from b58check to hex and back I am manipulating (with pybitcointools) some regular addresses as well as some P2SH and stumbled upon the following:

Case 1:

&gt;&gt;&gt; b58check_to_hex('17rq8m5SW8XoZ9dHEkQyBfc5w2XQnQvznz')
'4b3d51202139ff109f6d10b637081f2de48d4b35'
&gt;&gt;&gt; hex_to_b58check('4b3d51202139ff109f6d10b637081f2de48d4b35')
'17rq8m5SW8XoZ9dHEkQyBfc5w2XQnQvznz'


second command is the inverse mapping of the first one 

Case 2: 

&gt;&gt;&gt; b58check_to_hex('3BRu7EhouApLkW1EZ64T9o9yMuX5Rexz6f')
'6ad55439d8faab476bbc0f89183ce689f8f6985b'
&gt;&gt;&gt; hex_to_b58check('6ad55439d8faab476bbc0f89183ce689f8f6985b')
'1AjtBhDNMGVxfLJoRzPrjAo3DPEMps6xEd'


second command is NOT the inverse mapping of the first one 

This surprises me a lot. Can someone give me some hints as to why this is the case please?

Thank you
The second case is a P2SH address. You must supply the version number/magic byte for the address, which is 5 for P2SH.

&gt;&gt;&gt; bitcoin.b58check_to_hex('3BRu7EhouApLkW1EZ64T9o9yMuX5Rexz6f')
'6ad55439d8faab476bbc0f89183ce689f8f6985b'
&gt;&gt;&gt; bitcoin.hex_to_b58check('6ad55439d8faab476bbc0f89183ce689f8f6985b', magicbyte=5)
'3BRu7EhouApLkW1EZ64T9o9yMuX5Rexz6f'


To figure out what the magic byte of an address is, use this:

&gt;&gt;&gt; bitcoin.get_version_byte('3BRu7EhouApLkW1EZ64T9o9yMuX5Rexz6f')
5