Reading the fileset from a torrent - parsing

Question

I want to (quickly) put a program/script together to read the fileset from a .torrent file. I want to then use that set to delete any files from a specific directory that do not belong to the torrent.
Any recommendations on a handy library for reading this index from the .torrent file? Whilst I don't object to it, I don't want to be digging deep into the bittorrent spec and rolling a load of code from scratch for this simple purpose.
I have no preference on language.

Answer 1

I would use rasterbar's libtorrent which is a small and fast C++ library.
To iterate over the files you could use the torrent_info class (begin_files(), end_files()).
There's also a python interface for libtorrent:
import libtorrent
info = libtorrent.torrent_info('test.torrent')
for f in info.files():
print "%s - %s" % (f.path, f.size)

Answer 2

Effbot has your question answered. Here is the complete code to read the list of files from .torrent file (Python 2.4+):
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"])

Answer 3

Here's the code from Constantine's answer above, slightly modified to handle Unicode characters in torrent filenames and fileset filenames in torrent info:
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
n = 0
if __name__ == "__main__":
data = open("C:\\Torrents\\test.torrent", "rb").read()
torrent = decode(data)
for file in torrent["info"]["files"]:
n = n + 1
filenamepath = file["path"]
print str(n) + " -- " + ', '.join(map(str, filenamepath))
fname = ', '.join(map(str, filenamepath))
print fname + " -- " + str(file["length"])

Answer 4

bencode.py from the original Mainline BitTorrent 5.x client (http://download.bittorrent.com/dl/BitTorrent-5.2.2.tar.gz) would give you pretty much the reference implementation in Python.
It has an import dependency on the BTL package but that's trivially easy to remove. You'd then look at bencode.bdecode(filecontent)['info']['files'].

Answer 5

Expanding on the ideas above, I did the following:
~> cd ~/bin
~/bin> ls torrent*
torrent-parse.py torrent-parse.sh
~/bin> cat torrent-parse.py
# torrent-parse.py
import sys
import libtorrent
# get the input torrent file
if (len(sys.argv) > 1):
torrent = sys.argv[1]
else:
print "Missing param: torrent filename"
sys.exit()
# get names of files in the torrent file
info = libtorrent.torrent_info(torrent);
for f in info.files():
print "%s - %s" % (f.path, f.size)
~/bin> cat torrent-parse.sh
#!/bin/bash
if [ $# -lt 1 ]; then
echo "Missing param: torrent filename"
exit 0
fi
python torrent-parse.py "$*"
You'll want to set permissions appropriately to make the shell script executable:
~/bin> chmod a+x torrent-parse.sh
Hope this helps someone :)