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.
I have access to the size and offset of the field through the .size and .offset attribute of the field. How can I get the 'type' attribute of the field? Using type(value) does not print the ctypes datatype for the particular field. If I print value then I do see the ctypes datatype but there doesn't seem to be an attribute to access this directly.
How can I access the type field descriptor directly?
from ctypes import *
class A(Structure):
_fields_ = [("one", c_long),
("two", c_char),
("three", c_byte)]
>>> A.one
<Field type=c_long, ofs=0, size=4>
>>> A.one.offset
0
>>> A.one.size
4
>>> type(A.one)
<class '_ctypes.CField'>
Ideally I would like to get the field type similar to the snippet below...
>>> A.one.type
c_long
This doesn't appear to be supported in the ctypes API. When the Field
repr <Field type=c_long ..>
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:
name
-> type
.<Field type=X
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:
<class 'ctypes.c_long'> <class 'ctypes.c_char'> <class 'ctypes.c_byte'>
Just use the _fields_
list:
>>> for f,t in A._fields_:
... a = getattr(A,f)
... print a,a.offset,a.size,t
...
<Field type=c_long, ofs=0, size=4> 0 4 <class 'ctypes.c_long'>
<Field type=c_char, ofs=4, size=1> 4 1 <class 'ctypes.c_char'>
<Field type=c_byte, ofs=5, size=1> 5 1 <class 'ctypes.c_byte'>
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With