This page hosts a formal specification of .dat file format of Fallout 2 using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.
The Python runtime library can be installed from PyPI:
python3 -m pip install kaitaistruct
Parse a local file and get structure in memory:
data = Fallout2Dat.from_file("path/to/local/file.dat")
Or parse structure from a bytes:
from kaitaistruct import KaitaiStream, BytesIO
raw = b"\x00\x01\x02..."
data = Fallout2Dat(KaitaiStream(BytesIO(raw)))
After that, one can get various attributes from the structure by invoking getter methods like:
data.footer # => get footer
# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
# type: ignore
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
from enum import IntEnum
import zlib
if getattr(kaitaistruct, 'API_VERSION', (0, 9)) < (0, 11):
    raise Exception("Incompatible Kaitai Struct Python API: 0.11 or later is required, but you have %s" % (kaitaistruct.__version__))
class Fallout2Dat(KaitaiStruct):
    class Compression(IntEnum):
        none = 0
        zlib = 1
    def __init__(self, _io, _parent=None, _root=None):
        super(Fallout2Dat, self).__init__(_io)
        self._parent = _parent
        self._root = _root or self
        self._read()
    def _read(self):
        pass
    def _fetch_instances(self):
        pass
        _ = self.footer
        if hasattr(self, '_m_footer'):
            pass
            self._m_footer._fetch_instances()
        _ = self.index
        if hasattr(self, '_m_index'):
            pass
            self._m_index._fetch_instances()
    class File(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Fallout2Dat.File, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()
        def _read(self):
            self.name = Fallout2Dat.Pstr(self._io, self, self._root)
            self.flags = KaitaiStream.resolve_enum(Fallout2Dat.Compression, self._io.read_u1())
            self.size_unpacked = self._io.read_u4le()
            self.size_packed = self._io.read_u4le()
            self.offset = self._io.read_u4le()
        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            _ = self.contents_raw
            if hasattr(self, '_m_contents_raw'):
                pass
            _ = self.contents_zlib
            if hasattr(self, '_m_contents_zlib'):
                pass
        @property
        def contents(self):
            if hasattr(self, '_m_contents'):
                return self._m_contents
            if  ((self.flags == Fallout2Dat.Compression.zlib) or (self.flags == Fallout2Dat.Compression.none)) :
                pass
                self._m_contents = (self.contents_zlib if self.flags == Fallout2Dat.Compression.zlib else self.contents_raw)
            return getattr(self, '_m_contents', None)
        @property
        def contents_raw(self):
            if hasattr(self, '_m_contents_raw'):
                return self._m_contents_raw
            if self.flags == Fallout2Dat.Compression.none:
                pass
                io = self._root._io
                _pos = io.pos()
                io.seek(self.offset)
                self._m_contents_raw = io.read_bytes(self.size_unpacked)
                io.seek(_pos)
            return getattr(self, '_m_contents_raw', None)
        @property
        def contents_zlib(self):
            if hasattr(self, '_m_contents_zlib'):
                return self._m_contents_zlib
            if self.flags == Fallout2Dat.Compression.zlib:
                pass
                io = self._root._io
                _pos = io.pos()
                io.seek(self.offset)
                self._raw__m_contents_zlib = io.read_bytes(self.size_packed)
                self._m_contents_zlib = zlib.decompress(self._raw__m_contents_zlib)
                io.seek(_pos)
            return getattr(self, '_m_contents_zlib', None)
    class Footer(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Fallout2Dat.Footer, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()
        def _read(self):
            self.index_size = self._io.read_u4le()
            self.file_size = self._io.read_u4le()
        def _fetch_instances(self):
            pass
    class Index(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Fallout2Dat.Index, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()
        def _read(self):
            self.file_count = self._io.read_u4le()
            self.files = []
            for i in range(self.file_count):
                self.files.append(Fallout2Dat.File(self._io, self, self._root))
        def _fetch_instances(self):
            pass
            for i in range(len(self.files)):
                pass
                self.files[i]._fetch_instances()
    class Pstr(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Fallout2Dat.Pstr, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()
        def _read(self):
            self.size = self._io.read_u4le()
            self.str = (self._io.read_bytes(self.size)).decode(u"ASCII")
        def _fetch_instances(self):
            pass
    @property
    def footer(self):
        if hasattr(self, '_m_footer'):
            return self._m_footer
        _pos = self._io.pos()
        self._io.seek(self._io.size() - 8)
        self._m_footer = Fallout2Dat.Footer(self._io, self, self._root)
        self._io.seek(_pos)
        return getattr(self, '_m_footer', None)
    @property
    def index(self):
        if hasattr(self, '_m_index'):
            return self._m_index
        _pos = self._io.pos()
        self._io.seek((self._io.size() - 8) - self.footer.index_size)
        self._m_index = Fallout2Dat.Index(self._io, self, self._root)
        self._io.seek(_pos)
        return getattr(self, '_m_index', None)