namedstruct

Define complicated C/C++ structs in Python with variable length arrays, header/extend relations, extensible structures in one time, parse/rebuild them with a single line of code

This library is designed for Openflow structures parsing in VLCP project. It is also used to parse GZIP header, HTTP2 frames, etc.

Some documents are hosted on readthedocs or pythonhosted.org. For a quick guide see Basic Tutorial.

Some examples are given in misc package, including a complete definition of Openflow 1.0, 1.3 and Nicira extensions. They are not part of the library but can be used freely. The Openflow structures are very complicated C/C++ structures, usually you will need more than 6000 lines of hard-coded, not extensible, uneasy to maintain Python code to parse them. With namedstruct, I just modified the openflow.h, nicira_ext.h headers with some rules and created the types in several days. I even keep the license and remarks unchanged.

Another example parses GZIP file header, which is a little-endian format. It is used in HTTP implementation in VLCP to extract the deflate data from a GZIP-encoded content without creating a in-memory file.

The project name in PyPI is nstruct. Install with PyPI::

pip install nstruct

This project supports both Python2.x and Python3.x.

Some quick examples:

###### BASIC STRUCT #####

from namedstruct import *
mytype = nstruct((uint16, 'myshort'),  # unsigned short int
                (uint8, 'mybyte'),       # unsigned char
                (uint8,),               # a padding byte of unsigned char
                (char[5], 'mystr'),   # a 16-byte bytes string
                (uint8,),
                (uint16[5], 'myarray'),    # 
                name = 'mytype',
                padding = 1)
# Create an object
obj0 = mytype()
# Access a field
s = obj0.myshort
obj0.myshort = 12
# Create an object with the specified fields initialized
obj1 = mytype(myshort = 2, mystr = b'123', myarray = [1,2,3,4,5]) 
# Unpack an object from stream, return the object and bytes size used
obj2,size = mytype.parse(b'\x00\x02\x01\x00123\x00\x00\x00\x00\x01\x00\x02\x00\x03\x00\x04\x00\x05')
# Unpack an object from packed bytes
obj3 = mytype.create(b'\x00\x02\x01\x00123\x00\x00\x00\x00\x01\x00\x02\x00\x03\x00\x04\x00\x05')
# Estimate the struct size
size = len(obj0)
# Estimate the struct size excludes automatic padding bytes
size2 = obj0._realsize()
# Pack the object
b = obj0._tobytes()
b2 = mytype.tobytes(obj0)

# Use the type in other structs

mytype2 = nstruct((mytype, 'mydata'),
                (mytype[4], 'mystructarray'),
                name = 'mytype2',
                padding = 1)

obj4 = mytype2()
obj4.mydata.myshort = 12
obj4.mystructarray[0].mybyte = 7
b3 = obj4._tobytes()

###### VARIABLE LENGTH TYPES #####

my_unsize_struct = nstruct((uint16, 'length'),
                        (raw, 'data'),
                        padding = 1,
                        name = 'my_unsize_struct')

"""
>>> my_unsize_struct.create(b'\x00\x07abcde').data
b'abcde'
>>> my_unsize_struct.parse(b'\x00\x07abcde')[0].data
b''
"""

my_size_struct = nstruct((uint16, 'length'),
                        (raw, 'data'),
                        padding = 1,
                        name = 'my_size_struct',
                        prepack = packrealsize('length'),
                        size = lambda x: x.length)
"""
packrealsize('length') is equivalent to:
    
    def _packsize(x):
        x.length = x._realsize()
"""

"""
>>> my_unsize_struct(data = b'abcde')._tobytes()
b'\x00\x07abcde'
>>> my_unsize_struct.parse(b'\x00\x07abcde')[0].data
b'abcde'
"""

##### EXTENDING #####

my_base = nstruct((uint16, 'length'),
                 (uint8, 'type'),
                 (uint8, 'basedata'),
                 name = 'my_base',
                 size = lambda x: x.length,
                 prepack = packrealsize('length'),
                 padding = 4)

my_child1 = nstruct((uint16, 'data1'),
                    (uint8, 'data2'),
                    name = 'my_child1',
                    base = my_base,
                    criteria = lambda x: x.type == 1,
                    init = packvalue(1, 'type'))

my_child2 = nstruct((uint32, 'data3'),
                   name = 'my_child2',
                   base = my_base,
                   criteria = lambda x: x.type == 2,
                   init = packvalue(2, 'type'))

"""
Fields and most base class options are inherited, e.g. size, prepack, padding
>>> my_child1(basedata = 1, data1 = 2, data2 = 3)._tobytes()
b'\x00\x07\x01\x01\x00\x02\x03\x00'
>>> my_child2(basedata = 1, data3 = 4)._tobytes()
b'\x00\x08\x02\x01\x00\x00\x00\x04'
"""

# Fields in child classes are automatically parsed when the type is determined
obj1, _ = my_base.parse(b'\x00\x07\x01\x01\x00\x02\x03\x00')
"""
>>> obj1.basedata
1
>>> obj1.data1
2
>>> obj1.data2
3
>>> obj1._gettype()
my_child1
"""

# Base type can be used in fields or arrays of other structs

my_base_array = nstruct((uint16, 'total_len'),
                       (my_base[0], 'array'),
                       name = 'my_base_array',
                       padding = 1,
                       size = lambda x: x.total_len,
                       prepack = packrealsize('total_len'))

obj2 = my_base_array()
obj2.array.append(my_child1(data1 = 1, data2 = 2, basedata = 3))
obj2.array.append(my_child2(data3 = 4, basedata = 5))
"""
>>> obj2._tobytes()
b'\x00\x12\x00\x07\x01\x03\x00\x01\x02\x00\x00\x08\x02\x05\x00\x00\x00\x04'
"""
obj3, _ = my_base_array.parse(b'\x00\x12\x00\x07\x01\x03\x00\x01\x02\x00\x00\x08\x02\x05\x00\x00\x00\x04')
"""
>>> obj3.array[0].data2
2
"""

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