Interop between asyncio and Qt for Python
Project description
qtinter — Interop between asyncio and Qt for Python
qtinter
is a Python module that brings together asyncio and Qt
for Python, allowing you to use one from the other seamlessly.
Quickstart
Installation
$ pip install qtinter
Using asyncio from Qt
To use asyncio-based libraries in Qt for Python, enclose app.exec()
inside context manager qtinter.using_asyncio_from_qt()
, and optionally
connect Qt signals to coroutine functions using qtinter.asyncslot()
.
Minimal example (taken from examples/sleep.py
):
import asyncio
import qtinter # <-- import module
from PyQt6 import QtWidgets
async def sleep():
button.setEnabled(False)
await asyncio.sleep(1)
button.setEnabled(True)
if __name__ == "__main__":
app = QtWidgets.QApplication([])
button = QtWidgets.QPushButton()
button.setText('Sleep for one second')
button.clicked.connect(qtinter.asyncslot(sleep)) # <-- wrap coroutine function
button.show()
with qtinter.using_asyncio_from_qt(): # <-- enclose in context manager
app.exec()
Using Qt from asyncio
To use Qt components from asyncio-based code, enclose the asyncio
entry-point inside context manager qtinter.using_qt_from_asyncio()
,
and optionally wait for Qt signals using qtinter.asyncsignal()
.
Minimal example (taken from examples/color.py
):
import asyncio
import qtinter # <-- import module
from PyQt6 import QtWidgets
async def choose_color():
dialog = QtWidgets.QColorDialog()
dialog.show()
result = await qtinter.asyncsignal(dialog.finished) # <-- wait for signal
if result == QtWidgets.QDialog.DialogCode.Accepted:
return dialog.selectedColor().name()
else:
return None
if __name__ == "__main__":
app = QtWidgets.QApplication([])
with qtinter.using_qt_from_asyncio(): # <-- enclose in context manager
color = asyncio.run(choose_color())
if color is not None:
print(color)
Documentation
See full documentation at qtinter.readthedocs.io.
Requirements
qtinter
supports the following:
- Python version: 3.7 or higher
- Qt binding: PyQt5, PyQt6, PySide2, PySide6
- Operating system: Linux, MacOS, Windows
Details
qtinter
embeds a logical asyncio event loop
within a physical Qt event loop (QEventLoop
), so that Python libraries
written for asyncio can be used by a Python for Qt application.
Running Modes
An QiBaseEventLoop
may be run in attached mode or nested mode.
Use QiRunner
context manager to run it in attached mode.
This mode only installs the logical asyncio event loop; the physical Qt
event loop must still be run as usual, e.g. by app.exec()
. This is the
preferred workflow as it integrates seamlessly with an existing Qt app.
Call run_forever
to run it in nested mode. This starts
a (possibly nested) Qt event loop using QEventLoop.exec()
and waits until
it exits. This is the standard asyncio workflow and is convenient for
unit testing, but it is not recommended for integration with an existing Qt
app as nested event loops are advised against by Qt.
For either mode, a (global) QCoreApplication
(or QApplication
/
QGuiApplication
) instance must exist before running any coroutine,
as is required by Qt.
Clean-up
To properly release the resources of the event loop after it stops, you
should call shutdown_asyncgens
and shutdown_default_executor
, followed
by close
. The first two methods are actually coroutines and therefore
must be run from within the event loop.
For attached mode, use the QiRunner
context manager,
which handles clean-up automatically. Note, however, that it actually runs
the first two coroutines in nested mode, i.e. a Qt event loop is started.
Your code should be prepared for this.
For nested mode, asyncio.run()
handles clean-up automatically.
The asyncslot
Adaptor
qtinter.asyncslot
wraps a coroutine function (one defined by async def
)
to make it usable as a Qt slot. Without wrapping, a coroutine function
(whether decorated with QtCore.Slot
/PyQt6.pyqtSlot
or not)
generally cannot be used as a slot because calling it merely returns a
coroutine object instead of performing real work.
Under the hood, qtinter.asyncslot
calls QiBaseEventLoop.run_task
,
a custom method which creates a Task wrapping the coroutine and executes
it immediately until the first suspension point.
This is designed to work with a common pattern where some work has to be
performed immediately in response to a signal. For example, the clicked
handler of a "Send Order" button normally disables the button on entry
before actually sending the order over network, to avoid sending duplicate
orders. For this to work correctly, the code until the first suspension
point must be executed synchronously.
An QiBaseEventLoop
must be running when a coroutine wrapped by
asyncslot
is called, or a RuntimeError
will be raised.
It is not recommended to decorate a coroutine function with asyncslot
as that would make an async def
function into a normal function, which
is confusing.
Cancellation
To cancel a running coroutine from within itself, raise
asyncio.CancelledError
.
To retrieve the Task
object from within the running coroutine and store
it somewhere to be used later, call asyncio.current_task()
from within
the running coroutine.
Implementation Notes
By embedding a (logical) asyncio event loop inside a (physical) Qt event
loop, what's not changed (from the perspective of the asyncio event loop) is
that all calls (other than call_soon_threadsafe
) are still made from the
same thread. This frees us from multi-threading complexities.
What has changed, however, is that in a standalone asyncio event loop, no
code can run when the scheduler (specifically, _run_once
) is blocked in
select()
, while in an embedded asyncio event loop, a select()
call
that would otherwise block yields, allowing any code to run while the loop
is "logically" blocked in select
.
For example, BaseEventLoop.stop()
is implemented by setting the flag
_stopping
to True
, which is then checked before the next iteration of
_run_once
to stop the loop. This works because stop
can only ever be
called from a callback, and a callback can only ever be called after
select
returns and before the next iteration of _run_once
. The behavior
changes if select
yields and stop
is called -- the event loop wait not
wake up until some IO is available.
We refer to code that runs (from the Qt event loop) after select
yields
and before _run_once
is called again as injected code. We must
examine and handle the implications of such code.
We do this by fitting injected code execution into the standalone asyncio
event loop model. Specifically, we treat injected code as if they were
scheduled with call_soon_threadsafe
, which wakes up the selector and
executes the code. With some loss of generality, we assume no IO event
nor timed callback is ready at the exact same time, so that the scheduler
will be put back into blocking select
immediately after the code finishes
running (unless the code calls stop
). This simplification is acceptable
because the precise timing of multiple IO or timer events should not be
relied upon.
In practice, we cannot actually wake up the asyncio scheduler every time injected code is executed, firstly because there's no way to detect their execution and secondly because doing so would be highly inefficient. Instead, we assume that injected code which does not access the event loop object or its selector is benign enough to be treated as independent from the asyncio event loop ecosystem and may be safely ignored.
This leaves us to just consider injected code that accesses the event loop
object or its selector and examine its impact on scheduling. The scheduler
depends on three things: the _ready
queue for "soon" callbacks, the
_scheduled
queue for timer callbacks, and _selector
for IO events.
If the injected code touches any of these things, it needs to be handled.
While the public interface of AbstractEventLoop
has numerous methods, the
methods that modify those three things boil down to call_soon
, call_at
,
call_later
, (arguably) stop
, and anything that modifies the selector
(proactor). When any of these happens, we physically or logically wake up
the selector to simulate a call_soon_threadsafe
call.
History
asyncslot
is derived from
qasync but rewritten from
scratch. qasync is derived from
asyncqt, which is derived from
quamash.
License
BSD License.
Project details
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