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14.14.2.8 Structured data types

class `Union`( | *args, **kw)

  Abstract base class for unions in native byte order.

class `BigEndianStructure`( | *args, **kw)

  Abstract base class for structures in big endian byte order.

class `LittleEndianStructure`( | *args, **kw)

  Abstract base class for structures in little endian byte order.

Structures with non-native byte order cannot contain pointer type fields, or any other data types containing pointer type fields.

class `Structure`( | *args, **kw)

  Abstract base class for structures in native byte order.

Concrete structure and union types must be created by subclassing one of these types, and at least define a `_fields_` class variable. `ctypes` will create descriptors which allow reading and writing the fields by direct attribute accesses. These are the

`_fields_`
  A sequence defining the structure fields. The items must be 2-tuples or 3-tuples. The first item is the name of the field, the second item specifies the type of the field; it can be any ctypes data type.

  For integer type fields like `c_int`, a third optional item can be given. It must be a small positive integer defining the bit width of the field.

  Field names must be unique within one structure or union. This is not checked, only one field can be accessed when names are repeated.

  It is possible to define the `_fields_` class variable after the class statement that defines the Structure subclass, this allows to create data types that directly or indirectly reference themselves:

      class List(Structure):
          pass
      List._fields_ = [("pnext", POINTER(List)),
                       ...
                      ]

  The `_fields_` class variable must, however, be defined before the type is first used (an instance is created, `sizeof()` is called on it, and so on). Later assignments to the `_fields_` class variable will raise an AttributeError.

  Structure and union subclass constructors accept both positional and named arguments. Positional arguments are used to initialize the fields in the same order as they appear in the `_fields_` definition, named arguments are used to initialize the fields with the corresponding name.

  It is possible to defined sub-subclasses of structure types, they inherit the fields of the base class plus the `_fields_` defined in the sub-subclass, if any.

`_pack_`
  An optional small integer that allows to override the alignment of structure fields in the instance. `_pack_` must already be defined when `_fields_` is assigned, otherwise it will have no effect.

`_anonymous_`
  An optional sequence that lists the names of unnamed (anonymous) fields. `_anonymous_` must be already defined when `_fields_` is assigned, otherwise it will have no effect.

  The fields listed in this variable must be structure or union type fields. `ctypes` will create descriptors in the structure type that allows to access the nested fields directly, without the need to create the structure or union field.

  Here is an example type (Windows):

      class _U(Union):
          _fields_ = [("lptdesc", POINTER(TYPEDESC)),
                      ("lpadesc", POINTER(ARRAYDESC)),
                      ("hreftype", HREFTYPE)]

      class TYPEDESC(Structure):
          _fields_ = [("u", _U),
                      ("vt", VARTYPE)]

          _anonymous_ = ("u",)

  The `TYPEDESC` structure describes a COM data type, the `vt` field specifies which one of the union fields is valid. Since the `u` field is defined as anonymous field, it is now possible to access the members directly off the TYPEDESC instance. `td.lptdesc` and `td.u.lptdesc` are equivalent, but the former is faster since it does not need to create a temporary union instance:

      td = TYPEDESC()
      td.vt = VT_PTR
      td.lptdesc = POINTER(some_type)
      td.u.lptdesc = POINTER(some_type)

It is possible to defined sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate `_fields_` variable, the fields specified in this are appended to the fields of the base class.

Structure and union constructors accept both positional and keyword arguments. Positional arguments are used to initialize member fields in the same order as they are appear in `_fields_`. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize `_fields_` with the same name, or create new attributes for names not present in `_fields_`.