Given a .torrent file how do I generate a magnet link in python?
Question: I need a way to convert .torrents into magnet links. Would like a way to do so
in python. Are there any libraries that already do this?


Answer: You can do this with the [bencode](http://pypi.python.org/pypi/bencode/1.0)
module, extracted from BitTorrent.

To show you an example, I downloaded a torrent ISO of Ubuntu from here:

    
    
    http://releases.ubuntu.com/12.04/ubuntu-12.04.1-desktop-i386.iso.torrent
    

Then, you can parse it in Python like this:

    
    
    >>> import bencode
    >>> torrent = open('ubuntu-12.04.1-desktop-i386.iso.torrent', 'r').read()
    >>> metadata = bencode.bdecode(torrent)
    

A magnet hash is calculated from only the "info" section of the torrent
metadata and then encoded in base32, like this:

    
    
    >>> hashcontents = bencode.bencode(metadata['info'])
    >>> import hashlib
    >>> digest = hashlib.sha1(hashcontents).digest()
    >>> import base64
    >>> b32hash = base64.b32encode(digest)
    >>> b32hash
    'CT76LXJDDCH5LS2TUHKH6EUJ3NYKX4Y6'
    

You can verify that this is correct by looking
[here](http://www.linux23.com/torrent/ubuntu-12-04-1-desktop-i386-iso:14ffe5dd23188fd5cb53a1d47f1289db70abf31e)
and you will see the magnet link is:

    
    
    magnet:?xt=urn:btih:CT76LXJDDCH5LS2TUHKH6EUJ3NYKX4Y6
    

If you want to fill in some extra parameters to the magnet URI:

    
    
    >>> params = {'xt': 'urn:btih:%s' % b32hash,
    ...           'dn': metadata['info']['name'],
    ...           'tr': metadata['announce'],
    ...           'xl': metadata['info']['length']}
    >>> import urllib
    >>> paramstr = urllib.urlencode(params)
    >>> magneturi = 'magnet:?%s' % paramstr
    >>> magneturi
    'magnet:?dn=ubuntu-12.04.1-desktop-i386.iso&tr=http%3A%2F%2Ftorrent.ubuntu.com%3A6969%2Fannounce&xl=729067520&xt=urn%3Abtih%3ACT76LXJDDCH5LS2TUHKH6EUJ3NYKX4Y6'
    

