EStruct 0.1.3

Python package to extend the capabilities of the standard library's Struct package.

EStruct provides an extended packaging/unpackaging library that that provided by the standard library’s Struct module. It provides packaging of conditional elements and arrays. The library was originally written by the author to simplify packing and unpacking binary data within mpeg4 file.

Typical usage:

#!/usr/bin/env python

from estruct.estruct import EStruct
import sys
from datetime import date

packager = EStruct("Student", "Student.Name Student.Address Student.DOB", "![50s50sQ]")
records=[]
with open(sys.argv[1],"rb") as f:
        records=packager.unpack(f.read())

for r in records:
        print r.Student.Name, r.Student.Address, date.fromordinal(r.Student.DOB)

EStruct is also hosted on GitHub at https://github.com/simblack/EStruct

Installation

pip install EStruct

EStruct object

The EStruct object __init__ method takes three parameters, the unpacked class name, the fields within the class and the packaging format.

EStruct.__init__(self, name, fields_string, format_string)

name

This is simply the name of the object that is created when unpacking values.

fields_string

The fields string is a description that contains fields and sub fields. The string needs to be a space separated list of fields. Nested structures can be created by using a dot notation ‘.’, i.e. object.field or object.sub_object.field.

Example:

'f1 f2 f3.a1 f3.a2 f4 f5.a1.b1 f5.a1.b2 f5.a2.b1 f5.a2.b2'
This would create the object with fields:
    f1,f2,f3,f4,f5
f3 is of a type of an object with fields:
    a1, a2
f5 is of a type of an object with fields:
    a1, a2 with both fields of an object of type with fields b1 and b2

format_string

format= byteorder + format_string
byte_order= '@'|'='|'<'|'>'|'!'
    @ native byte-order    native size        native alignment
    = native byte-order    standard size      no alignment
    < little endian        standard size      no alignment
    > big endian           standard size      no alignment
    ! network              standard size      no alignment

format_string=      conditonal_format | array_format | normal_format | empty_format

emptry_format=

conditionl_format=  '(' + condition + '?' + true_format + ':' + false_format + ')'
condition=          [python evaluation]
true_format=        format_string
false_format=       format_string
An empty format_string implies that no field is required for packinging.

array_format=       count[format_string]

count=              empty_count | constant_count | field_count
empty_count=
constant_count=    (0..9)*
field_count=       '{' + [field_name] + '}'

The [field name] is any field defined prior to this packing element. The [python evaluation] is a valid python statement equating to a True or False result. Any field name prior to this packing element may be used. Also for evaulation of array’s the index value INDEX may be used to specify the current index.

Examples

Network endian, 32 bit integers followed by a 64 but value if f1==1 else another 32 bit integer:

!II(f1==1?Q|I)

Network endian, 32 bit integers followed by an array with the number of elements as specified in the field f1. The array elements are a 32 bit integer followed by a number of characters, where that number is set in the field f2[INDEX].a1, the index is the current iteration of the outer array.:

!I{f1}[I{f2[INDEX].a1}[c]]

Unpacking

The unpack method is used to unpack binary data into an object

EStruct.unpack(self, data, **kargs)

Keyword arguments may also be passed if they are required for evaluations. The method returns an object as described by the fields string of the constructor.

struct=EStruct('Test','f1 f2','!I{f1}[I]')

data=[0,0,0,3,0,0,0,2,0,0,0,1,0,0,0,0]
data_str="".join([chr(x) for x in data])
obj=struct.unpack(data_str)

print obj.f1, obj.f2

Packing

The pack method is used to pack values into a binary string

EStruct.pack(self, *args, **kargs)

The correct number of arguments must be passed for packing, according to the packing format. Keyword arguments may also be passed if they are required for evaluations. The method returns the packed binary string.

struct=EStruct('Test','f1 f2','!I{f1}[I]')
packed_result=self.struct.pack(3,[2,1,0])

print packed_result.encode("hex")