In Python 2 the distinction between open() and codes.open() was clear because 'encoding' and 'errors' args were provided by codecs.open only.
This is no longer the case in Python 3 since both args are provided also by open().

I'm probably missing something but regardless I think codecs.open doc [1] should be more clear as to when and why (say) codecs.open(file, encoding='utf8', errors='ignore') should be preferred over open(file, encoding='utf8', errors='ignore').

[1] http://docs.python.org/3/library/codecs.html#codecs.open