Passing integer by reference in python

问题

I'm attempting to use PyQt to talk to an ActiveX COM object in Python. This method works in Python:

myobj.dynamicCall('SetAbsMovePos(int, double)', [0, 10])

However, I can't send arguments by reference. From the docs, the method I wish to call is:

int GetNumHWUnits (int lHWType, int& plNumUnits)

and the basic example code is:

QVariantList params = ...
int result = object->dynamicCall("GetNumHWUnits(int, int&)", params).toInt();

I presume the equivalent code in Python, using ctypes, should be:

num = ctypes.c_int()
myobj.dynamicCall('GetNumHWUnits(int, int&)', [6, ctypes.byref(num)])

but num is never updated.

What's the equivalent code or workaround in Python, and how can I send and later read an argument of type int& using Python?

回答1:

You could try to pass a list with just one element:

def funct(a):
    a[0] = 4

b = [3]
funct(b)
print b[0]

Output: 4

回答2:

QVariant is involved:

    QVariant dynamicCall(const char *, QList<QVariant> & /GetWrapper/);
%MethodCode
        Py_BEGIN_ALLOW_THREADS
        sipRes = new QVariant(sipCpp->dynamicCall(a0, *a1));
        Py_END_ALLOW_THREADS

        // Update the input list with the (possibly) new values.
        for (SIP_SSIZE_T i = 0; i < PyList_GET_SIZE(a1Wrapper); ++i)
        {
            QVariant *v = new QVariant(a1->at(i));
            PyObject *v_obj = sipConvertFromNewType(v, sipType_QVariant, NULL);

In a nutshell, QVariant is a wrapper to pass a value by reference.

Which means by passing c_int, you have wrapped the value twice. Try to pass a real QVariant instead of c_int as second argument. Without this, the automatic type coercion in PyQt would take your c_int, wrap its value in a QVariant, call dynamicCall. During the call, the new QVariant is updated but there is no connection to the original c_int so your instance won't change.

This should work:

num = QVariant(0)
myobj.dynamicCall('GetNumHWUnits(int, int&)', [6, num])
print repr(num.value())

回答3:

Got the solution from the PyQt mailing list. A reference to the list of arguments is needed, as only that list is updated; the original QVariant objects never are. Seems fairly obvious in retrospect!

typ = QVariant(6)
num = QVariant(0)
args_list = [typ, num]

self.dynamicCall('GetNumHWUnits(int, int&)', args_list)

print 'Num of HW units =', args_list[1].toInt()[0]

A full code example is shown in this post.

来源：https://stackoverflow.com/questions/25379146/passing-integer-by-reference-in-python

标签

python

pyqt

activex

ctypes