Decoding if it's not unicode

问题:

I want my function to take an argument that could be an unicode object or a utf-8 encoded string. Inside my function, I want to convert the argument to unicode. I have something like this:

def myfunction(text):
    if not isinstance(text, unicode):
        text = unicode(text, 'utf-8')

    ...

Is it possible to avoid the use of isinstance? I was looking for something more duck-typing friendly.

During my experiments with decoding, I have run into several weird behaviours of Python. For instance:

>>> u'hello'.decode('utf-8')
u'hello'
>>> u'cer\xf3n'.decode('utf-8')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/usr/lib/python2.6/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xf3' in po
sition 3: ordinal not in range(128)

Or

>>> u'hello'.decode('utf-8')
u'hello' 12:11
>>> unicode(u'hello', 'utf-8')
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: decoding Unicode is not supported

By the way. I'm using Python 2.6

回答1:

You could just try decoding it with the 'utf-8' codec, and if that does not work, then return the object.

def myfunction(text):
    try:
        text = unicode(text, 'utf-8')
    except TypeError:
        return text

print(myfunction(u'cer\xf3n'))
# cerón

When you take a unicode object and call its decode method with the 'utf-8' codec, Python first tries to convert the unicode object to a string object, and then it calls the string object's decode('utf-8') method.

Sometimes the conversion from unicode object to string object fails because Python2 uses the ascii codec by default.

So, in general, never try to decode unicode objects. Or, if you must try, trap it in a try..except block. There may be a few codecs for which decoding unicode objects works in Python2 (see below), but they have been removed in Python3.

See this Python bug ticket for an interesting discussion of the issue, and also Guido van Rossum's blog:

"We are adopting a slightly different approach to codecs: while in Python 2, codecs can accept either Unicode or 8-bits as input and produce either as output, in Py3k, encoding is always a translation from a Unicode (text) string to an array of bytes, and decoding always goes the opposite direction. This means that we had to drop a few codecs that don't fit in this model, for example rot13, base64 and bz2 (those conversions are still supported, just not through the encode/decode API)."

回答2:

I'm not aware of any good way to avoid the isinstance check in your function, but maybe someone else will be. I can point out that the two weirdnesses you cite are because you're doing something that doesn't make sense: Trying to decode into Unicode something that's already decoded into Unicode.

The first should instead look like this, which decodes the UTF-8 encoding of that string into the Unicode version:

>>> 'cer\xc3\xb3n'.decode('utf-8')
u'cer\xf3n'

And your second should look like this (not using a u'' Unicode string literal):

>>> unicode('hello', 'utf-8')
u'hello'