that such calls are okay. Is there a consensus on

Dima's mock.patch_all_known_blocking_calls() is an interesting idea
and seems like it would work for the case I mentioned. Has anyone
started writing such a method (e.g. for certain standard lib modules)?


On Mon, Feb 12, 2018 at 11:24 PM, Andrew Svetlov
<> wrote:
> loop.slow_callback_duration + PYTHONASYNCIODEBUG=1
> Does it fork for you?
> Do you need extra info?
> On Tue, Feb 13, 2018 at 7:07 AM Dima Tisnek <> wrote:
>> Let me try to answer the question behind the question.
>> Like any code validation, it's a healthy mix of:
>> * unit tests [perhaps with
>> mock.patch_all_known_blocking_calls(side_effect=Exception)]
>> * good judgement [open("foo").read() technically blocks, but only a
>> problem on network filesystems]
>> * monitoring prod [at least to ensure that tests correspond to prod]
>> * peer review
>> Specific issues you've raised:
>> re: detecting blocking calls ahead of time.
>> That's a hard problem (insert quip about Turing completeness)
>> Perhaps it can be alleviated by function annotations, but that's
>> dangerously close to Java way...
>> re: detecting blocking calls at run time.
>> Let's say there's a per-thread global "blocking lock" that's managed
>> by event loop executor and it's in "allowed" state when executor has
>> no runnables and wishes to select/poll/epoll and it's in "denied"
>> state when user code is running.
>> Some calls can be caught (e.g. check fd fcntl bits before any
>> recv/send/...)
>> Some calls can never be caught (anything via ctypes or extensions in
>> general)
>> Case in point: what do you propose to do about socket.gethostbyname()?
>> re: after the fact.
>> hack1. run a separate thread/process that inspects event loop thread's
>> `wchan` value (Linux)
>> hack2. same via taskstats
>> hack3. detect event loop lag (false positives are cpu-bound tasks and
>> overloaded system, but I think you'd like to detect those too)
>> On 13 February 2018 at 12:10, Chris Jerdonek <>
>> wrote:
>> > Hi,
>> >
>> > This is a general Python async question I've had that I haven't seen a
>> > discussion of. Do people know of any techniques other than manually
>> > inspecting code line by line to find out if you're accidentally making
>> > a raw call to a blocking function in a coroutine?
>> >
>> > Related to this, again, outside of inspecting the external source
>> > code, is there any way to know if a function you're calling (e.g. in
>> > someone else's library or in the standard lib) has the potential of
>> > blocking?
>> >
>> > What would be the way of detecting something like this if it made its
>> > way into "production"? What would be some of the symptoms?
>> >
>> > --Chris
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>> >
>> >
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> --
> Thanks,
> Andrew Svetlov
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