一个简单的 BitTorrent "bencode" 解码器

原文来自于 这里

这是一个 BitTorrent Torrent 编码格式 bencode 的简单解码器。它使用了来自于 Simple Iterator-based Parsing 基于迭代器的方法，从而产生了可读且高效的代码。

import re


def tokenize(text, match=re.compile("([idel])|(\d+):|(-?\d+)").match):
    i = 0
    while i < len(text):
        m = match(text, i)
        s = m.group(m.lastindex)
        i = m.end()
        if m.lastindex == 2:
            yield "s"
            yield text[i:i+int(s)]
            i = i + int(s)
        else:
            yield s


def decode_item(next, token):
    if token == "i":
        # integer: "i" value "e"
        data = int(next())
        if next() != "e":
            raise ValueError
    elif token == "s":
        # string: "s" value (virtual tokens)
        data = next()
    elif token == "l" or token == "d":
        # container: "l" (or "d") values "e"
        data = []
        tok = next()
        while tok != "e":
            data.append(decode_item(next, tok))
            tok = next()
        if token == "d":
            data = dict(zip(data[0::2], data[1::2]))
    else:
        raise ValueError
    return data


def decode(text):
    try:
        src = tokenize(text)
        data = decode_item(src.next, src.next())
        for token in src: # look for more tokens
            raise SyntaxError("trailing junk")
    except (AttributeError, ValueError, StopIteration):
        raise SyntaxError("syntax error")
    return data


if __name__ == '__main__':
    data = open("test.torrent", "rb").read()

    torrent = decode(data)

    for file in torrent["info"]["files"]:
        print "%r - %d bytes" % ("/".join(file["path"]), file["length"])

大多数编码对象由类型代码，对象值和结束代码组成；而例外是编码字符串，其前缀为长度。为了简化解析器，tokenize 函数将字符串作为两个“虚拟”令牌返回；一个类型代码，后跟着字符串值。一些例子：

>>> list(tokenize("4:spam"))
['s', 'spam']
>>> list(tokenize("i3e")) # int
['i', '3', 'e']
>>> list(tokenize("i-3e"))
['i', '-3', 'e']
>>> list(tokenize("l4:spam4:eggse")) # list
['l', 's', 'spam', 's', 'eggs', 'e']
>>> list(tokenize("d3:cow3:moo4:spam4:eggse")) # dict
['d', 's', 'cow', 's', 'moo', 's', 'spam', 's', 'eggs', 'e']

请注意，越简单的格式通常可以使用 finditer 方法进行拆分（或甚至于使用 findall ）。

encode_item 函数使用类型信息将数据转换为合适的 Python 对象。 它调用自身来处理嵌套的容器类型。

使用方法

这是一个代码清单，列出了种子文件中包含的所有文件：

data = open("test.torrent", "rb").read()

torrent = decode(data)

for file in torrent["info"]["files"]:
    print "%r - %d bytes" % ("/".join(file["path"]), file["length"])

运行此命令将产生类似以下内容：

'Little Earthquakes/01 Crucify.m4a' - 4845721 bytes
'Little Earthquakes/02 Girl.m4a' - 4012517 bytes
'Little Earthquakes/03 Silent All These Years.m4a' - 4076790 bytes
'Little Earthquakes/04 Precious Things.m4a' - 4328948 bytes
'Little Earthquakes/05 Winter.m4a' - 5538530 bytes
'Little Earthquakes/06 Happy Phantom.m4a' - 3204091 bytes
'Little Earthquakes/07 China.m4a' - 4859246 bytes
'Little Earthquakes/08 Leather.m4a' - 3125716 bytes
'Little Earthquakes/09 Mother.m4a' - 6785591 bytes
'Little Earthquakes/10 Tear In Your Hand.m4a' - 4515482 bytes
'Little Earthquakes/11 Me And A Gun.m4a' - 3649914 bytes
'Little Earthquakes/12 Little Earthquakes.m4a' - 6663794 bytes