The Format Specification Language

bread reads binary data according to a format specification. Format specifications are just lists. Each element in the list is called a field descriptor. Field descriptors describe how to consume a piece of binary data from the input, usually to create a field in the resulting object. Field descriptors are consumed from the format specification one at a time until the entire list has been consumed.

Field Descriptors

Most field descriptors will consume a certain amount of binary data and produce a value of a certain basic type.


intX(num_bits, signed) - the next num_bits bits represent an integer. If signed is True, the integer is interpreted as a signed, twos-complement number.

For convenience and improved readability, the following shorthands are defined:

Field Descriptor Width (Bits) Signed
bit 1 no
semi_nibble 2 no
nibble 4 no
byte 8 no
uint8 8 no
uint16 16 no
uint32 32 no
uint64 64 no
int8 8 yes
int16 16 yes
int32 32 yes
int64 64 yes


string(length) - the next length bytes represent a string of the given length


boolean - the next bit represents a boolean value. 0 is False, 1 is True


enum(length, values, default=None) - the next length bits represent one of a set of values, whose values are given by the dictionary values. If default is specified, it will be returned if the bits do not correspond to any value in values. Otherwise, it raises a ValueError.

Here is an example of a 2-bit field representing a card suit:

import bread as b

("suit", b.enum(2, {
    0: "diamonds",
    1: "hearts",
    2: "spades",
    3: "clubs"


array(count, field_or_struct) - the next piece of data is count occurrences of field_or_struct which, as the name might imply, can be either a field (including another array) or a format specification.

Here’s an example way of representing a deck of playing cards:

import bread as b

# A card is made up of a 2-bit suit and a 4-bit card number

card = [
    ("suit", b.enum(2, {
        0: "diamonds",
        1: "hearts",
        2: "spades",
        3: "clubs"
    ("number", b.intX(4))]

# A deck consists of 52 cards, for a total of 312 bits or 39 bytes of data

deck = [("cards", b.array(52, card))]

Field Options

A dictionary of field options can be specified as the last argument to any field. A dictionary of global field options can also be defined at the beginning of the format spec (before any fields). Options defined on fields override these global options.

The following field options are defined:

  • str_format - the function that should be used to format a field in the structure’s human-readable representation. For example:

    >>> import bread as b
    # Format spec without str_format ...
    >>> simple_spec = [('addr', b.uint8)]
    >>> parsed_data = b.parse(bytearray([42]), simple_spec)
    >>> print parsed_data
    addr: 42
    # ... and with str_format
    >>> simple_spec_hex = [('addr', b.uint8, {"str_format": hex})]
    >>> parsed_data = b.parse(bytearray([42]), simple_spec_hex)
    >>> print parsed_data
    addr: 0x2a
  • endianness - for integer types, the endianness of the bytes that make up that integer. Can either be LITTLE_ENDIAN or BIG_ENDIAN. Default is little-endian.

    A simple example:

    endianness_test = [
      ("big_endian", b.uint32, {"endianness" : b.BIG_ENDIAN}),
      ("little_endian", b.uint32, {"endianness" : b.LITTLE_ENDIAN}),
      ("default_endian", b.uint32)]
    data = bytearray([0x01, 0x02, 0x03, 0x04] * 3)
    test = b.parse(data, endianness_test)
    >>> test.big_endian == 0x01020304
    >>> test.little_endian == 0x04030201
    >>> test.default_endian == test.little_endian
  • offset - for integer types, the amount to add to the number after it has been parsed. Specifying a negative number will subtract that amount from the number.


Conditionals allow the format specification to branch based on the value of a previous field. Conditional field descriptors are specified as follows:

(CONDITIONAL "field_name", options)

where field_name is the name of the field whose value determines the course of the conditional, and options is a dictionary giving format specifications to evaluate based on the field’s value.

This is perhaps best illustrated by example:

import bread as b

# There are three kinds of widgets: type A, type B and type C. Each has
# its own format spec.

widget_A = [...]
widget_B = [...]
widget_C = [...]

# A widget may be of any of the three types, determined by its type field

widget = [
    ("type", b.string(1)),
    (b.CONDITIONAL, "type", {
        "A": widget_A,
        "B": widget_B,
        "C": widget_C


padding(num_bits) - indicates that the next num_bits bits should be ignored. Useful in situations where only the first few bits of a byte are meaningful, or where the format skips multiple bits or bytes.