string - Python throws UnicodeEncodeError although I am doing str.decode(). Why?

Question

Consider this function:

def escape(text):
    print repr(text)
    escaped_chars = []
    for c in text:
        try:
            c = c.decode('ascii')
        except UnicodeDecodeError:
            c = '&{};'.format(htmlentitydefs.codepoint2name[ord(c)])
        escaped_chars.append(c)
    return ''.join(escaped_chars)

It should escape all non ascii characters by the corresponding htmlentitydefs. Unfortunately python throws

UnicodeEncodeError: 'ascii' codec can't encode character u'xe1' in position 0: ordinal not in range(128)

when the variable text contains the string whose repr() is u'Tamxe1s Horvxe1th'.

But, I don't use str.encode(). I only use str.decode(). Do I miss something?

Answer 1

It's a misleading error-report which comes from the way python handles the de/encoding process. You tried to decode an already decoded String a second time and that confuses the Python function which retaliates by confusing you in turn! ;-) The encoding/decoding process takes place as far as i know, by the codecs-module. And somewhere there lies the origin for this misleading Exception messages.

You may check for yourself: either

u'x80'.encode('ascii')

or

u'x80'.decode('ascii')

will throw a UnicodeEncodeError, where a

u'x80'.encode('utf8')

will not, but

u'x80'.decode('utf8')

again will!

I guess you are confused by the meaning of encoding and decoding. To put it simple:

                     decode             encode    
ByteString (ascii)  --------> UNICODE  --------->  ByteString (utf8)
            codec                                              codec

But why is there a codec-argument for the decode method? Well, the underlying function can not guess which codec the ByteString was encoded with, so as a hint it takes codec as an argument. If not provided it assumes you mean the sys.getdefaultencoding() to be implicitly used.

so when you use c.decode('ascii') you a) have a (encoded) ByteString (thats why you use decode) b) you want to get a unicode-representation-object (thats what you use decode for) and c) the codec in which the ByteString is encoded is ascii.

See also: https://stackoverflow.com/a/370199/1107807
http://docs.python.org/howto/unicode.html
http://www.cl.cam.ac.uk/~mgk25/unicode.html#utf-8
http://www.stereoplex.com/blog/python-unicode-and-unicodedecodeerror