Dpkt是一个python包创建和解析库https://code.google.com/p/dpkt/ 该项目缺乏初学者的文档。我正在尝试记录它并为所有人制作示例示例代码。根据我对python的了解，我很难理解一些源代码。这里例如是python RTP（实时传输协议）模块
https://code.google.com/p/dpkt/source/browse/trunk/dpkt/rtp.py＃rtp.py源代码
# $Id$
"""Real-Time Transport Protocol"""
from dpkt import Packet
# version 1100 0000 0000 0000 ! 0xC000 14
# p 0010 0000 0000 0000 ! 0x2000 13
# x 0001 0000 0000 0000 ! 0x1000 12
# cc 0000 1111 0000 0000 ! 0x0F00 8
# m 0000 0000 1000 0000 ! 0x0080 7
# pt 0000 0000 0111 1111 ! 0x007F 0
#
_VERSION_MASK= 0xC000
_P_MASK = 0x2000
_X_MASK = 0x1000
_CC_MASK = 0x0F00
_M_MASK = 0x0080
_PT_MASK = 0x007F
_VERSION_SHIFT=14
_P_SHIFT = 13
_X_SHIFT = 12
_CC_SHIFT = 8
_M_SHIFT = 7
_PT_SHIFT = 0
VERSION = 2
class RTP(Packet):
__hdr__ = (
('_type', 'H', 0x8000),
('seq', 'H', 0),
('ts', 'I', 0),
('ssrc', 'I', 0),
)
csrc = ''
def _get_version(self): return (self._type&_VERSION_MASK)>>_VERSION_SHIFT
def _set_version(self, ver):
self._type = (ver << _VERSION_SHIFT) | (self._type & ~_VERSION_MASK)
def _get_p(self): return (self._type & _P_MASK) >> _P_SHIFT
def _set_p(self, p): self._type = (p << _P_SHIFT) | (self._type & ~_P_MASK)
def _get_x(self): return (self._type & _X_MASK) >> _X_SHIFT
def _set_x(self, x): self._type = (x << _X_SHIFT) | (self._type & ~_X_MASK)
def _get_cc(self): return (self._type & _CC_MASK) >> _CC_SHIFT
def _set_cc(self, cc): self._type = (cc<<_CC_SHIFT)|(self._type&~_CC_MASK)
def _get_m(self): return (self._type & _M_MASK) >> _M_SHIFT
def _set_m(self, m): self._type = (m << _M_SHIFT) | (self._type & ~_M_MASK)
def _get_pt(self): return (self._type & _PT_MASK) >> _PT_SHIFT
def _set_pt(self, m): self._type = (m << _PT_SHIFT)|(self._type&~_PT_MASK)
version = property(_get_version, _set_version)
p = property(_get_p, _set_p)
x = property(_get_x, _set_x)
cc = property(_get_cc, _set_cc)
m = property(_get_m, _set_m)
pt = property(_get_pt, _set_pt)
def __len__(self):
return self.__hdr_len__ + len(self.csrc) + len(self.data)
def __str__(self):
return self.pack_hdr() + self.csrc + str(self.data)
def unpack(self, buf):
super(RTP, self).unpack(buf)
self.csrc = buf[self.__hdr_len__:self.__hdr_len__ + self.cc * 4]
self.data = buf[self.__hdr_len__ + self.cc * 4:]
使用此代码，我能够在IPython Shell中执行以下操作
[37] import dpkt
[38] rtp_pkt=dpkt.rtp.RTP()
[39] rtp_pkt.pack_hdr()
Out[39]: '\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
[47] rtp_pkt.data="HelloWorld"
[48] rtp_pkt.pack()
Out[48]: '\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00HelloWorld'
基于我对Python中的类的理解，我在rtp.py中没有看到类中的“ init ”函数？我想知道上面的Ipython Shell命令是如何工作的？为什么rtp.py中的hdr变量以双下划线“__”开头，为什么每个类方法都以单下划线“_”开头。我知道这可能是私有或半私有但是它必须是那样吗？
我知道RTP类派生自Packet，为方便起见，它的源代码也粘贴在这里。
# $Id$
"""Simple packet creation and parsing."""
import copy, itertools, socket, struct
class Error(Exception): pass
class UnpackError(Error): pass
class NeedData(UnpackError): pass
class PackError(Error): pass
class _MetaPacket(type):
def __new__(cls, clsname, clsbases, clsdict):
t = type.__new__(cls, clsname, clsbases, clsdict)
st = getattr(t, '__hdr__', None)
if st is not None:
# XXX - __slots__ only created in __new__()
clsdict['__slots__'] = [ x[0] for x in st ] + [ 'data' ]
t = type.__new__(cls, clsname, clsbases, clsdict)
t.__hdr_fields__ = [ x[0] for x in st ]
t.__hdr_fmt__ = getattr(t, '__byte_order__', '>') + \
''.join([ x[1] for x in st ])
t.__hdr_len__ = struct.calcsize(t.__hdr_fmt__)
t.__hdr_defaults__ = dict(zip(
t.__hdr_fields__, [ x[2] for x in st ]))
return t
class Packet(object):
"""Base packet class, with metaclass magic to generate members from
self.__hdr__.
__hdr__ should be defined as a list of (name, structfmt, default) tuples
__byte_order__ can be set to override the default ('>')
Example::
>>> class Foo(Packet):
... __hdr__ = (('foo', 'I', 1), ('bar', 'H', 2), ('baz', '4s', 'quux'))
...
>>> foo = Foo(bar=3)
>>> foo
Foo(bar=3)
>>> str(foo)
'\x00\x00\x00\x01\x00\x03quux'
>>> foo.bar
3
>>> foo.baz
'quux'
>>> foo.foo = 7
>>> foo.baz = 'whee'
>>> foo
Foo(baz='whee', foo=7, bar=3)
>>> Foo('hello, world!')
Foo(baz=' wor', foo=1751477356L, bar=28460, data='ld!')
"""
__metaclass__ = _MetaPacket
def __init__(self, *args, **kwargs):
"""Packet constructor with ([buf], [field=val,...]) prototype.
Arguments:
buf -- optional packet buffer to unpack
Optional keyword arguments correspond to members to set
(matching fields in self.__hdr__, or 'data').
"""
self.data = ''
if args:
try:
self.unpack(args[0])
except struct.error:
if len(args[0]) < self.__hdr_len__:
raise NeedData
raise UnpackError('invalid %s: %r' %
(self.__class__.__name__, args[0]))
else:
for k in self.__hdr_fields__:
setattr(self, k, copy.copy(self.__hdr_defaults__[k]))
for k, v in kwargs.iteritems():
setattr(self, k, v)
def __len__(self):
return self.__hdr_len__ + len(self.data)
def __getitem__(self, k):
try: return getattr(self, k)
except AttributeError: raise KeyError
def __repr__(self):
l = [ '%s=%r' % (k, getattr(self, k))
for k in self.__hdr_defaults__
if getattr(self, k) != self.__hdr_defaults__[k] ]
if self.data:
l.append('data=%r' % self.data)
return '%s(%s)' % (self.__class__.__name__, ', '.join(l))
def __str__(self):
return self.pack_hdr() + str(self.data)
def pack_hdr(self):
"""Return packed header string."""
try:
return struct.pack(self.__hdr_fmt__,
*[ getattr(self, k) for k in self.__hdr_fields__ ])
except struct.error:
vals = []
for k in self.__hdr_fields__:
v = getattr(self, k)
if isinstance(v, tuple):
vals.extend(v)
else:
vals.append(v)
try:
return struct.pack(self.__hdr_fmt__, *vals)
except struct.error, e:
raise PackError(str(e))
def pack(self):
"""Return packed header + self.data string."""
return str(self)
def unpack(self, buf):
"""Unpack packet header fields from buf, and set self.data."""
for k, v in itertools.izip(self.__hdr_fields__,
struct.unpack(self.__hdr_fmt__, buf[:self.__hdr_len__])):
setattr(self, k, v)
self.data = buf[self.__hdr_len__:]
# XXX - ''.join([(len(`chr(x)`)==3) and chr(x) or '.' for x in range(256)])
__vis_filter = """................................ !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[.]^_`abcdefghijklmnopqrstuvwxyz{|}~................................................................................................................................."""
def hexdump(buf, length=16):
"""Return a hexdump output string of the given buffer."""
n = 0
res = []
while buf:
line, buf = buf[:length], buf[length:]
hexa = ' '.join(['%02x' % ord(x) for x in line])
line = line.translate(__vis_filter)
res.append(' %04d: %-*s %s' % (n, length * 3, hexa, line))
n += length
return '\n'.join(res)
try:
import dnet
def in_cksum_add(s, buf):
return dnet.ip_cksum_add(buf, s)
def in_cksum_done(s):
return socket.ntohs(dnet.ip_cksum_carry(s))
except ImportError:
import array
def in_cksum_add(s, buf):
n = len(buf)
cnt = (n / 2) * 2
a = array.array('H', buf[:cnt])
if cnt != n:
a.append(struct.unpack('H', buf[-1] + '\x00')[0])
return s + sum(a)
def in_cksum_done(s):
s = (s >> 16) + (s & 0xffff)
s += (s >> 16)
return socket.ntohs(~s & 0xffff)
def in_cksum(buf):
"""Return computed Internet checksum."""
return in_cksum_done(in_cksum_add(0, buf))
问题是如何真正理解源代码以使其文档正确完成？