Exceptions in virtual methods

New in version 1.1.

It is common in Qt programming to override virtual C++ methods to customize behavior, like listening for mouse events, implement drawing routines, etc.

Fortunately, all Python bindings for Qt support overriding these virtual methods naturally in your Python code:

class MyWidget(QWidget):
    # mouseReleaseEvent
    def mouseReleaseEvent(self, ev):
        print(f"mouse released at: {ev.pos()}")

In PyQt5 and PyQt6, exceptions in virtual methods will by default call abort(), which will crash the interpreter. All other Qt wrappers will print the exception stacktrace and return a default value back to C++/Qt (if a return value is required).

This might be surprising for Python users which are used to exceptions being raised at the calling point: For example, the following code will just print a stack trace without raising any exception:

class MyWidget(QWidget):
    def mouseReleaseEvent(self, ev):
        raise RuntimeError("unexpected error")


w = MyWidget()
QTest.mouseClick(w, QtCore.Qt.LeftButton)

To make testing Qt code less surprising, pytest-qt automatically installs an exception hook which captures errors and fails tests when exceptions are raised inside virtual methods, like this:

E           Failed: Qt exceptions in virtual methods:
E           ________________________________________________________________________________
E             File "x:\pytest-qt\pytestqt\_tests\test_exceptions.py", line 14, in event
E               raise RuntimeError('unexpected error')
E
E           RuntimeError: unexpected error

Disabling the automatic exception hook

You can disable the automatic exception hook on individual tests by using a qt_no_exception_capture marker:

@pytest.mark.qt_no_exception_capture
def test_buttons(qtbot):
    ...

Or even disable it for your entire project in your pytest.ini file:

[pytest]
qt_no_exception_capture = 1

This might be desirable if you plan to install a custom exception hook.

Note

Starting with PyQt5.5, exceptions raised during virtual methods will actually trigger an abort(), crashing the Python interpreter. For this reason, disabling exception capture in PyQt5.5+ and PyQt6 is not recommended unless you install your own exception hook.