# 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, both `PyQt` and `PySide` support overriding this virtual methods naturally in your python code:

    class MyWidget(QWidget):

        # mouseReleaseEvent
        def mouseReleaseEvent(self, ev):
            print('mouse released at: %s' % ev.pos())

This works fine, but if python code in Qt virtual methods raise an exception `PyQt4` and `PySide` will just print the exception traceback to standard error, since this method is called deep within Qt’s event loop handling and exceptions are not allowed at that point. In `PyQt5.5+`, exceptions in virtual methods will by default call `abort()`, which will crash the interpreter.

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+` is not recommended unless you install your own exception hook.