Python: Sanitize a string for unicode?

Question

I have a string that I'm trying to make safe for the unicode() function:

>>> s = " foo “bar bar ” weasel"
>>> s.encode('utf-8', 'ignore')

Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    s.encode('utf-8', 'ignore')
UnicodeDecodeError: 'ascii' codec can't decode byte 0x93 in position 5: ordinal not in range(128)
>>> unicode(s)

Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    unicode(s)
UnicodeDecodeError: 'ascii' codec can't decode byte 0x93 in position 5: ordinal not in range(128)

I'm mostly flailing around here. What do I need to do to remove the unsafe characters from the string?

Somewhat related to this question, although I was unable to solve my problem from it.

This also fails:

>>> s
' foo \x93bar bar \x94 weasel'
>>> s.decode('utf-8')

Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    s.decode('utf-8')
  File "C:\Python25\254\lib\encodings\utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x93 in position 5: unexpected code byte

Answer 1

EDIT. Looks like your string is encoded in such a way that “ (LEFT DOUBLE QUOTATION MARK) becomes \x93 and ” (RIGHT DOUBLE QUOTATION MARK) becomes \x94. There is a number of codepages with such a mapping, CP1250 is one of them, so you may use this:

s = s.decode('cp1250')

For all the codepages which map “ to \x93 see here (all of them also map ” to \x94, which can be verified here).

Answer 2

Good question. Encoding issues are tricky. Let's start with "I have a string." Strings in Python 2.x aren't really "strings," they're byte arrays. So your string, where did it come from and what encoding is it in? Your example shows curly quotes in the literal, and I'm not even sure how you did that. I try to paste it into a Python interpreter, or type it on OS X with Option-[, and it doesn't come through.

Looking at your second example though, you have a character of hex 93. That can't be UTF-8, because in UTF-8, any byte higher than 127 is part of a multibyte sequence. So I'm guessing it's supposed to be Latin-1. The problem is, x93 isn't a character in the Latin-1 character set. There's this "invalid" range in Latin-1 from x7f to x9f that's considered illegal. However, Microsoft saw that unused range and decided to put "curly quotes" in there. In doing so they created this similar encoding called "windows-1252", which is like Latin-1 with stuff in that invalid range.

So, let's assume it is windows-1252. What now? String.decode converts bytes into Unicode, so that's the one you want. Your second example was on the right track, but it failed because the string wasn't UTF-8. Try:

>>> uni = 'foo \x93bar bar\x94 weasel'.decode("windows-1252")
u'foo \u201cbar bar\u201d weasel'
>>> print uni
foo “bar bar” weasel
>>> type(uni)
<type 'unicode'>

That's correct, because opening curly quote is Unicode U+201C. Now that you have Unicode, you can serialize it to bytes in any encoding you choose (if you need to pass it across the wire) or just keep it as Unicode if it's staying within Python. If you want to convert to UTF-8, use the oppose function, string.encode.

>>> uni.encode("utf-8")
'foo \xe2\x80\x9cbar bar \xe2\x80\x9d weasel'

Curly quotes take 3 bytes to encode in UTF-8. You could use UTF-16 and they'd only be two bytes. You can't encode as ASCII or Latin-1 though, because those don't have curly quotes.

Answer 3

Please check this answer to a related question: “Python UnicodeDecodeError - Am I misunderstanding encode?”