Parsing a string from a BitStruct

[sigusr]

I am using the following code to parse incoming binary data.

from construct import BitStruct, BitField, Padding, String

struct = BitStruct("foo",
  BitField("bar", 8),
  BitField("baz", 16),
  Padding(4),
  BitField("bat", 4)
)

struct2 = BitStruct("foo",
  BitField("bar", 8),
  String("baz", 16),
  Padding(4),
  BitField("bat", 4)
)

data = "\x01AB\xCD"

print struct.parse(data)
print struct2.parse(data)

This prints the output:

Container:
    bar = 1
    baz = 16706
    bat = 13
Container:
    bar = 1
    baz = '\x00\x01\x00\x00\x00\x00\x00\x01\x00\x01\x00\x00\x00\x00\x01\x00'
    bat = 13

I was expecting that String would give me back AB as an actual string. However it is returning the equivalent binary string instead.
How can I persuade construct to return me the actual ASCII string?
Thanks.

[Chris Chow]

I had a problem with this recently, and I solved it by using something like 

String('magic', 3, encoding='ascii')

instead of 

String("baz", 16)

[sigusr]

Unfortunately that doesn't seem to work either.  With a length of 2 the following exception is raised:

 File "C:\Python27\lib\site-packages\construct\macros.py", line 354, in resizer

    raise SizeofError("size must be a multiple of 8", length)

construct.core.SizeofError: ('size must be a multiple of 8', 18)

This makes sense because it's wrapped in a BitStruct.

Using encoding="ascii" with a 16 bit length coerces the string to unicode but doesn't change anything else - it's still a string representation of binary values.

David

[Steven Michalske]

The BitStruct container works on bits.  I can see how the string representation of a sequence of bits is printed as that...  Not that I agree.  I would want your desired behavior.

You might need to use a union at the byte level for the string.

You might need to write a subclass of string to recompose the data.   Think 7bit ascii :-)

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[sigusr]

Thanks Steven - this pointed me along the right lines.  I ended up creating an adapter, a small successful test case looks like:

from binascii import hexlify
from construct import BitStruct, BitField, Padding, Array, Octet, Adapter

class BitStringAdapter(Adapter):
  def _encode(self, obj, context):
    return list(ord(b) for b in obj)
  def _decode(self, obj, context):
    return "".join(chr(b) for b in obj)

struct = BitStruct("foo",
  BitField("bar", 8),

  BitStringAdapter(Array(2, Octet("baz"))),

  Padding(4),
  BitField("bat", 4)
)

data = "\x01AB\xCD"

out = struct.parse(data)
print hexlify(struct.build(out))

I also documented this on Stackoverflow[1].

David

1 - http://stackoverflow.com/questions/33723833/obtaining-strings-from-bitstruct-in-pythons-construct-module/

[Arek Bulski]

Look at new Bytewise wrapper. Should do it for ya.