Feature Name
Summary
Make Fedora's implementation of Python use a locale-aware default string encoding (generally "UTF-8"), rather than hardcoding "ascii", thus avoiding exceptions of the form
UnicodeEncodeError: 'ascii' codec can't encode characters in position ...: ordinal not in range(128)
Owner
- Name: Dave Malcolm
- Email: <dmalcolm@redhat.com>
Current status
- Targeted release: Fedora 41
- Last updated: (DATE)
- Percentage of completion: XX%
Detailed Description
Python's site.py
includes this fragment of code:
def setencoding(): """Set the string encoding used by the Unicode implementation. The default is 'ascii', but if you're willing to experiment, you can change this.""" encoding = "ascii" # Default value set by _PyUnicode_Init() if 0: # Enable to support locale aware default string encodings. import locale loc = locale.getdefaultlocale() if loc[1]: encoding = loc[1] if 0: # Enable to switch off string to Unicode coercion and implicit # Unicode to string conversion. encoding = "undefined" if encoding != "ascii": # On Non-Unicode builds this will raise an AttributeError... sys.setdefaultencoding(encoding) # Needs Python Unicode build !
It is proposed to change the first conditional to if 1:
so that Fedora's Python by default reads the locale from the environment and uses that encoding. This will generally mean UTF-8
is used, rather than ascii
.
Background
CPython's "default encoding"
The C implementation of Python 2 has two ways it can represent text strings:
- the classic legacy
str
object in which each character is represented as a single byte in an undefined character set. This is represented internally as astruct PyStringObject
unicode
objects where each character is represented as either 16-bit or 32-bit word in the Unicode character set (UCS). This is represented internally as astruct PyUnicodeObject
. We use UCS4 (32-bit) in Fedora's builds of Python.
Python 2 will encode and decode between unicode objects and str objects based on what Python believes the character set and character encoding are for the str object.
CPython 2's implementation has an internal read-only variable called unicode_default_encoding
which is returned by sys.getdefaultencoding()
(for brevity sake I'm going to refer to this variable as default_encoding). Whenever Python passes a string to an external API or receives a string from an external API, e.g. any string ultimately passed to a C function and the C binding has not explicitly specified its encode/decode requirements then Python consults the unicode_default_encoding variable to decide how to encode/decode that string. That means any time you print a string, open a file, call a function in a CPython binding it is subject to the default encoding.
(In Python 3, the str
object became a struct PyUnicodeObject
, and struct PyStringObject
became a bytes
object)
The unicode_default_encoding
is set in site.py
to ascii
for historical reasons. Then site.py
makes the default_encoding read-only by removing it from the sys
module name space. This means you cannot call sys.setdefaultencoding()
without generating an exception. This also means Python's default encoding is locked to ascii
.
The reason for this appears to be an optimization within CPython: at the C level a struct PyUnicodeObject
actually carries two copies of the string:
- its UCS-{2,4} representation (this is the
Py_UNICODE *str
field), and - its encoded representation after encoding it according to the value in the global
unicode_default_encoding
variable; this is thePyObject *defenc
field.
Think of this as a cached value of the string in the default encoding. The first time a unicode object is subject to encode/decode it caches the encoded value of the string to avoid having to encode/decode every time the unicode object needs to accessed in its encoded form. This cached value is invalidated when the unicode string content changes but there is no mechanism to invalidate it when the default encoding changes (hence, I believe, the restrictions on changing the default encoding, and the possibility that any struct PyUnicodeObject
instances created prior to the modification of the default encoding may exhibit incorrect behavior with respect to encoding).
The system locale's encoding
In Fedora there is the notion of the "locale", embodying various localization parameters for the whole operating system. From the perspective of the "operating system locale", there is an "encoding", separate to that of the CPython runtime. From this perspective of operating system locale, our default encoding is UTF-8. This is normally set via login scripts in /etc/profile.d
. The user if they wish may choose to override the system default.
In both instances the default language and encoding is exported via an environment variable:
[david@brick ~]$ echo $LANG en_US.utf8
It's possible to query this locale information from Python using the locale
module:
>>> import locale >>> print locale.getdefaultlocale() ('en_US', 'UTF8')
The encoding of stdout/stderr/stdin varies with TTY-connectivity
To add to the confusion, Py_InitializeEx sets up the encoding of each of stdout, stderr, stdin to the default locale encoding (typically UTF-8), _provided_ they are connected to a tty:
#0 PyFile_SetEncodingAndErrors (f=0xb7fc5020, enc=0x80edc28 "UTF-8", errors=0x0) at Objects/fileobject.c:458 #1 0x04fbdd49 in Py_InitializeEx (install_sigs=<value optimized out>) at Python/pythonrun.c:322 #2 0x04fbe29e in Py_Initialize () at Python/pythonrun.c:359 #3 0x04fc9886 in Py_Main (argc=<value optimized out>, argv=<value optimized out>) at Modules/main.c:512 #4 0x080485c7 in main (argc=<value optimized out>, argv=<value optimized out>) at Modules/python.c:23
so that the python interpreter run interactively from a terminal uses UTF-8 for the standard streams:
>>> sys.getdefaultencoding() 'ascii' >>> sys.stdin.encoding 'UTF-8' >>> sys.stdout.encoding 'UTF-8' >>> sys.stderr.encoding 'UTF-8'
This means that a simple case (printing lower case greek alpha, beta, gamma) works when run directly:
[david@brick ~]$ python -c 'print u"\u03b1\u03b2\u03b3"' αβγ
...but fails if you pipe it to a file or redirected into "less", despite the fact that the system locale is UTF-8, and thus "less" expects UTF-8 data:
[david@brick ~]$ python -c 'print u"\u03b1\u03b2\u03b3"' > foo.txt Traceback (most recent call last): File "<string>", line 1, in <module> UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128) [david@brick ~]$ python -c 'print u"\u03b1\u03b2\u03b3"' | less Traceback (most recent call last): File "<string>", line 1, in <module> UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
PyGTK and Pango
A significant "gotcha" here is that the pango
Python module forces the global default encoding variable to be 'utf-8'. It can do this because it's implemented in CPython where there are no restrictions; it directly calls PyUnicode_SetDefaultEncoding
/* set the default python encoding to utf-8 */ PyUnicode_SetDefaultEncoding("utf-8");
Let's take a little test drive and see things in action for ourselves:
$ python Python 2.5.1 (r251:54863, Jun 15 2008, 18:24:51) [GCC 4.3.0 20080428 (Red Hat 4.3.0-8)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import sys >>> sys.getdefaultencoding() 'ascii' >>> sys.setdefaultencoding('utf-8') Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'module' object has no attribute 'setdefaultencoding' >>> import pango >>> sys.getdefaultencoding() 'utf-8'
This hidden global side-effect can be particularly confusing, since the module is typically imported implicitly by other modules (e.g. by the gtk
module)
This was first introduced in pygtk in a 2000-10-25 commit, and was moved from the pygtk module to the pango module in a 2006-04-01 commit in response to https://bugzilla.gnome.org/show_bug.cgi?id=328031
site.py
Looking over the source history in upstream's Subversion:
- the site.py hook to set the default encoding from the locale was added on June 7th 2000 in rev 15634:
'Added support to set the default encoding of strings at startup time to the values defined by the C locale...'
- the code was disabled by default 5 weeks later on July 15th 2000 in rev 16374 by effbot (Fredrik Lundh):
-- changed default encoding to "ascii". you can still change the default via site.py...:
- and the code was optimized two months later on Sept 18th 2000 in rev 17513, to only set it if it's changed:
Looking over upstream mailing list archives for this period:
- Python-Dev changing the locale.py interface?: Fredrik Lundh <effbot@telia.com>
- followed by: "ascii default encoding":
- http://mail.python.org/pipermail/python-dev/2000-July/006724.html
(unfortunately side-tracked into a debate of "deprecated" vs "depreciated"); I may have missed some of the discussion though.
sys.setdefaultencoding
The function sys.setdefaultencoding
is defined in Python/sysmodule.c, it calls
PyUnicode_SetDefaultEncoding(encoding) on the string "encoding"
PyUnicode_SetDefaultEncoding is defined in Objects/unicodeobject.c; it has this code:
/* Make sure the encoding is valid. As side effect, this also loads the encoding into the codec registry cache. */ v = _PyCodec_Lookup(encoding);
then copies the encoding into the buffer: "unicode_default_encoding"; this buffer supplies the return value for PyUnicode_GetDefaultEncoding(), which is used in many places inside the unicode implementation, plus in bytearrayobject.c: bytearray_decode() and in stringobject.c: PyString_AsDecodedObject() and PyString_AsEncodedObject() so it would seem that there's at least some risk in changing this setting.
"ascii" and "UTF-8"
Note that UTF-8 is identical by design to ASCII when the set of characters is composed only from the ASCII character set: code points 0-127 are all represented in UTF-8 as bytes 0-127, identical to ASCII. So any string which was encodable in "ascii" will also be encodable in "utf-8", and the encodings will be byte-for-byte identical. Data containing bytes in the range 128-255 were not valid "ascii", and attempts to decode them to unicode would have failed.
Applications which used i18n unicode strings previously could only have worked correctly if they were manually encoding to UTF-8 on every output call, they should also see no regression. Applications which load unicode strings from translation catalogs would never have worked correctly and will now work.
Material I'm assembling
(Probably should discuss the usage of the PyArg_ API, and the effect of the encoding on converting between PyObject*
and char*
for calling into libraries)
(Quoting jdennis from https://bugzilla.redhat.com/show_bug.cgi?id=243541)
Python when it outputs unicode strings will automatically translate them into the default system encoding. The default encoding is set in site.py and cannot be overriden by the user, once set in site.py it is locked. In Fedora and RHEL our default encoding is UTF-8. This is normally set via login scripts in /etc/profile.d. Thu user if they wish may choose to override the system default. In both instances the default language and encoding is exported via an environment variable. In site.py there is code to allow the default encoding to be set from the locale information discussed above, however this functionality is turned off and instead is hardcoded to be ascii. This is clearly wrong IMHO. A typical consequence of this is a i18n python application using unicode strings will fault with encoding exceptions when it tries to output any of its unicode strings. The reason string output will throw exceptions is because the default encoding is ascii, internally CPython will convert the unicode string using the default codec (ascii) which of course will fail if the unicode string contains characters outside the asckii character set, which is highly likely in non-latin languages. If the default encoding was UTF-8, as it should be by default to match the rest of our environment the the encoding translations from Pythons internal UCS-4 Unicode to UTF-8 would succeed. I have personally tested and verified this works . Note, the only way existing applications could have worked correctly is: 1) They load unicode strings and manuall convert to UTF-8 on output (correct default encoding removes the need for manual conversion on every output call). 2) The load their i18n strings from message catalog in UTF-8 format. This is typically specified as the codeset parameter in gettext.bind_textdomain_codeset() or gettext.install(). In this case the strings loaded from the catelog ARE NOT UNIICODE (python has an explicit string type called unicode which in our builds is UCS-4) normal python strings are represented as 'str' objects. When gettext is told to return strings via _() using the UTF-8 codeset python represents them as 'str' not 'unicode', in other words they are sequences of octets. When output the default encoding is not not applied because they are not unicode strings, rather they are vanilla strings. Thus output works in our environment because their entire lifetime in python is as UTF-8. However, there are many good reasons to work with i18n strings as unicode, not byte sequences which happen to be represented as UTF-8 (e.g. can't count the number of characters, can't concatenate, etc.). Thus applications should be able to represent their i18n strings as unicode (internally as UCS-4) and output correctly with correct translation to UTF-8 automatically applied by python, not manually. This is from site.py. Note the hardcoding of 'ascii'. If the first 'if 0:' test allowed locale.getdefaultlocale() to be called it would allow the default encoding to be correctly set from the environment. Site.py should be patched to allow this. def setencoding(): """Set the string encoding used by the Unicode implementation. The default is 'ascii', but if you're willing to experiment, you can change this.""" encoding = "ascii" # Default value set by _PyUnicode_Init() if 0: # Enable to support locale aware default string encodings. import locale loc = locale.getdefaultlocale() if loc[1]: encoding = loc[1] if 0: # Enable to switch off string to Unicode coercion and implicit # Unicode to string conversion. encoding = "undefined" if encoding != "ascii": # On Non-Unicode builds this will raise an AttributeError... sys.setdefaultencoding(encoding) # Needs Python Unicode build !
(end quote)
Currently Fedora's python implementation uses ascii
Benefit to Fedora
Scope
(should survey other distributions, and discuss things with them)
In theory this is just a one-byte change in the site.py
shipped in the python
rpm.
Need to repeat the above analysis for the python3
rpm, which has the same code in its site.py
file.
How To Test
Given that this makes a deep and subtle change to the internals of Python, the best way of testing this is to get it into Rawhide ASAP and for people to test their Python code on a version of Python with the change.
Smoketest
- Run
python -c "import sys; print(sys.getdefaultencoding())"
- It should report
UTF-8
, notascii
- The same test should be runnable with
python3
, and reportutf-8
User Experience
Dependencies
None: this is a one-line change in our python and python3 rpms.
Contingency Plan
In theory this is a one-line change in the site.py file shipped in our python rpm, and so it can be backed out by reverting that one line change.
(It may be that Python applications develop a dependency on our Python having made this change and so would be broken by reverting)