A pure-python universal errors-and-erasures Reed-Solomon Codec , based on the wonderful tutorial at wikiversity, written by "Bobmath" and "LRQ3000".

The code of wikiversity is here consolidated into a nice API with exceptions handling. The algorithm can correct up to 2*e+v <= nsym, where e is the number of errors, v the number of erasures and nsym = n-k = the number of ECC (error correction code) symbols. This means that you can either correct exactly floor(nsym/2) errors, or nsym erasures (errors where you know the position), and a combination of both errors and erasures. The code should work on pretty much any reasonable version of python (2.4-3.2), but I'm only testing on 2.5 - 3.2.

The codec has quite reasonable performances if you either use PyPy on the pure-python implementation (reedsolo.py) or either if you compile the Cython extension creedsolo.py (which is about 2x faster than PyPy). You can expect encoding rate of several MB/s.

>>> rs = RSCodec(10)
>>> rs.encode([1,2,3,4])
b'\x01\x02\x03\x04,\x9d\x1c+=\xf8h\xfa\x98M'
>>> rs.encode(b'hello world')
b'hello world\xed%T\xc4\xfd\xfd\x89\xf3\xa8\xaa'
>>> rs.decode(b'hello world\xed%T\xc4\xfd\xfd\x89\xf3\xa8\xaa')
b'hello world'
>>> rs.decode(b'heXlo worXd\xed%T\xc4\xfdX\x89\xf3\xa8\xaa')     # 3 errors
b'hello world'
>>> rs.decode(b'hXXXo worXd\xed%T\xc4\xfdX\x89\xf3\xa8\xaa')     # 5 errors
b'hello world'
>>> rs.decode(b'hXXXo worXd\xed%T\xc4\xfdXX\xf3\xa8\xaa')        # 6 errors - fail
Traceback (most recent call last):
  ...
ReedSolomonError: Could not locate error

>>> rs = RSCodec(12)
>>> rs.encode(b'hello world')
b'hello world?Ay\xb2\xbc\xdc\x01q\xb9\xe3\xe2='
>>> rs.decode(b'hello worXXXXy\xb2XX\x01q\xb9\xe3\xe2=')         # 6 errors - ok
b'hello world'

If you want full control, you can skip the API and directly use the library as-is. Here's how:

First you need to init the precomputed tables:
>> init_tables(0x11d)
Pro tip: if you get the error: ValueError: byte must be in range(0, 256), please check that your prime polynomial is correct for your field.

Then to encode:
>> mesecc = rs_encode_msg(mes, n-k)

To decode:
>> mes, ecc = rs_correct_msg(mes + ecc, n-k, erase_pos=erase_pos)

If the decoding fails, it will normally automatically check and raise a ReedSolomonError exception that you can handle.
However if you want to manually check if the repaired message is correct, you can do so:
>> rsman.check(rmes + recc, k=k)

Read the sourcecode's comments for more infos about how it works, and for the various parameters you can setup if
you need to interface with other RS codecs.