Misleading error message when AttributeError is raised in property of class deriving from QObject

Question

Let's say I make a regular Python class that has a property, and in the implementation of that property I make a mistake that leads to an AttributeError. An MVCE is as follows:

class MyClass():

    @property
    def myProp(self):
        raise AttributeError("my mistake")

def main():
    # Gives a 'expected-error-message' as expected
    myObject = MyClass()
    print("Regular object property: {}".format(myObject.myProp))


if __name__ == "__main__":
    main()

This gives the following error, as expected:

Traceback (most recent call last):
  File "prop_regular.py", line 14, in <module>
    main()
  File "prop_regular.py", line 10, in main
    print("Regular object property: {}".format(myObject.myProp))
  File "prop_regular.py", line 5, in myProp
    raise AttributeError("my mistake")
AttributeError: my mistake

However, if I let the class inherit from QObject, the error is confusing. For instance running the following code

from PyQt5 import QtCore

class MyQtClass(QtCore.QObject):

    @property
    def myProp(self):
        raise AttributeError("my-mistake")


def main():
    app = QtCore.QCoreApplication([])

    # Gives confusing error message: 'MyQtClass' object has no attribute 'myProp'
    qc = MyQtClass()
    print("Qt object property: {}".format(qc.myProp))


if __name__ == "__main__":
    main()

gives

Traceback (most recent call last):
  File "prop_qt.py", line 19, in <module>
    main()
  File "prop_qt.py", line 15, in main
    print("Qt object property: {}".format(qc.myProp))
AttributeError: 'MyQtClass' object has no attribute 'myProp'

But the MyQtClass class does have a myProp property, it just contains a bug! This took me a while to debug in a real application.

So my question is: what is going on here? Is this a bug in PyQt? Or am I doing something wrong?

EDIT:

Ekhumoro's answer prompted me to look in the PyQt (5.6) source. It seems that the error originates from QtCore/qpycore_qobject_getattr.cpp, which defines the following function

// See if we can find an attribute in the Qt meta-type system.  This is
// primarily to support access to JavaScript (e.g. QQuickItem) so we don't
// support overloads.
PyObject *qpycore_qobject_getattr(const QObject *qobj, PyObject *py_qobj,
        const char *name)
{
    const QMetaObject *mo = qobj->metaObject();

    // Try and find a method with the name.
    QMetaMethod method;
    int method_index = -1;

    // Count down to allow overrides (assuming they are possible).
    for (int m = mo->methodCount() - 1; m >= 0; --m)
    {
        method = mo->method(m);

        if (method.methodType() == QMetaMethod::Constructor)
            continue;

        // Get the method name.
        QByteArray mname(method.methodSignature());
        int idx = mname.indexOf('(');

        if (idx >= 0)
            mname.truncate(idx);

        if (mname == name)
        {
            method_index = m;
            break;
        }
    }

    if (method_index >= 0)
    {
        // Get the value to return.  Note that this is recreated each time.  We
        // could put a descriptor in the type dictionary to satisfy the request
        // in future but the typical use case is getting a value from a C++
        // proxy (e.g. QDeclarativeItem) and we can't assume that what is being
        // proxied is the same each time.
        if (method.methodType() == QMetaMethod::Signal)
        {
            // We need to keep explicit references to the unbound signals
            // (because we don't use the type dictionary to do so) because they
            // own the parsed signature which may be needed by a PyQtSlotProxy
            // at some point.
            typedef QHash<QByteArray, PyObject *> SignalHash;

            static SignalHash *sig_hash = 0;

            // For crappy compilers.
            if (!sig_hash)
                sig_hash = new SignalHash;

            PyObject *sig_obj;

            QByteArray sig_str = method.methodSignature();

            SignalHash::const_iterator it = sig_hash->find(sig_str);

            if (it == sig_hash->end())
            {
                sig_obj = (PyObject *)qpycore_pyqtSignal_New(
                        sig_str.constData());

                if (!sig_obj)
                    return 0;

                sig_hash->insert(sig_str, sig_obj);
            }
            else
            {
                sig_obj = it.value();
            }

            return qpycore_pyqtBoundSignal_New((qpycore_pyqtSignal *)sig_obj,
                    py_qobj, const_cast<QObject *>(qobj));
        }

        // Respect the 'private' nature of __ names.
        if (name[0] != '_' || name[1] != '_')
        {
            QByteArray py_name(Py_TYPE(py_qobj)->tp_name);
            py_name.append('.');
            py_name.append(name);

            return qpycore_pyqtMethodProxy_New(const_cast<QObject *>(qobj),
                    method_index, py_name);
        }
    }

    // Replicate the standard Python exception.
    PyErr_Format(PyExc_AttributeError, "'%s' object has no attribute '%s'",
            Py_TYPE(py_qobj)->tp_name, name);

    return 0;
}

If no method of that name can be found in the Qt meta-type system, that error message is raised. I guess it would be difficult to do something else indeed.

Answer 1

This is normal python behaviour when a class defines __getattr__, since it must be called whenever an AttributeError is raised:

>>> class MyClass():
...     @property
...     def myProp(self):
...         raise AttributeError("my mistake")
...     def __getattr__(self, name):
...         raise AttributeError("no attribute %r" % name)
...
>>> x = MyClass()
>>> x.myProp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __getattr__
AttributeError: no attribute 'myProp'

The original exception is propagated if there is no __getattr__ defined; otherwise, the original is swallowed and the exception raised by __getattr__ is propagated instead.

Which implies all PyQt classes derived from QObject must define __getattr__.