FAQ
Hi!

The following program in an UTF-8 encoded file:


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

FIELDS = ("F?cher", )
FROZEN_FIELDS = frozenset(FIELDS)
FIELDS_SET = set(FIELDS)

print u"F?cher" in FROZEN_FIELDS
print u"F?cher" in FIELDS_SET
print u"F?cher" in FIELDS


gives this output


False
False
Traceback (most recent call last):
File "test.py", line 9, in ?
print u"F??cher" 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?

Why does the use of set/frozenset remove the exception?


Thanks,
Dennis

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  • Serge Orlov at Jun 27, 2006 at 8:05 pm

    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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.
    Why does the use of set/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.
  • Dennis Benzinger at Jun 27, 2006 at 9:04 pm

    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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?
    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.


    Dennis
  • Serge Orlov at Jun 27, 2006 at 9:59 pm

    On 6/27/06, Dennis Benzinger wrote:
    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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>.
    Yes, but later it's written that non-ascii characters do not have
    universal meaning assigned to them. In other words if you put byte
    0xE4 into a bytes string all python knows about it is that it's *some*
    character. If you put character U+00E4 into a unicode string python
    knows it's a "latin small letter a with diaeresis". Trying to compare
    *some* character with a specific character is obviously undefined.
    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.
    Because byte strings can come from different sources (network, files,
    etc) not only from the sources of your program python cannot assume
    all of them are utf-8. It assumes they are ascii, because most of
    wide-spread text encodings are ascii bases. Actually it's a guess,
    since there are utf-16, utf-32 and other non-ascii encodings. If you
    want to experience the life without guesses put
    sys.setdefaultencoding("undefined") into site.py
  • Robert Kern at Jun 27, 2006 at 10:05 pm

    Dennis Benzinger wrote:
    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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
  • Laurent Pointal at Jun 28, 2006 at 8:02 am

    Dennis Benzinger a ?crit :
    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.
    [addendum to others replies]

    The file encoding directive is used by Python to convert u"xxx" strings
    into unicode objects using right conversion rules when compiling the code.
    When a string is written simply with "xxx", its a 8 bits string with NO
    encoding data associated. When these strings must be converted they are
    considered to be using sys.getdefaultencoding() [generally ascii -
    forced ascii in python 2.5]

    So a short reply: the utf8 directive has no effect on 8 bits strings,
    use unicode strings to manage correctly non-ascii texts.

    A+

    Laurent.
  • Dennis Benzinger at Jun 28, 2006 at 4:40 pm

    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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>.
    Yes, but later it's written that non-ascii characters do not have
    universal meaning assigned to them. In other words if you put byte
    0xE4 into a bytes string all python knows about it is that it's *some*
    character. If you put character U+00E4 into a unicode string python
    knows it's a "latin small letter a with diaeresis". Trying to compare
    *some* character with a specific character is obviously undefined.
    [...]
    But <http://docs.python.org/ref/comparisons.html> says:

    Strings are compared lexicographically using the numeric equivalents
    (the result of the built-in function ord()) of their characters. Unicode
    and 8-bit strings are fully interoperable in this behavior.

    Doesn't this mean that Unicode and 8-bit strings can be compared and
    this comparison is well defined? (even if it's is not meaningful)



    Thanks for your anwsers,
    Dennis
  • Diez B. Roggisch at Jun 28, 2006 at 4:59 pm

    But <http://docs.python.org/ref/comparisons.html> says:

    Strings are compared lexicographically using the numeric equivalents
    (the result of the built-in function ord()) of their characters. Unicode
    and 8-bit strings are fully interoperable in this behavior.

    Doesn't this mean that Unicode and 8-bit strings can be compared and
    this comparison is well defined? (even if it's is not meaningful)
    Obviously not - otherwise you wouldn't have the problems you'd observed,
    wouldn't you?

    What happens of course is that in case of string to unicode-comparison, the
    string gets coerced to an unicode value - using the default encoding!


    # -*- coding: latin1 -*-

    print "?".decode("latin1") == u"?"
    print "?" == u"?"



    So - they are fully interoperable and the comparison is well defined - when
    the coercion is successful.

    Diez
  • Dennis Benzinger at Jun 28, 2006 at 4:48 pm

    Robert Kern wrote:
    Dennis Benzinger wrote:
    Serge Orlov wrote:
    On 6/27/06, Dennis Benzinger wrote:
    Hi!

    The following program in an UTF-8 encoded file:


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

    FIELDS = ("F?cher", )
    FROZEN_FIELDS = frozenset(FIELDS)
    FIELDS_SET = set(FIELDS)

    print u"F?cher" in FROZEN_FIELDS
    print u"F?cher" in FIELDS_SET
    print u"F?cher" in FIELDS


    gives this output


    False
    False
    Traceback (most recent call last):
    File "test.py", line 9, in ?
    print u"F??cher" 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.
    [...]
    But I'd say that it's not intuitive that for sets x in y can be false
    (without raising an exception!) while the doing the same with a tuple
    raises an exception. Where is this difference documented?


    Thanks,
    Dennis
  • Diez B. Roggisch at Jun 28, 2006 at 5:04 pm

    But I'd say that it's not intuitive that for sets x in y can be false
    (without raising an exception!) while the doing the same with a tuple
    raises an exception. Where is this difference documented?
    2.3.7 Set Types -- set, frozenset

    ...

    Set elements are like dictionary keys; they need to define both __hash__ and
    __eq__ methods.
    ...

    And it has to hold that

    a == b => hash(a) == hash(b)

    but NOT

    hash(a) == hash(b) => a == b

    Thus if the hashes vary, the set doesn't bother to actually compare the
    values.

    Diez
  • Dennis Benzinger at Jun 29, 2006 at 6:38 pm

    Diez B. Roggisch wrote:
    But I'd say that it's not intuitive that for sets x in y can be false
    (without raising an exception!) while the doing the same with a tuple
    raises an exception. Where is this difference documented?
    2.3.7 Set Types -- set, frozenset

    ...

    Set elements are like dictionary keys; they need to define both __hash__ and
    __eq__ methods.
    ...

    And it has to hold that

    a == b => hash(a) == hash(b)

    but NOT

    hash(a) == hash(b) => a == b

    Thus if the hashes vary, the set doesn't bother to actually compare the
    values.
    [...]
    Ok, I understand.
    But isn't it a (minor) problem that using a set like this:

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

    FIELDS_SET = set(("F?cher", ))


    print u"F?cher" in FIELDS_SET
    print u"F?cher" == "F?cher"


    shadows the error of not setting sys.defaultencoding()?


    Dennis
  • Robert Kern at Jun 29, 2006 at 7:00 pm

    Dennis Benzinger wrote:
    Ok, I understand.
    But isn't it a (minor) problem that using a set like this:

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

    FIELDS_SET = set(("F?cher", ))

    print u"F?cher" in FIELDS_SET
    print u"F?cher" == "F?cher"

    shadows the error of not setting sys.defaultencoding()?
    You can't set the default encoding. If you could, then scripts that run on your
    machine wouldn't run on mine.

    If there's an error, it's the fact that you use a regular string at the
    beginning ("F?cher") and a unicode string later (u"F?cher"). But set objects
    can't know that that's the problem or even if it *is* a problem.

    --
    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
  • Dennis Benzinger at Jun 29, 2006 at 7:19 pm

    Robert Kern wrote:
    Dennis Benzinger wrote:
    Ok, I understand.
    But isn't it a (minor) problem that using a set like this:

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

    FIELDS_SET = set(("F?cher", ))

    print u"F?cher" in FIELDS_SET
    print u"F?cher" == "F?cher"

    shadows the error of not setting sys.defaultencoding()?
    You can't set the default encoding. If you could, then scripts that run
    on your machine wouldn't run on mine.
    [...]
    As Serge Orlov wrote in one of his posts you _can_ set the default
    encoding (at least in site.py). See
    <http://docs.python.org/lib/module-sys.html>


    Bye,
    Dennis
  • Robert Kern at Jun 29, 2006 at 7:46 pm

    Dennis Benzinger wrote:
    Robert Kern wrote:
    Dennis Benzinger wrote:
    Ok, I understand.
    But isn't it a (minor) problem that using a set like this:

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

    FIELDS_SET = set(("F?cher", ))

    print u"F?cher" in FIELDS_SET
    print u"F?cher" == "F?cher"

    shadows the error of not setting sys.defaultencoding()?
    You can't set the default encoding. If you could, then scripts that run
    on your machine wouldn't run on mine.
    [...]
    As Serge Orlov wrote in one of his posts you _can_ set the default
    encoding (at least in site.py). See
    <http://docs.python.org/lib/module-sys.html>
    Okay, *don't* set the default encoding to anything other than 'ascii'. Doing so
    would be an error, not the other way around.

    --
    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
  • Jean-Paul Calderone at Jun 29, 2006 at 7:50 pm

    On Thu, 29 Jun 2006 21:19:30 +0200, Dennis Benzinger wrote:
    Robert Kern wrote:
    Dennis Benzinger wrote:
    Ok, I understand.
    But isn't it a (minor) problem that using a set like this:

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

    FIELDS_SET = set(("F?cher", ))

    print u"F?cher" in FIELDS_SET
    print u"F?cher" == "F?cher"

    shadows the error of not setting sys.defaultencoding()?
    You can't set the default encoding. If you could, then scripts that run
    on your machine wouldn't run on mine.
    [...]
    As Serge Orlov wrote in one of his posts you _can_ set the default
    encoding (at least in site.py). See
    <http://docs.python.org/lib/module-sys.html>
    But doing so is not useful so one should generally never do it. You
    cannot set the default encoding on any computers you don't directly
    control, so any software you write which depends on this will not be
    easily distributable. Additionally, if you decide to use two packages
    which use this feature and go to the trouble of modifying your own
    site.py for them, you won't be able to, since there can only be one
    default system encoding. Only one will be able to work at a time.

    The default encoding is ascii and should always be ascii. If you want
    another encoding, specify it in a call to .encode() or .decode().

    Jean-Paul
  • Fredrik Lundh at Jun 29, 2006 at 9:02 pm

    Dennis Benzinger wrote:

    shadows the error of not setting sys.defaultencoding()?
    You can't set the default encoding. If you could, then scripts that run
    on your machine wouldn't run on mine.
    [...]
    As Serge Orlov wrote in one of his posts you _can_ set the default
    encoding (at least in site.py). See
    <http://docs.python.org/lib/module-sys.html>
    yes, but you're not supposed to do that, for several reasons, including
    the reasons Robert provided: if you mess with the interpreter defaults,
    code you write isn't portable, and code written by others may not work
    on your machine.

    the interpreter isn't fully encoding agnostic either; things are not
    guaranteed to work properly if you're not using the default.

    </F>

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