cs-binary 20250501
Facilities associated with binary data parsing and transcription. The classes in this module support easy parsing of binary data structures, returning instances with the binary data decoded into attributes and capable of transcribing themselves in binary form (trivially via `bytes(instance)` and also otherwise).
Requires Python: >=3.6
Keywords: python3

Facilities associated with binary data parsing and transcription.
The classes in this module support easy parsing of binary data
structures,
returning instances with the binary data decoded into attributes
and capable of transcribing themselves in binary form
(trivially via `bytes(instance)` and also otherwise).

*Latest release 20250501*:
* @binclass: a dataclass-like way to specify binary structures.
* Rename BinaryByteses to BinaryBytes.
* Rename BinaryMultiStruct and BinarySingleStruct to BinaryStruct.
* New bs(bytes) subclass with a compact repr().
* BinarySingleValue classes now expect a type specification for the value.
* BinarySingleValue now subclasses cs.deco.Promotable.
* New struct_field_types(struct_format,field_names) to return a mapping of field names to struct.unpack-returned types.
* New is_single_value(obj) to test for single value binary objects.
* New ListOfBinary, a class holding a list of instances of some binary class.
* Many internal updates and improvements.

See `cs.iso14496` for an ISO 14496 (eg MPEG4) parser
built using this module.

Note: this module requires Python 3.6+ because various default
behaviours rely on `dict`s preserving their insert order.

Terminology used below:
* buffer:
  an instance of `cs.buffer.CornuCopyBuffer`,
  which manages an iterable of bytes-like values
  and has various useful methods for parsing.
* chunk:
  a piece of binary data obeying the buffer protocol,
  i.e. a `collections.abc.Buffer`;
  almost always a `bytes` instance or a `memoryview`,
  but in principle also things like `bytearray`.

The `CornuCopyBuffer` is the basis for all parsing, as it manages
a variety of input sources such as files, memory, sockets etc.
It also has a factory methods to make one from a variety of sources
such as bytes, iterables, binary files, `mmap`ped files,
TCP data streams, etc.

All the binary classes subclass `AbstractBinary`,
Amongst other things, this means that the binary transcription
can be had simply from `bytes(instance)`,
although there are more transcription methods provided
for when greater flexibility is desired.
It also means that all classes have `parse`* and `scan`* methods
for parsing binary data streams.

The `.parse(cls,bfr)` class method reads binary data from a
buffer and returns an instance.

The `.transcribe(self)` method may be a regular function or a
generator which returns or yields things which can be transcribed
as bytes via the `flatten` function.
See the `AbstractBinary.transcribe` docstring for specifics; this might:
- return a `bytes`
- return an ASCII string
- be a generator which yields various values such as bytes,
  ASCII strings, other `AbstractBinary` instances such as each
  field (which get transcribed in turn) or an iterable of these
  things

There are 6 main ways an implementor might base their data structures:
* `BinaryStruct`: a factory for classes based
  on a `struct.struct` format string with multiple values;
  this also builds a `namedtuple` subclass
* `@binclass`: a dataclass-like specification of a binary structure
* `BinarySingleValue`: a base class for subclasses
  parsing and transcribing a single value, such as `UInt8` or
  `BinaryUTF8NUL`
* `BinaryMultiValue`: a factory for subclasses
  parsing and transcribing multiple values
  with no variation
* `SimpleBinary`: a base class for subclasses
  with custom `.parse` and `.transcribe` methods,
  for structures with variable fields;
  this makes a `SimpleNamespace` subclass

These can all be mixed as appropriate to your needs.

You can also instantiate objects directly;
there's no requirement for the source information to be binary.

There are several presupplied subclasses for common basic types
such as `UInt32BE` (an unsigned 32 bit big endian integer).

## Some Examples

### A `BinaryStruct`, from `cs.iso14496`

A simple `struct` style definitiion for 9 longs:

    Matrix9Long = BinaryStruct(
        'Matrix9Long', '>lllllllll', 'v0 v1 v2 v3 v4 v5 v6 v7 v8'
    )

Per the `struct.struct` format string, this parses 9 big endian longs
and returns a `namedtuple` with 9 fields.
Like all the `AbstractBinary` subclasses, parsing an instance from a
stream can be done like this:

    m9 = Matrix9Long.parse(bfr)
    print("m9.v3", m9.v3)

and writing its binary form to a file like this:

    f.write(bytes(m9))

### A `@binclass`, also from `cs.iso14496`

For reasons to do with the larger MP4 parser this uses an extra
decorator `@boxbodyclass` which is just a shim for the `@binclass`
decorator with an addition step.

    @boxbodyclass
    class FullBoxBody2(BoxBody):
      """ A common extension of a basic `BoxBody`, with a version and flags field.
          ISO14496 section 4.2.
      """
      version: UInt8
      flags0: UInt8
      flags1: UInt8
      flags2: UInt8

      @property
      def flags(self):
        """ The flags value, computed from the 3 flag bytes.
        """
        return (self.flags0 << 16) | (self.flags1 << 8) | self.flags2

This has 4 fields, each an unsigned 8 bit value (one bytes),
and a property `.flags` which is the overall flags value for
the box header.

You should look at the source code for the `TKHDBoxBody` from
that module for an example of a `@binclass` with a variable
collection of fields based on an earlier `version` field value.

### A `BinarySingleValue`, the `BSUInt` from thos module

The `BSUint` transcribes an unsigned integera of arbitrary size
as a big endian variable sizes sequence of bytes.
I understand this is the same scheme MIDI uses.

You can define a `BinarySingleValue` with conventional `.parse()`
and `.transribe()` methods but it is usually expedient to instead
provide `.parse_value()` and `transcribe_value()` methods, which
return or transcibe the core value (the unsigned integer in
this case).

    class BSUInt(BinarySingleValue, value_type=int):
      """ A binary serialised unsigned `int`.

          This uses a big endian byte encoding where continuation octets
          have their high bit set. The bits contributing to the value
          are in the low order 7 bits.
      """

      @staticmethod
      def parse_value(bfr: CornuCopyBuffer) -> int:
        """ Parse an extensible byte serialised unsigned `int` from a buffer.

            Continuation octets have their high bit set.
            The value is big-endian.

            This is the go for reading from a stream. If you already have
            a bare bytes instance then the `.decode_bytes` static method
            is probably most efficient;
            there is of course the usual `AbstractBinary.parse_bytes`
            but that constructs a buffer to obtain the individual bytes.
        """
        n = 0
        b = 0x80
        while b & 0x80:
          b = bfr.byte0()
          n = (n << 7) | (b & 0x7f)
        return n

      # pylint: disable=arguments-renamed
      @staticmethod
      def transcribe_value(n):
        """ Encode an unsigned int as an entensible byte serialised octet
            sequence for decode. Return the bytes object.
        """
        bs = [n & 0x7f]
        n >>= 7
        while n > 0:
          bs.append(0x80 | (n & 0x7f))
          n >>= 7
        return bytes(reversed(bs))

### A `BinaryMultiValue`

A `BinaryMultiValue` s a class factory for making a multi field
`AbstractBinary` from variable field descriptions.
You're probably better off using `@binclass` these days.
See the `BinaryMutliValue` docstring for details and an example.

An MP4 ELST box:

    class ELSTBoxBody(FullBoxBody):
      """ An 'elst' Edit List FullBoxBody - section 8.6.6.
      """

      V0EditEntry = BinaryStruct(
          'ELSTBoxBody_V0EditEntry', '>Llhh',
          'segment_duration media_time media_rate_integer media_rate_fraction'
      )
      V1EditEntry = BinaryStruct(
          'ELSTBoxBody_V1EditEntry', '>Qqhh',
          'segment_duration media_time media_rate_integer media_rate_fraction'
      )

      @property
      def entry_class(self):
        """ The class representing each entry.
        """
        return self.V1EditEntry if self.version == 1 else self.V0EditEntry

      @property
      def entry_count(self):
        """ The number of entries.
        """
        return len(self.entries)

      def parse_fields(self, bfr: CornuCopyBuffer):
        """ Parse the fields of an `ELSTBoxBody`.
        """
        super().parse_fields(bfr)
        assert self.version in (0, 1)
        entry_count = UInt32BE.parse_value(bfr)
        self.entries = list(self.entry_class.scan(bfr, count=entry_count))

      def transcribe(self):
        """ Transcribe an `ELSTBoxBody`.
        """
        yield super().transcribe()
        yield UInt32BE.transcribe_value(self.entry_count)
        yield map(self.entry_class.transcribe, self.entries)

A Edit List box comes in a version 0 and version 1 form, differing
in the field sizes in the edit entries.  This defines two
flavours of edit entry structure and a property to return the
suitable class based on the version field.  The `parse_fields()`
method is called from the base `BoxBody` class' `parse()` method
to collect addition fields for any box.  For this box it collectsa
32 bit `entry_count` and then a list of that many edit entries.
The transcription yields corresponding values.

Short summary:
* `AbstractBinary`: Abstract class for all `Binary`* implementations, specifying the abstract `parse` and `transcribe` methods and providing various helper methods.
* `BinaryBytes`: A list of `bytes` parsed directly from the native iteration of the buffer. Subclasses are initialised with a `consume=` class parameter indicating how many bytes to console on parse; the default is `...` meaning to consume the entire remaining buffer, but a positive integer can also be supplied to consume exactly that many bytes.
* `BinaryFixedBytes`: Factory for an `AbstractBinary` subclass matching `length` bytes of data. The bytes are saved as the attribute `.data`.
* `BinaryListValues`: A list of values with a common parse specification, such as sample or Boxes in an ISO14496 Box structure.
* `BinaryMultiStruct`: A class factory for `AbstractBinary` `namedtuple` subclasses built around potentially complex `struct` formats.
* `BinaryMultiValue`: Construct a `SimpleBinary` subclass named `class_name` whose fields are specified by the mapping `field_map`.
* `BinarySingleStruct`: OBSOLETE BinaryStruct.
* `BinarySingleValue`: A representation of a single value as the attribute `.value`.
* `BinaryStruct`: OBSOLETE BinaryStruct.
* `BinaryUTF16NUL`: A NUL terminated UTF-16 string.
* `BinaryUTF8NUL`: A NUL terminated UTF-8 string.
* `binclass`: A decorator for `dataclass`-like binary classes.
* `bs`: A `bytes subclass with a compact `repr()`.
* `BSData`: A run length encoded data chunk, with the length encoded as a `BSUInt`.
* `BSSFloat`: A float transcribed as a `BSString` of `str(float)`.
* `BSString`: A run length encoded string, with the length encoded as a BSUInt.
* `BSUInt`: A binary serialised unsigned `int`.
* `flatten`: Flatten `transcription` into an iterable of `Buffer`s. None of the `Buffer`s will be empty.
* `Float64BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>d'` and presents the attributes ['value'].
* `Float64LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<d'` and presents the attributes ['value'].
* `Int16BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>h'` and presents the attributes ['value'].
* `Int16LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<h'` and presents the attributes ['value'].
* `Int32BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>l'` and presents the attributes ['value'].
* `Int32LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<l'` and presents the attributes ['value'].
* `is_single_value`: Test whether `obj` is a single value binary object.
* `parse_offsets`: Decorate `parse` (usually an `AbstractBinary` class method) to record the buffer starting offset as `self.offset` and the buffer post parse offset as `self.end_offset`. If the decorator parameter `report` is true, call `bfr.report_offset()` with the starting offset at the end of the parse.
* `pt_spec`: Convert a parse/transcribe specification `pt` into an `AbstractBinary` subclass.
* `SimpleBinary`: Abstract binary class based on a `SimpleNamespace`, thus providing a nice `__str__` and a keyword based `__init__`. Implementors must still define `.parse` and `.transcribe`.
* `struct_field_types`: Construct a `dict` mapping field names to struct return types.
* `UInt16BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>H'` and presents the attributes ['value'].
* `UInt16LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<H'` and presents the attributes ['value'].
* `UInt32BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>L'` and presents the attributes ['value'].
* `UInt32LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<L'` and presents the attributes ['value'].
* `UInt64BE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'>Q'` and presents the attributes ['value'].
* `UInt64LE`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'<Q'` and presents the attributes ['value'].
* `UInt8`: An `AbstractBinary` `namedtuple` which parses and transcribes the struct format `'B'` and presents the attributes ['value'].

Module contents:
- <a name="AbstractBinary"></a>`Class `AbstractBinary(cs.deco.Promotable)`: Abstract class for all `Binary`* implementations,
  specifying the abstract `parse` and `transcribe` methods
  and providing various helper methods.

  Naming conventions:
  - `parse`* methods parse a single instance from a buffer
  - `scan`* methods are generators yielding successive instances from a buffer

*`AbstractBinary.__bytes__(self)`*:
The binary transcription as a single `bytes` object.

*`AbstractBinary.__len__(self)`*:
Compute the length by running a transcription and measuring it.

*`AbstractBinary.__str__(self, attr_names=None, attr_choose=None, str_func=None)`*:
The string summary of this object.
If called explicitly rather than via `str()` the following
optional parametsrs may be supplied:
* `attr_names`: an iterable of `str` naming the attributes to include;
  the default if the keys of `self.__dict__`
* `attr_choose`: a callable to select amongst the attribute names names;
  the default is to choose names which do not start with an underscore
* `str_func`: a callable returning the string form of an attribute value;
  the default returns `cropped_repr(v)` where `v` is the value's `.value`
  attribute for single value objects otherwise the object itself

*`AbstractBinary.from_bytes(bs, **parse_bytes_kw)`*:
Factory to parse an instance from the
bytes `bs` starting at `offset`.
Returns the new instance.

Raises `ValueError` if `bs` is not entirely consumed.
Raises `EOFError` if `bs` has insufficient data.

Keyword parameters are passed to the `.parse_bytes` method.

This relies on the `cls.parse` method for the parse.

*`AbstractBinary.load(f)`*:
Load an instance from the file `f`
which may be a filename or an open file as for `AbstractBinary.scan`.
Return the instance or `None` if the file is empty.

*`AbstractBinary.parse(bfr: cs.buffer.CornuCopyBuffer)`*:
Parse an instance of `cls` from the buffer `bfr`.

*`AbstractBinary.parse_bytes(bs, offset=0, length=None, **parse_kw)`*:
Factory to parse an instance from the
bytes `bs` starting at `offset`.
Returns `(instance,offset)` being the new instance and the post offset.

Raises `EOFError` if `bs` has insufficient data.

The parameters `offset` and `length` are passed to the
`CornuCopyBuffer.from_bytes` factory.

Other keyword parameters are passed to the `.parse` method.

This relies on the `cls.parse` method for the parse.

*`AbstractBinary.save(self, f)`*:
Save this instance to the file `f`
which may be a filename or an open file.
Return the length of the transcription.

*`AbstractBinary.scan(bfr: cs.buffer.CornuCopyBuffer, count=None, *, min_count=None, max_count=None, with_offsets=False, **parse_kw)`*:
A generator to scan the buffer `bfr` for repeated instances of `cls`
until end of input, and yield them.

Note that if `bfr` is not already a `CornuCopyBuffer`
it is promoted to `CornuCopyBuffer` from several types
such as filenames etc; see `CornuCopyBuffer.promote`.

Parameters:
* `bfr`: the buffer to scan, or any object suitable for `CornuCopyBuffer.promote`
* `count`: the required number of instances to scan,
  equivalent to setting `min_count=count` and `max_count=count`
* `min_count`: the minimum number of instances to scan
* `max_count`: the maximum number of instances to scan
* `with_offsets`: optional flag, default `False`;
  if true yield `(pre_offset,obj,post_offset)`, otherwise just `obj`
It is in error to specify both `count` and one of `min_count` or `max_count`.

Other keyword arguments are passed to `self.parse()`.

Scanning stops after `max_count` instances (if specified).
If fewer than `min_count` instances (if specified) are scanned
a warning is issued.
This is to accomodate nonconformant streams without raising exceptions.
Callers wanting to validate `max_count` may want to probe `bfr.at_eof()`
after return.
Callers not wanting a warning over `min_count` should not specify it,
and instead check the number of instances returned themselves.

*`AbstractBinary.scan_fspath(fspath: str, *, with_offsets=False, **kw)`*:
Open the file with filesystenm path `fspath` for read
and yield from `self.scan(..,**kw)` or
`self.scan_with_offsets(..,**kw)` according to the
`with_offsets` parameter.

*Deprecated; please just call `scan` with a filesystem pathname.*

Parameters:
* `fspath`: the filesystem path of the file to scan
* `with_offsets`: optional flag, default `False`;
  if true then scan with `scan_with_offsets` instead of
  with `scan`
Other keyword parameters are passed to `scan` or
`scan_with_offsets`.

*`AbstractBinary.scan_with_offsets(bfr: cs.buffer.CornuCopyBuffer, count=None, min_count=None, max_count=None)`*:
Wrapper for `scan()` which yields `(pre_offset,instance,post_offset)`
indicating the start and end offsets of the yielded instances.
All parameters are as for `scan()`.

*Deprecated; please just call `scan` with the `with_offsets=True` parameter.

*`AbstractBinary.self_check(self, *, field_types=None)`*:
Internal self check. Returns `True` if passed.

If the structure has a `FIELD_TYPES` attribute, normally a
class attribute, then check the fields against it.

The `FIELD_TYPES` attribute is a mapping of `field_name` to
a specification of `required` and `types`. The specification
may take one of 2 forms:
* a tuple of `(required,types)`
* a single `type`; this is equivalent to `(True,(type,))`
Their meanings are as follows:
* `required`: a Boolean. If true, the field must be present
  in the packet `field_map`, otherwise it need not be present.
* `types`: a tuple of acceptable field types

There are some special semantics involved here.

An implementation of a structure may choose to make some
fields plain instance attributes instead of binary objects
in the `field_map` mapping, particularly variable structures
such as a `cs.iso14496.BoxHeader`, whose `.length` may be parsed
directly from its binary form or computed from other fields
depending on the `box_size` value. Therefore, checking for
a field is first done via the `field_map` mapping, then by
`getattr`, and as such the acceptable `types` may include
nonstructure types such as `int`.

Here is the `cs.iso14496` `Box.FIELD_TYPES` definition as an example:

    FIELD_TYPES = {
        'header': BoxHeader,
        'body': BoxBody,
        'unparsed': list,
        'offset': int,
        'unparsed_offset': int,
        'end_offset': int,
    }

Note that `length` includes some nonstructure types,
and that it is written as a tuple of `(True,types)` because
it has more than one acceptable type.

*`AbstractBinary.transcribe(self)`*:
Return or yield `bytes`, ASCII string, `None` or iterables
comprising the binary form of this instance.

This aims for maximum convenience when transcribing a data structure.

This may be implemented as a generator, yielding parts of the structure.

This may be implemented as a normal function, returning:
* `None`: no bytes of data,
  for example for an omitted or empty structure
* a `bytes`-like object: the full data bytes for the structure
* an ASCII compatible string:
  this will be encoded with the `'ascii'` encoding to make `bytes`
* an iterable:
  the components of the structure,
  including substranscriptions which themselves
  adhere to this protocol - they may be `None`, `bytes`-like objects,
  ASCII compatible strings or iterables.
  This supports directly returning or yielding the result of a field's
  `.transcribe` method.

*`AbstractBinary.transcribe_flat(self)`*:
Return a flat iterable of chunks transcribing this field.

*`AbstractBinary.transcribed_length(self)`*:
Compute the length by running a transcription and measuring it.

*`AbstractBinary.write(self, file, *, flush=False)`*:
Write this instance to `file`, a file-like object supporting
`.write(bytes)` and `.flush()`.
Return the number of bytes written.
- <a name="BinaryBytes"></a>`Class `BinaryBytes(BinarySingleValue)`: A list of `bytes` parsed directly from the native iteration of the buffer.
  Subclasses are initialised with a `consume=` class parameter
  indicating how many bytes to console on parse; the default
  is `...` meaning to consume the entire remaining buffer, but
  a positive integer can also be supplied to consume exactly
  that many bytes.

*`BinaryBytes.parse(bfr: cs.buffer.CornuCopyBuffer)`*:
Consume `cls.PARSE_SIZE` bytes from the buffer and instantiate a new instance.

*`BinaryBytes.promote(obj)`*:
Promote `obj` to a `BinaryBytes` instance.
Other instances of `AbstractBinary` will be transcribed into the buffers.
Otherwise use `BinarySingleValue.promote(obj)`.

*`BinaryBytes.transcribe(self)`*:
Transcribe each value.

*`BinaryBytes.value`*:
The internal list of `bytes` instances joined together.
This is a property and may be expensive to compute for a large list.
- <a name="BinaryFixedBytes"></a>`BinaryFixedBytes(class_name: str, length: int)`: Factory for an `AbstractBinary` subclass matching `length` bytes of data.
  The bytes are saved as the attribute `.data`.
- <a name="BinaryListValues"></a>`Class `BinaryListValues(AbstractBinary)`: A list of values with a common parse specification,
  such as sample or Boxes in an ISO14496 Box structure.

*`BinaryListValues.parse(bfr: cs.buffer.CornuCopyBuffer, count=None, *, end_offset=None, min_count=None, max_count=None, pt)`*:
Read values from `bfr`.
Return a `BinaryListValue` containing the values.

Parameters:
* `count`: optional count of values to read;
  if specified, exactly this many values are expected.
* `end_offset`: an optional bounding end offset of the buffer.
* `min_count`: the least acceptable number of values.
* `max_count`: the most acceptable number of values.
* `pt`: a parse/transcribe specification
  as accepted by the `pt_spec()` factory.
  The values will be returned by its parse function.

*`BinaryListValues.transcribe(self)`*:
Transcribe all the values.
- <a name="BinaryMultiStruct"></a>`BinaryMultiStruct(class_name: str, struct_format: str, field_names: Union[str, List[str]] = 'value')`: A class factory for `AbstractBinary` `namedtuple` subclasses
  built around potentially complex `struct` formats.

  Parameters:
  * `class_name`: name for the generated class
  * `struct_format`: the `struct` format string
  * `field_names`: optional field name list,
    a space separated string or an interable of strings;
    the default is `'value'`, intended for single field structs

  Example:

      # an "access point" record from the .ap file
      Enigma2APInfo = BinaryStruct('Enigma2APInfo', '>QQ', 'pts offset')

      # a "cut" record from the .cuts file
      Enigma2Cut = BinaryStruct('Enigma2Cut', '>QL', 'pts type')

      >>> UInt16BE = BinaryStruct('UInt16BE', '>H')
      >>> UInt16BE.__name__
      'UInt16BE'
      >>> UInt16BE.format
      '>H'
      >>> UInt16BE.struct   #doctest: +ELLIPSIS
      <_struct.Struct object at ...>
      >>> field = UInt16BE.from_bytes(bytes((2,3)))
      >>> field
      UInt16BE('>H',value=515)
      >>> field.value
      515
- <a name="BinaryMultiValue"></a>`BinaryMultiValue(class_name, field_map, field_order=None)`: Construct a `SimpleBinary` subclass named `class_name`
  whose fields are specified by the mapping `field_map`.

  The `field_map` is a mapping of field name
  to parse/trasncribe specifications suitable for `pt_spec()`;
  these are all promoted by `pt_spec` into `AbstractBinary` subclasses.

  The `field_order` is an optional ordering of the field names;
  the default comes from the iteration order of `field_map`.

  *Note* for Python <3.6:
  if `field_order` is not specified
  it is constructed by iterating over `field_map`.
  Prior to Python 3.6, `dict`s do not provide a reliable order
  and should be accompanied by an explicit `field_order`.
  From 3.6 onward a `dict` is enough and its insertion order
  will dictate the default `field_order`.

  For a fixed record structure
  the default `.parse` and `.transcribe` methods will suffice;
  they parse or transcribe each field in turn.
  Subclasses with variable records should override
  the `.parse` and `.transcribe` methods
  accordingly.

  If the class has both `parse_value` and `transcribe_value` methods
  then the value itself will be directly stored.
  Otherwise the class it presumed to be more complex subclass
  of `AbstractBinary` and the instance is stored.

  Here is an example exhibiting various ways of defining each field:
  * `n1`: defined with the *`_value` methods of `UInt8`,
    which return or transcribe the `int` from an unsigned 8 bit value;
    this stores a `BinarySingleValue` whose `.value` is an `int`
  * `n2`: defined from the `UInt8` class,
    which parses an unsigned 8 bit value;
    this stores an `UInt8` instance
    (also a `BinarySingleValue` whole `.value` is an `int`)
  * `n3`: like `n2`
  * `data1`: defined with the *`_value` methods of `BSData`,
    which return or transcribe the data `bytes`
    from a run length encoded data chunk;
    this stores a `BinarySingleValue` whose `.value` is a `bytes`
  * `data2`: defined from the `BSData` class
    which parses a run length encoded data chunk;
    this is a `BinarySingleValue` so we store its `bytes` value directly.

        >>> class BMV(BinaryMultiValue("BMV", {
        ...         'n1': (UInt8.parse_value, UInt8.transcribe_value),
        ...         'n2': UInt8,
        ...         'n3': UInt8,
        ...         'nd': ('>H4s', 'short bs'),
        ...         'data1': (
        ...             BSData.parse_value,
        ...             BSData.transcribe_value,
        ...         ),
        ...         'data2': BSData,
        ... })):
        ...     pass
        >>> BMV.FIELD_ORDER
        ['n1', 'n2', 'n3', 'nd', 'data1', 'data2']
        >>> bmv = BMV.from_bytes(b'\x11\x22\x77\x81\x82zyxw\x02AB\x04DEFG')
        >>> bmv.n1  #doctest: +ELLIPSIS
        17
        >>> bmv.n2
        34
        >>> bmv  #doctest: +ELLIPSIS
        BMV(n1=17, n2=34, n3=119, nd=nd('>H4s',short=33154,bs=b'zyxw'), data1=b'AB', data2=b'DEFG')
        >>> bmv.nd  #doctest: +ELLIPSIS
        nd('>H4s',short=33154,bs=b'zyxw')
        >>> bmv.nd.bs
        b'zyxw'
        >>> bytes(bmv.nd)
        b'zyxw'
        >>> bmv.data1
        b'AB'
        >>> bmv.data2
        b'DEFG'
        >>> bytes(bmv)
        b'\x11"w\x81\x82zyxw\x02AB\x04DEFG'
        >>> list(bmv.transcribe_flat())
        [b'\x11', b'"', b'w', b'\x81\x82zyxw', b'\x02', b'AB', b'\x04', b'DEFG']
- <a name="BinarySingleStruct"></a>`BinarySingleStruct(class_name: str, struct_format: str, field_names: Union[str, List[str]] = 'value')`: OBSOLETE BinaryStruct

  A class factory for `AbstractBinary` `namedtuple` subclasses
  built around potentially complex `struct` formats.

  Parameters:
  * `class_name`: name for the generated class
  * `struct_format`: the `struct` format string
  * `field_names`: optional field name list,
    a space separated string or an interable of strings;
    the default is `'value'`, intended for single field structs

  Example:

      # an "access point" record from the .ap file
      Enigma2APInfo = BinaryStruct('Enigma2APInfo', '>QQ', 'pts offset')

      # a "cut" record from the .cuts file
      Enigma2Cut = BinaryStruct('Enigma2Cut', '>QL', 'pts type')

      >>> UInt16BE = BinaryStruct('UInt16BE', '>H')
      >>> UInt16BE.__name__
      'UInt16BE'
      >>> UInt16BE.format
      '>H'
      >>> UInt16BE.struct   #doctest: +ELLIPSIS
      <_struct.Struct object at ...>
      >>> field = UInt16BE.from_bytes(bytes((2,3)))
      >>> field
      UInt16BE('>H',value=515)
      >>> field.value
      515
- <a name="BinarySingleValue"></a>`Class `BinarySingleValue(AbstractBinary)`: A representation of a single value as the attribute `.value`.

  Subclasses must implement:
  * `parse` or `parse_value`
  * `transcribe` or `transcribe_value`

*`BinarySingleValue.__init__(self, value)`*:
Initialise `self` with `value`.

*`BinarySingleValue.parse(bfr: cs.buffer.CornuCopyBuffer)`*:
Parse an instance from `bfr`.

Subclasses must implement this method or `parse_value`.

*`BinarySingleValue.parse_value(bfr: cs.buffer.CornuCopyBuffer)`*:
Parse a value from `bfr` based on this class.

Subclasses must implement this method or `parse`.

*`BinarySingleValue.parse_value_from_bytes(bs, offset=0, length=None, **kw)`*:
Parse a value from the bytes `bs` based on this class.
Return `(value,offset)`.

*`BinarySingleValue.scan_values(bfr: cs.buffer.CornuCopyBuffer, **kw)`*:
Scan `bfr`, yield values.

*`BinarySingleValue.transcribe(self)`*:
Transcribe this instance as bytes.

Subclasses must implement this method or `transcribe_value`.

*`BinarySingleValue.transcribe_value(value)`*:
Transcribe `value` as bytes based on this class.

Subclasses must implement this method or `transcribe`.

*`BinarySingleValue.value_from_bytes(bs, **from_bytes_kw)`*:
Decode an instance from `bs` using `.from_bytes`
and return the `.value` attribute.
Keyword arguments are passed to `cls.from_bytes`.
- <a name="BinaryStruct"></a>`BinaryStruct(class_name: str, struct_format: str, field_names: Union[str, List[str]] = 'value')`: OBSOLETE BinaryStruct

  OBSOLETE BinaryStruct

  A class factory for `AbstractBinary` `namedtuple` subclasses
  built around potentially complex `struct` formats.

  Parameters:
  * `class_name`: name for the generated class
  * `struct_format`: the `struct` format string
  * `field_names`: optional field name list,
    a space separated string or an interable of strings;
    the default is `'value'`, intended for single field structs

  Example:

      # an "access point" record from the .ap file
      Enigma2APInfo = BinaryStruct('Enigma2APInfo', '>QQ', 'pts offset')

      # a "cut" record from the .cuts file
      Enigma2Cut = BinaryStruct('Enigma2Cut', '>QL', 'pts type')

      >>> UInt16BE = BinaryStruct('UInt16BE', '>H')
      >>> UInt16BE.__name__
      'UInt16BE'
      >>> UInt16BE.format
      '>H'
      >>> UInt16BE.struct   #doctest: +ELLIPSIS
      <_struct.Struct object at ...>
      >>> field = UInt16BE.from_bytes(bytes((2,3)))
      >>> field
      UInt16BE('>H',value=515)
      >>> field.value
      515
- <a name="BinaryUTF16NUL"></a>`Class `BinaryUTF16NUL(BinarySingleValue)`: A NUL terminated UTF-16 string.

*`BinaryUTF16NUL.__init__(self, value: str, *, encoding: str)`*:
pylint: disable=super-init-not-called

*`BinaryUTF16NUL.VALUE_TYPE`*

*`BinaryUTF16NUL.parse(bfr: cs.buffer.CornuCopyBuffer, *, encoding: str)`*:
Parse the encoding and value and construct an instance.

*`BinaryUTF16NUL.parse_value(bfr: cs.buffer.CornuCopyBuffer, *, encoding: str) -> str`*:
Read a NUL terminated UTF-16 string from `bfr`, return a `UTF16NULField`.
The mandatory parameter `encoding` specifies the UTF16 encoding to use
(`'utf_16_be'` or `'utf_16_le'`).

*`BinaryUTF16NUL.transcribe(self)`*:
Transcribe `self.value` in UTF-16 with a terminating NUL.

*`BinaryUTF16NUL.transcribe_value(value: str, encoding='utf-16')`*:
Transcribe `value` in UTF-16 with a terminating NUL.
- <a name="BinaryUTF8NUL"></a>`Class `BinaryUTF8NUL(BinarySingleValue)`: A NUL terminated UTF-8 string.

*`BinaryUTF8NUL.VALUE_TYPE`*

*`BinaryUTF8NUL.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> str`*:
Read a NUL terminated UTF-8 string from `bfr`, return field.

*`BinaryUTF8NUL.transcribe_value(s)`*:
Transcribe the `value` in UTF-8 with a terminating NUL.
- <a name="binclass"></a>`binclass(*da, **dkw)`: A decorator for `dataclass`-like binary classes.

  Example use:

      >>> @binclass
      ... class SomeStruct:
      ...     """A struct containing a count and some flags."""
      ...     count : UInt32BE
      ...     flags : UInt8
      >>> ss = SomeStruct(count=3, flags=0x04)
      >>> ss
      SomeStruct:SomeStruct__dataclass(count=UInt32BE('>L',value=3),flags=UInt8('B',value=4))
      >>> print(ss)
      SomeStruct(count=3,flags=4)
      >>> bytes(ss)
      b'\x00\x00\x00\x03\x04'
      >>> SomeStruct.promote(b'\x00\x00\x00\x03\x04')
      SomeStruct:SomeStruct__dataclass(count=UInt32BE('>L',value=3),flags=UInt8('B',value=4))

  Extending an existing `@binclass` class, for example to add
  the body of a structure to some header part:

      >>> @binclass
      ... class HeaderStruct:
      ...     """A header containing a count and some flags."""
      ...     count : UInt32BE
      ...     flags : UInt8
      >>> @binclass
      ... class Packet(HeaderStruct):
      ...     body_text : BSString
      ...     body_data : BSData
      ...     body_longs : BinaryStruct(
      ...         'longs', '>LL', 'long1 long2'
      ...     )
      >>> packet = Packet(
      ...     count=5, flags=0x03,
      ...     body_text="hello",
      ...     body_data=b'xyzabc',
      ...     body_longs=(10,20),
      ... )
      >>> packet
      Packet:Packet__dataclass(count=UInt32BE('>L',value=5),flags=UInt8('B',value=3),body_text=BSString('hello'),body_data=BSData(b'xyzabc'),body_longs=longs('>LL',long1=10,long2=20))
      >>> print(packet)
      Packet(count=5,flags=3,body_text=hello,body_data=b'xyzabc',body_longs=longs(long1=10,long2=20))
      >>> packet.body_data
      b'xyzabc'
- <a name="bs"></a>`Class `bs(builtins.bytes)`: A `bytes subclass with a compact `repr()`.

*`bs.join(self, chunks)`*:
`bytes.join` but returning a `bs`.

*`bs.promote(obj)`*:
Promote `bytes` or `memoryview` to a `bs`.
- <a name="BSData"></a>`Class `BSData(BinarySingleValue)`: A run length encoded data chunk, with the length encoded as a `BSUInt`.

*`BSData.data`*:
An alias for the `.value` attribute.

*`BSData.data_offset`*:
The length of the length indicator,
useful for computing the location of the raw data.

*`BSData.data_offset_for(bs) -> int`*:
Compute the `data_offset` which would obtain for the bytes `bs`.

*`BSData.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> bytes`*:
Parse the data from `bfr`.

*`BSData.transcribe_value(data)`*:
Transcribe the payload length and then the payload.
- <a name="BSSFloat"></a>`Class `BSSFloat(BinarySingleValue)`: A float transcribed as a `BSString` of `str(float)`.

*`BSSFloat.VALUE_TYPE`*

*`BSSFloat.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> float`*:
Parse a `BSSFloat` from a buffer and return the `float`.

*`BSSFloat.transcribe_value(f)`*:
Transcribe a `float`.
- <a name="BSString"></a>`Class `BSString(BinarySingleValue)`: A run length encoded string, with the length encoded as a BSUInt.

*`BSString.VALUE_TYPE`*

*`BSString.parse_value(bfr: cs.buffer.CornuCopyBuffer, encoding='utf-8', errors='strict') -> str`*:
Parse a run length encoded string from `bfr`.

*`BSString.transcribe_value(value: str, encoding='utf-8')`*:
Transcribe a string.
- <a name="BSUInt"></a>`Class `BSUInt(BinarySingleValue)`: A binary serialised unsigned `int`.

  This uses a big endian byte encoding where continuation octets
  have their high bit set. The bits contributing to the value
  are in the low order 7 bits.

*`BSUInt.VALUE_TYPE`*

*`BSUInt.decode_bytes(data, offset=0) -> Tuple[int, int]`*:
Decode an extensible byte serialised unsigned `int` from `data` at `offset`.
Return value and new offset.

Continuation octets have their high bit set.
The octets are big-endian.

If you just have a `bytes` instance, this is the go. If you're
reading from a stream you're better off with `parse` or `parse_value`.

Examples:

    >>> BSUInt.decode_bytes(b'\0')
    (0, 1)

Note: there is of course the usual `AbstractBinary.parse_bytes`
but that constructs a buffer to obtain the individual bytes;
this static method will be more performant
if all you are doing is reading this serialisation
and do not already have a buffer.

*`BSUInt.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> int`*:
Parse an extensible byte serialised unsigned `int` from a buffer.

Continuation octets have their high bit set.
The value is big-endian.

This is the go for reading from a stream. If you already have
a bare bytes instance then the `.decode_bytes` static method
is probably most efficient;
there is of course the usual `AbstractBinary.parse_bytes`
but that constructs a buffer to obtain the individual bytes.

*`BSUInt.transcribe_value(n)`*:
Encode an unsigned int as an entensible byte serialised octet
sequence for decode. Return the bytes object.
- <a name="flatten"></a>`flatten(transcription) -> Iterable[bytes]`: Flatten `transcription` into an iterable of `Buffer`s.
  None of the `Buffer`s will be empty.

  This exists to allow subclass methods to easily return
  transcribable things (having a `.transcribe` method), ASCII
  strings or bytes or iterables or even `None`, in turn allowing
  them simply to return their superclass' chunks iterators
  directly instead of having to unpack them.

  The supplied `transcription` may be any of the following:
  - `None`: yield nothing
  - an object with a `.transcribe` method: yield from
    `flatten(transcription.transcribe())`
  - a `Buffer`: yield the `Buffer` if it is not empty
  - a `str`: yield `transcription.encode('ascii')`
  - an iterable: yield from `flatten(item)` for each item in `transcription`

  An example from the `cs.iso14496.METABoxBody` class:

      def transcribe(self):
          yield super().transcribe()
          yield self.theHandler
          yield self.boxes

  The binary classes `flatten` the result of the `.transcribe`
  method to obtain `bytes` instances for the object's binary
  transcription.
- <a name="Float64BE"></a>`Class `Float64BE(Float64BE, AbstractBinary)`: An `AbstractBinary` `namedtuple` which parses and transcribes
  the struct format `'>d'` and presents the attributes ['value'].

*`Float64BE.parse(bfr: cs.buffer.CornuCopyBuffer)`*:
Parse from `bfr` via `struct.unpack`.

*`Float64BE.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> float`*:
Parse a value from `bfr`, return the value.

*`Float64BE.promote(obj)`*:
Promote a single value to an instance of `cls`.

*`Float64BE.transcribe(self)`*:
Transcribe via `struct.pack`.

*`Float64BE.transcribe_value(value)`*:
Transcribe a value back into bytes.
- <a name="Float64LE"></a>`Class `Float64LE(Float64LE, AbstractBinary)`: An `AbstractBinary` `namedtuple` which parses and transcribes
  the struct format `'<d'` and presents the attributes ['value'].

*`Float64LE.parse(bfr: cs.buffer.CornuCopyBuffer)`*:
Parse from `bfr` via `struct.unpack`.

*`Float64LE.parse_value(bfr: cs.buffer.CornuCopyBuffer) -> float`*:
Parse a value from `