Armin Ronacher's Thoughts and Writings

Codec Confusion in Python

written on Saturday, August 11, 2012

Alright, I admit Alex Gaynor is a pretty clever guy but I was very close to strangling him today for this tweet:

@alex_gaynor: WTF does str.encode or unicode.decode even do on Python2?

And that's because on the way to Python 3 these functions were removed because they cause confusion with people, but this broke a lot of really good use cases for them in the process. On top of that I truly believe the mere presence of these function did not actually cause confusion, but an unintended side effect did. And all in all I believe that's a really sad example of where wrong conclusions were drawn.

One thing I believe is pretty true is that you should never ask people about their opinions directly. You should observe them, figure out what's wrong and then slowly figure out where the true problem lies. In this particular case it seems like many people missed the true problem and stopped noticing that the true solution was much simpler. I can't blame anyone for that and I did not notice it either until the damage was already done.

So what do str.encode and str.decode actually do in Python 2.x? They are roughly implemented like this:

import codecs

class basestring(object):
    ...
    def encode(self, encoding, errors='strict'):
        rv = codecs.lookup(encoding).encode(self, errors)[0]
        if not isinstance(rv, basestring):
            raise TypeError('encoder did not return a string/unicode object')
        return rv

    def decode(self, encoding, errors='strict'):
        rv = codecs.lookup(encoding).decode(self, errors)[0]
        if not isinstance(rv, basestring):
            raise TypeError('encoder did not return a string/unicode object')
        return rv

class str(basestring):
    ...

class unicode(basestring):
    ...

Alright, so they are just shortcuts for functionality hidden in another module. One important thing of note here is that this is talking about encodings, not about charsets. Very important difference!

The most important part here is that codecs in Python are not restricted to strings at all. If you look into the original Python sources you will see that the codecs module talks about objects and not strings.

So what is actually the confusing part? The confusing part is not the codecs API but the Python default encoding. Let me show you the problem on a simple example. Alex was arguing against str.encode because of this:

>>> '\xe2\x98\x83'.encode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)

What's happening here? The call did not make any sense. The utf-8 codec encodes from Unicode to bytes, not from bytes to bytes like in the given example. Now that might let you believe the correct solution is just to get rid of the encode function on strings. However there are legitimate cases for string to string encodings:

>>> 'foo'.encode('base64')
'Zm9v\n'

Alright, so we want to preserve that. So why exactly is the first example confusing anyways? I mean, it gives an exception. The problem is the wording of the exception. Where is the ascii codec coming from all the sudden? That's actually coming from something completely unrelated and that is Python's default encoding which caused us all the problems in the first place. What's happening in the above code is that the codec function is written in a way that it looks at the incoming object and it sees that it expects an unicode object but it got a bytestring object. As the next step that function takes that bytestring object and asks the interpreter state what it should do with that. The interpreter has a special setting which defines the default encoding. In Python 2.x this historically has been set to ascii. Now the function will ask the ascii codec to decode the string to Unicode. Because the string did not fit into ASCII range it will error out with that horrible error message.

Not only is that error message misleading, it also does not show up at all if the string does indeed fit into ascii:

>>> 'foo'.encode('utf-8')
'foo'

There it does foo (bytes) -> ascii decode -> foo (Unicode) -> utf-8 encode -> foo (bytes).

Now let me blow your mind: this was actually envisioned when the module was created initially. You can in fact still take a stock Python 2.x interpreter and disable that behavior:

>>> import sys
>>> reload(sys)
<module 'sys' (built-in)>
>>> sys.setdefaultencoding('undefined')

>>> '\xe2\x98\x83'.encode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeError: undefined encoding
>>> 'foo'.encode('utf-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeError: undefined encoding
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeError: undefined encoding

(The reload on sys is necessary because after site.py did it's job there is no way to change the default encoding any more).

So there you have it. If we would have just never started doing the implicit ASCII codec we would have solved so much confusion early on and everything would have been more explicit. When going to Python 3 all we would have had to do was to add a b prefix for bytestrings and made the u implied. And we would not now end up with inferior codec support in Python 3 because the byte to byte and Unicode to Unicode codecs were removed.

This entry was tagged codecs, python and thoughts