How to get 'type' field descriptor from ctypes Structure or Union field

Question

I have a structure with different datatype fields. I would like to iterate through the structure fields, check the datatype, and set the field with an appropriate value.

Answer 1

This doesn't appear to be supported in the ctypes API. When the Field repr is created, the name is retrieved from the embedded type like this:

name = ((PyTypeObject *)self->proto)->tp_name;

For your field the member self->proto points to c_long, yet I find no place in Python 2.7's cfield.c where you can retrieve the value of self->proto itself. You may be forced either to:

1. Create your own mapping from name -> type.
2. (yuck) Parse the repr for and use getattr(ctypes, X) to fetch the type object.

---

Just to follow up with an example of option (1), here's a class decorator which creates the type mapping for you, adding a _typeof(cls, fld) class method:

from ctypes import *

def typemap(cls):
    _types = dict((getattr(cls, t), v) for t, v in cls._fields_)
    setattr(cls, '_typeof', classmethod(lambda c, f: _types.get(f)))
    return cls

@typemap
class A(Structure):
    _fields_ = [("one", c_long),
                ("two", c_char),
                ("three", c_byte)]

print A._typeof(A.one), A._typeof(A.two), A._typeof(A.three)

Result: