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Python Library Reference |  |
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Structures and unions must derive from the Structure and Union
base classes which are defined in the ctypes module. Each subclass
must define a _fields_ attribute. _fields_ must be a list of
2-tuples, containing a field name and a field type.
The field type must be a ctypes type like c_int, or any other
derived ctypes type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two
integers named x and y, and also shows how to initialize a
structure in the constructor:
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = [("x", c_int),
... ("y", c_int)]
...
>>> point = POINT(10, 20)
>>> print point.x, point.y
10 20
>>> point = POINT(y=5)
>>> print point.x, point.y
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: too many initializers
>>>
You can, however, build much more complicated structures. Structures can itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft
and lowerright
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print rc.upperleft.x, rc.upperleft.y
0 5
>>> print rc.lowerright.x, rc.lowerright.y
0 0
>>>
Nested structures can also be initialized in the constructor in several ways:
>>> r = RECT(POINT(1, 2), POINT(3, 4)) >>> r = RECT((1, 2), (3, 4))
Fields descriptors can be retrieved from the class, they are useful for debugging because they can provide useful information:
>>> print POINT.x <Field type=c_long, ofs=0, size=4> >>> print POINT.y <Field type=c_long, ofs=4, size=4> >>>
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Python Library Reference | |
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