Question

In our codebase there is Python and C++ married by SWIG. The C++ classes are sometimes given Python extensions like so:

%pythoncode %{

def DiscreteKey_baseData(self, baseData):
    pass

def DiscreteKey_asSet(self):
    pass

DiscreteKey.baseData = new_instancemethod(DiscreteKey_baseData, None, DiscreteKey)
DiscreteKey.asSet = new_instancemethod(DiscreteKey_asSet, None, DiscreteKey)

%}

or

%pythoncode %{

def ParmID_hash(parmID):
    return hash(str(parmID))

ParmID.__hash__ = ParmID_hash

%}

What is the difference between using new.instancemethod to attach a method to a class, vs. simply assigning it, as in the second example? Could the first example be changed to simply

DiscreteKey.baseData = DiscreteKey_baseData
DiscreteKey.asSet = DiscreteKey_asSet

? (note that baseData takes another argument)

Or is the second example actually deficient in some way, and should be using new_instancemethod too?

Answer 1

Assigning a function to a class attribute, and doing the same via new_instancemethod() (which I assume is a Swig alias for the method constructor, e.g. types.MethodType) are exactly equivalent. You can tell by checking type(DiscreteKey.baseData). Regardless of how the method was assigned, it will be <type 'instancemethod'>.

The new_instancemethod() constructor is useful when assigning functions onto instances, however. This won’t work without the method “wrapper.”