Re: Problem with sets and Unicode strings

Dennis Benzinger wrote:
Serge Orlov wrote:
On 6/27/06, Dennis Benzinger <Dennis.Benzinger@xxxxxxx> wrote:
Hi!

The following program in an UTF-8 encoded file:


# -*- coding: UTF-8 -*-

FIELDS = ("Fächer", )
FROZEN_FIELDS = frozenset(FIELDS)
FIELDS_SET = set(FIELDS)

print u"Fächer" in FROZEN_FIELDS
print u"Fächer" in FIELDS_SET
print u"Fächer" in FIELDS


gives this output


False
False
Traceback (most recent call last):
File "test.py", line 9, in ?
print u"FÀcher" in FIELDS
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 1:
ordinal not in range(128)


Why do the first two print statements succeed and the third one fails
with an exception?
Actually all three statements fail to produce correct result.

So this is a bug in Python?

No.

frozenset remove the exception?

Because sets use hash algorithm to find matches, whereas the last
statement directly compares a unicode string with a byte string. Byte
strings can only contain ascii characters, that's why python raises an
exception. The problem is very easy to fix: use unicode strings for
all non-ascii strings.

No, byte strings contain characters which are at least 8-bit wide <http://docs.python.org/ref/types.html>. But I don't understand what Python is trying to decode and why the exception says something about the ASCII codec, because my file is encoded with UTF-8.

Please read

http://www.amk.ca/python/howto/unicode

The string in all of the containers (FIELDS, FROZEN_FIELDS, FIELDS_SET) is a regular byte string, not a Unicode string. The encoding declaration only controls how the file is parsed. The string literal that you use for FIELDS is a regular string literal, not a Unicode string literal, so the object it creates is an 8-bit byte string. The tuple containment test is attempting to compare your Unicode string object to the regular string object for equality. Python does these comparisons by attempting to decode the regular string into a Unicode string. Since there is no encoding information present on regular strings at this point (since the encoding declaration in your file only controls parsing, nothing else), Python assumes ASCII and throws an exception otherwise.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

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