### 14.14.1.10 Structures and unions

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>
>>>
```