Example

A Struct is a collection of ordered, named fields:

>>> format = Struct(
...     "signature" / Const(b"BMP"),
...     "width" / Int8ub,
...     "height" / Int8ub,
...     "pixels" / Array(this.width * this.height, Byte),
... )
>>> format.build(dict(width=3,height=2,pixels=[7,8,9,11,12,13]))
b'BMP\x03\x02\x07\x08\t\x0b\x0c\r'
>>> format.parse(b'BMP\x03\x02\x07\x08\t\x0b\x0c\r')
Container(signature=b'BMP')(width=3)(height=2)(pixels=[7, 8, 9, 11, 12, 13])

A Sequence is a collection of ordered fields, and differs from Array and GreedyRange in that those two are homogenous:

>>> format = Sequence(PascalString(Byte, "utf8"), GreedyRange(Byte))
>>> format.build([u"lalaland", [255,1,2]])
b'\nlalaland\xff\x01\x02'
>>> format.parse(b"\x004361789432197")
['', [52, 51, 54, 49, 55, 56, 57, 52, 51, 50, 49, 57, 55]]

Most constructs can be build into XML and parsed back from XML:

>>> s = Struct(
...    "a" / Int32ul,
...    "b" / Int32ul,
...    "s" / Struct(
...        "c" / Int32ul,
...        "d" / Int32ul,
...    ),
...    )
>>> data = {"a": 1, "b": 2, "s": {"c": 3, "d": 4}}
>>> xml = s.toET(obj=data, name="test")
>>> assert(ET.tostring(xml) == b'<test a="1" b="2"><s c="3" d="4" /></test>')

>>> s = "test" / Struct(
...     "a" / Int32ul,
...     "b" / Int32ul,
... )
>>> xml = ET.fromstring(b'<test a="1" b="2" />')
>>> obj = s.fromET(xml=xml)
>>> assert(obj == {"a": 1, "b": 2})

However some constructs, like Switch or FocusedSeq have some caveats, because they use the XML tag name for identifying the corresponding construct.

This is mainly build for easy and quick describing of datastructures with an easy, human readable and changeable XML representation, rather than completeness of all possible constructs.