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PEP: 538
Title: Coercing the legacy C locale to a UTF-8 based locale
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <>
BDFL-Delegate: INADA Naoki
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Dec-2016
Python-Version: 3.7
Post-History: 03-Jan-2017 (linux-sig),
07-Jan-2017 (python-ideas),
05-Mar-2017 (python-dev),
09-May-2017 (python-dev)
An ongoing challenge with Python 3 on \*nix systems is the conflict between
needing to use the configured locale encoding by default for consistency with
other locale-aware components in the same process or subprocesses,
and the fact that the standard C locale (as defined in POSIX:2001) typically
implies a default text encoding of ASCII, which is entirely inadequate for the
development of networked services and client applications in a multilingual
PEP 540 proposes a change to CPython's handling of the legacy C locale such
that CPython will assume the use of UTF-8 in such environments, rather than
persisting with the demonstrably problematic assumption of ASCII as an
appropriate encoding for communicating with operating system interfaces.
This is a good approach for cases where network encoding interoperability
is a more important concern than local encoding interoperability.
However, it comes at the cost of making CPython's encoding assumptions diverge
from those of other locale-aware components in the same process, as well as
those of components running in subprocesses that share the same environment.
This can cause interoperability problems with some extension modules (such as
GNU readline's command line history editing), as well as with components
running in subprocesses (such as older Python runtimes).
It also requires non-trivial changes to the internals of how CPython itself
works, rather than relying primarily on existing configuration settings that
are supported by Python versions prior to Python 3.7.
Accordingly, this PEP proposes that independently of the UTF-8 mode proposed
in PEP 540, the way the CPython implementation handles the default C locale be
changed to be roughly equivalent to the following existing configuration
settings (supported since Python 3.1)::
The exact target locale for coercion will be chosen from a predefined list at
runtime based on the actually available locales.
The reinterpreted locale settings will be written back to the environment so
they're visible to other components in the same process and in subprocesses,
but the changed ``PYTHONIOENCODING`` default will be made implicit in order to
avoid causing compatibility problems with Python 2 subprocesses that don't
provide the ``surrogateescape`` error handler.
The new legacy locale coercion behavior can be disabled either by setting
``LC_ALL`` (which may still lead to a Unicode compatibility warning) or by
setting the new ``PYTHONCOERCECLOCALE`` environment variable to ``0``.
With this change, any \*nix platform that does *not* offer at least one of the
``C.UTF-8``, ``C.utf8`` or ``UTF-8`` locales as part of its standard
configuration would only be considered a fully supported platform for CPython
3.7+ deployments when a suitable locale other than the default ``C`` locale is
configured explicitly (e.g. ``en_AU.UTF-8``, ``zh_CN.gb18030``). If PEP 540 is
accepted in addition to this PEP, then pure Python modules would also be
supported when using the proposed ``PYTHONUTF8`` mode, but expectations for
full Unicode compatibility in extension modules would continue to be limited
to the platforms covered by this PEP.
As it only reflects a change in default settings rather than a fundamentally
new capability, redistributors (such as Linux distributions) with a narrower
target audience than the upstream CPython development team may also choose to
opt in to this locale coercion behaviour for the Python 3.6.x series by
applying the necessary changes as a downstream patch.
Implementation Notes
Attempting to implement the PEP as originally accepted showed that the
proposal to emit locale coercion and compatibility warnings by default
simply wasn't practical (there were too many cases where previously working
code failed *because of the warnings*, rather than because of latent locale
handling defects in the affected code).
As a result, the ``PY_WARN_ON_C_LOCALE`` config flag was removed, and replaced
with a runtime ``PYTHONCOERCECLOCALE=warn`` environment variable setting
that allows developers and system integrators to opt-in to receiving locale
coercion and compatibility warnings, without emitting them by default.
The output examples in the PEP itself have also been updated to remove
the warnings and make them easier to read.
While the CPython interpreter is starting up, it may need to convert from
the ``char *`` format to the ``wchar_t *`` format, or from one of those formats
to ``PyUnicodeObject *``, in a way that's consistent with the locale settings
of the overall system. It handles these cases by relying on the operating
system to do the conversion and then ensuring that the text encoding name
reported by ``sys.getfilesystemencoding()`` matches the encoding used during
this early bootstrapping process.
On Windows, the limitations of the ``mbcs`` format used by default in these
conversions proved sufficiently problematic that PEP 528 and PEP 529 were
implemented to bypass the operating system supplied interfaces for binary data
handling and force the use of UTF-8 instead.
On Mac OS X, iOS, and Android, many components, including CPython, already
assume the use of UTF-8 as the system encoding, regardless of the locale
setting. However, this isn't the case for all components, and the discrepancy
can cause problems in some situations (for example, when using the GNU readline
module [16_]).
On non-Apple and non-Android \*nix systems, these operations are handled using
the C locale system in glibc, which has the following characteristics [4_]:
* by default, all processes start in the ``C`` locale, which uses ``ASCII``
for these conversions. This is almost never what anyone doing multilingual
text processing actually wants (including CPython and C/C++ GUI frameworks).
* calling ``setlocale(LC_ALL, "")`` reconfigures the active locale based on
the locale categories configured in the current process environment
* if the locale requested by the current environment is unknown, or no specific
locale is configured, then the default ``C`` locale will remain active
The specific locale category that covers the APIs that CPython depends on is
``LC_CTYPE``, which applies to "classification and conversion of characters,
and to multibyte and wide characters" [5_]. Accordingly, CPython includes the
following key calls to ``setlocale``:
* in the main ``python`` binary, CPython calls ``setlocale(LC_ALL, "")`` to
configure the entire C locale subsystem according to the process environment.
It does this prior to making any calls into the shared CPython library
* in ``Py_Initialize``, CPython calls ``setlocale(LC_CTYPE, "")``, such that
the configured locale settings for that category *always* match those set in
the environment. It does this unconditionally, and it *doesn't* revert the
process state change in ``Py_Finalize``
(This summary of the locale handling omits several technical details related
to exactly where and when the text encoding declared as part of the locale
settings is used - see PEP 540 for further discussion, as these particular
details matter more when decoupling CPython from the declared C locale than
they do when overriding the locale with one based on UTF-8)
These calls are usually sufficient to provide sensible behaviour, but they can
still fail in the following cases:
* SSH environment forwarding means that SSH clients may sometimes forward
client locale settings to servers that don't have that locale installed. This
leads to CPython running in the default ASCII-based C locale
* some process environments (such as Linux containers) may not have any
explicit locale configured at all. As with unknown locales, this leads to
CPython running in the default ASCII-based C locale
* on Android, rather than configuring the locale based on environment variables,
the empty locale ``""`` is treated as specifically requesting the ``"C"``
The simplest way to deal with this problem for currently released versions of
CPython is to explicitly set a more sensible locale when launching the
application. For example::
LC_CTYPE=C.UTF-8 python3 ...
The ``C.UTF-8`` locale is a full locale definition that uses ``UTF-8`` for the
``LC_CTYPE`` category, and the same settings as the ``C`` locale for all other
categories (including ``LC_COLLATE``). It is offered by a number of Linux
distributions (including Debian, Ubuntu, Fedora, Alpine and Android) as an
alternative to the ASCII-based C locale. Some other platforms (such as
``HP-UX``) offer an equivalent locale definition under the name ``C.utf8``.
Mac OS X and other \*BSD systems have taken a different approach: instead of
offering a ``C.UTF-8`` locale, they offer a partial ``UTF-8`` locale that only
defines the ``LC_CTYPE`` category. On such systems, the preferred
environmental locale adjustment is to set ``LC_CTYPE=UTF-8`` rather than to set
``LC_ALL`` or ``LANG``. [17_]
In the specific case of Docker containers and similar technologies, the
appropriate locale setting can be specified directly in the container image
Another common failure case is developers specifying ``LANG=C`` in order to
see otherwise translated user interface messages in English, rather than the
more narrowly scoped ``LC_MESSAGES=C`` or ``LANGUAGE=en``.
Relationship with other PEPs
This PEP shares a common problem statement with PEP 540 (improving Python 3's
behaviour in the default C locale), but diverges markedly in the proposed
* PEP 540 proposes to entirely decouple CPython's default text encoding from
the C locale system in that case, allowing text handling inconsistencies to
arise between CPython and other locale-aware components running in the same
process and in subprocesses. This approach aims to make CPython behave less
like a locale-aware application, and more like locale-independent language
runtimes like those for Go, Node.js (V8), and Rust
* this PEP proposes to override the legacy C locale with a more recently
defined locale that uses UTF-8 as its default text encoding. This means that
the text encoding override will apply not only to CPython, but also to any
locale-aware extension modules loaded into the current process, as well as to
locale-aware applications invoked in subprocesses that inherit their
environment from the parent process. This approach aims to retain CPython's
traditional strong support for integration with other locale-aware components
while also actively helping to push forward the adoption and standardisation
of the C.UTF-8 locale as a Unicode-aware replacement for the legacy C locale
in the wider C/C++ ecosystem
After reviewing both PEPs, it became clear that they didn't actually conflict
at a technical level, and the proposal in PEP 540 offered a superior option in
cases where no suitable locale was available, as well as offering a better
reference behaviour for platforms where the notion of a "locale encoding"
doesn't make sense (for example, embedded systems running MicroPython rather
than the CPython reference interpreter).
Meanwhile, this PEP offered improved compatibility with other locale-aware
components, and an approach more amenable to being backported to Python 3.6
by downstream redistributors.
As a result, this PEP was amended to refer to PEP 540 as a complementary
solution that offered improved behaviour when none of the standard UTF-8 based
locales were available, as well as extending the changes in the default
settings to APIs that aren't currently independently configurable (such as
the default encoding and error handler for ``open()``).
The availability of PEP 540 also meant that the ``LC_CTYPE=en_US.UTF-8`` legacy
fallback was removed from the list of UTF-8 locales tried as a coercion target,
with the expectation being that CPython will instead rely solely on the
proposed PYTHONUTF8 mode in such cases.
While Linux container technologies like Docker, Kubernetes, and OpenShift are
best known for their use in web service development, the related container
formats and execution models are also being adopted for Linux command line
application development. Technologies like Gnome Flatpak [7_] and
Ubuntu Snappy [8_] further aim to bring these same techniques to Linux GUI
application development.
When using Python 3 for application development in these contexts, it isn't
uncommon to see text encoding related errors akin to the following::
$ docker run --rm fedora:25 python3 -c 'print("ℙƴ☂ℌøἤ")'
Unable to decode the command from the command line:
UnicodeEncodeError: 'utf-8' codec can't encode character '\udce2' in position 7: surrogates not allowed
$ docker run --rm ncoghlan/debian-python python3 -c 'print("ℙƴ☂ℌøἤ")'
Unable to decode the command from the command line:
UnicodeEncodeError: 'utf-8' codec can't encode character '\udce2' in position 7: surrogates not allowed
Even though the same command is likely to work fine when run locally::
$ python3 -c 'print("ℙƴ☂ℌøἤ")'
The source of the problem can be seen by instead running the ``locale`` command
in the three environments::
$ locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
$ docker run --rm fedora:25 locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
$ docker run --rm ncoghlan/debian-python locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
In this particular example, we can see that the host system locale is set to
"en_AU.UTF-8", so CPython uses UTF-8 as the default text encoding. By contrast,
the base Docker images for Fedora and Debian don't have any specific locale
set, so they use the POSIX locale by default, which is an alias for the
ASCII-based default C locale.
The simplest way to get Python 3 (regardless of the exact version) to behave
sensibly in Fedora and Debian based containers is to run it in the ``C.UTF-8``
locale that both distros provide::
$ docker run --rm -e LC_CTYPE=C.UTF-8 fedora:25 python3 -c 'print("ℙƴ☂ℌøἤ")'
$ docker run --rm -e LC_CTYPE=C.UTF-8 ncoghlan/debian-python python3 -c 'print("ℙƴ☂ℌøἤ")'
$ docker run --rm -e LC_CTYPE=C.UTF-8 fedora:25 locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
$ docker run --rm -e LC_CTYPE=C.UTF-8 ncoghlan/debian-python locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
The Alpine Linux based Python images provided by Docker, Inc. already use the
C.UTF-8 locale by default::
$ docker run --rm python:3 python3 -c 'print("ℙƴ☂ℌøἤ")'
$ docker run --rm python:3 locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
Similarly, for custom container images (i.e. those adding additional content on
top of a base distro image), a more suitable locale can be set in the image
definition so everything just works by default. However, it would provide a much
nicer and more consistent user experience if CPython were able to just deal
with this problem automatically rather than relying on redistributors or end
users to handle it through system configuration changes.
While the glibc developers are working towards making the C.UTF-8 locale
universally available for use by glibc based applications like CPython [6_],
this unfortunately doesn't help on platforms that ship older versions of glibc
without that feature, and also don't provide C.UTF-8 (or an equivalent) as an
on-disk locale the way Debian and Fedora do. These platforms are considered
out of scope for this PEP - see PEP 540 for further discussion of possible
options for improving CPython's default behaviour in such environments.
Design Principles
The above motivation leads to the following core design principles for the
proposed solution:
* if a locale other than the default C locale is explicitly configured, we'll
continue to respect it
* as far as is feasible, any changes made will use *existing* configuration
* Python's runtime behaviour in potential coercion target locales should be
identical regardless of whether the locale was set explicitly in the
environment or implicitly as a locale coercion target
* for Python 3.7, if we're changing the locale setting without an explicit
config option, we'll emit a warning on stderr that we're doing so rather
than silently changing the process configuration. This will alert application
and system integrators to the change, even if they don't closely follow the
PEP process or Python release announcements. However, to minimize the chance
of introducing new problems for end users, we'll do this *without* using the
warnings system, so even running with ``-Werror`` won't turn it into a runtime
exception. (Note: these warnings ended up being silenced by default. See the
Implementation Note above for more details)
* for Python 3.7, any changed defaults will offer some form of explicit "off"
switch at build time, runtime, or both
Minimizing the negative impact on systems currently correctly configured to
use GB-18030 or another partially ASCII compatible universal encoding leads to
the following design principle:
* if a UTF-8 based Linux container is run on a host that is explicitly
configured to use a non-UTF-8 encoding, and tries to exchange locally
encoded data with that host rather than exchanging explicitly UTF-8 encoded
data, CPython will endeavour to correctly round-trip host provided data that
is concatenated or split solely at common ASCII compatible code points, but
may otherwise emit nonsensical results.
Minimizing the negative impact on systems and programs correctly configured to
use an explicit locale category like ``LC_TIME``, ``LC_MONETARY`` or
``LC_NUMERIC`` while otherwise running in the legacy C locale gives the
following design principles:
* don't make any environmental changes that would alter any existing settings
for locale categories other than ``LC_CTYPE`` (most notably: don't set
``LC_ALL`` or ``LANG``)
Finally, maintaining compatibility with running arbitrary subprocesses in
orchestration use cases leads to the following design principle:
* don't make any Python-specific environmental changes that might be
incompatible with any still supported version of CPython (including
CPython 2.7)
To better handle the cases where CPython would otherwise end up attempting
to operate in the ``C`` locale, this PEP proposes that CPython automatically
attempt to coerce the legacy ``C`` locale to a UTF-8 based locale for the
``LC_CTYPE`` category when it is run as a standalone command line application.
It further proposes to emit a warning on stderr if the legacy ``C`` locale
is in effect for the ``LC_CTYPE`` category at the point where the language
runtime itself is initialized,
and the explicit environmental flag to disable locale coercion is not set, in
order to warn system and application integrators that they're running CPython
in an unsupported configuration.
In addition to these general changes, some additional Android-specific changes
are proposed to handle the differences in the behaviour of ``setlocale`` on that
Legacy C locale coercion in the standalone Python interpreter binary
When run as a standalone application, CPython has the opportunity to
reconfigure the C locale before any locale dependent operations are executed
in the process.
This means that it can change the locale settings not only for the CPython
runtime, but also for any other locale-aware components running in the current
process (e.g. as part of extension modules), as well as in subprocesses that
inherit their environment from the current process.
After calling ``setlocale(LC_ALL, "")`` to initialize the locale settings in
the current process, the main interpreter binary will be updated to include
the following call::
const char *ctype_loc = setlocale(LC_CTYPE, NULL);
This cryptic invocation is the API that C provides to query the current locale
setting without changing it. Given that query, it is possible to check for
exactly the ``C`` locale with ``strcmp``::
ctype_loc != NULL && strcmp(ctype_loc, "C") == 0 # true only in the C locale
This call also returns ``"C"`` when either no particular locale is set, or the
nominal locale is set to an alias for the ``C`` locale (such as ``POSIX``).
Given this information, CPython can then attempt to coerce the locale to one
that uses UTF-8 rather than ASCII as the default encoding.
Three such locales will be tried:
* ``C.UTF-8`` (available at least in Debian, Ubuntu, Alpine, and Fedora 25+, and
expected to be available by default in a future version of glibc)
* ``C.utf8`` (available at least in HP-UX)
* ``UTF-8`` (available in at least some \*BSD variants, including Mac OS X)
The coercion will be implemented by setting the ``LC_CTYPE`` environment
variable to the candidate locale name, such that future calls to
``setlocale()`` will see it, as will other components looking for those
settings (such as GUI development frameworks and Python's own ``locale``
To allow for better cross-platform binary portability and to adjust
automatically to future changes in locale availability, these checks will be
implemented at runtime on all platforms other than Windows, rather than
attempting to determine which locales to try at compile time.
When this locale coercion is activated, the following warning will be
printed on stderr, with the warning containing whichever locale was
successfully configured::
Python detected LC_CTYPE=C: LC_CTYPE coerced to C.UTF-8 (set another
locale or PYTHONCOERCECLOCALE=0 to disable this locale coercion behaviour).
(Note: this warning ended up being silenced by default. See the
Implementation Note above for more details)
As long as the current platform provides at least one of the candidate UTF-8
based environments, this locale coercion will mean that the standard
Python binary *and* locale-aware extensions should once again "just work"
in the three main failure cases we're aware of (missing locale
settings, SSH forwarding of unknown locales via ``LANG`` or ``LC_CTYPE``, and
developers explicitly requesting ``LANG=C``).
The one case where failures may still occur is when ``stderr`` is specifically
being checked for no output, which can be resolved either by configuring
a locale other than the C locale, or else by using a mechanism other than
"there was no output on stderr" to check for subprocess errors (e.g. checking
process return codes).
If none of the candidate locales are successfully configured, or the ``LC_ALL``,
locale override is defined in the current process environment, then
initialization will continue in the C locale and the Unicode compatibility
warning described in the next section will be emitted just as it would for
any other application.
If ``PYTHONCOERCECLOCALE=0`` is explicitly set, initialization will continue in
the C locale and the Unicode compatibility warning described in the next
section will be automatically suppressed.
The interpreter will always check for the ``PYTHONCOERCECLOCALE`` environment
variable at startup (even when running under the ``-E`` or ``-I`` switches),
as the locale coercion check necessarily takes place before any command line
argument processing. For consistency, the runtime check to determine whether
or not to suppress the locale compatibility warning will be similarly
independent of these settings.
Legacy C locale warning during runtime initialization
By the time that ``Py_Initialize`` is called, arbitrary locale-dependent
operations may have taken place in the current process. This means that
by the time it is called, it is *too late* to reliably switch to a different
locale - doing so would introduce inconsistencies in decoded text, even in the
context of the standalone Python interpreter binary.
Accordingly, when ``Py_Initialize`` is called and CPython detects that the
configured locale is still the default ``C`` locale and
``PYTHONCOERCECLOCALE=0`` is not set, the following warning will be issued::
Python runtime initialized with LC_CTYPE=C (a locale with default ASCII
encoding), which may cause Unicode compatibility problems. Using C.UTF-8,
C.utf8, or UTF-8 (if available) as alternative Unicode-compatible
locales is recommended.
(Note: this warning ended up being silenced by default. See the
Implementation Note above for more details)
In this case, no actual change will be made to the locale settings.
Instead, the warning informs both system and application integrators that
they're running Python 3 in a configuration that we don't expect to work
The second sentence providing recommendations may eventually be conditionally
compiled based on the operating system (e.g. recommending ``LC_CTYPE=UTF-8``
on \*BSD systems), but the initial implementation will just use the common
generic message shown above.
New build-time configuration options
While both of the above behaviours would be enabled by default, they would
also have new associated configuration options and preprocessor definitions
for the benefit of redistributors that want to override those default settings.
The locale coercion behaviour would be controlled by the flag
``--with[out]-c-locale-coercion``, which would set the ``PY_COERCE_C_LOCALE``
preprocessor definition.
The locale warning behaviour would be controlled by the flag
``--with[out]-c-locale-warning``, which would set the ``PY_WARN_ON_C_LOCALE``
preprocessor definition.
(Note: this compile time warning option ended up being replaced by a runtime
``PYTHONCOERCECLOCALE=warn`` option. See the Implementation Note above for
more details)
On platforms which don't use the ``autotools`` based build system (i.e.
Windows) these preprocessor variables would always be undefined.
Changes to the default error handling on the standard streams
Since Python 3.5, CPython has defaulted to using ``surrogateescape`` on the
standard streams (``sys.stdin``, ``sys.stdout``) when it detects that the
current locale is ``C`` and no specific error handled has been set using
either the ``PYTHONIOENCODING`` environment variable or the
``Py_setStandardStreamEncoding`` API. For other locales, the default error
handler for the standard streams is ``strict``.
In order to preserve this behaviour without introducing any behavioural
discrepancies between locale coercion and explicitly configuring a locale, the
coercion target locales (``C.UTF-8``, ``C.utf8``, and ``UTF-8``) will be added
to the list of locales that use ``surrogateescape`` as their default error
handler for the standard streams.
No changes are proposed to the default error handler for ``sys.stderr``: that
will continue to be ``backslashreplace``.
Changes to locale settings on Android
Independently of the other changes in this PEP, CPython on Android systems
will be updated to call ``setlocale(LC_ALL, "C.UTF-8")`` where it currently
calls ``setlocale(LC_ALL, "")`` and ``setlocale(LC_CTYPE, "C.UTF-8")`` where
it currently calls ``setlocale(LC_CTYPE, "")``.
This Android-specific behaviour is being introduced due to the following
Android-specific details:
* on Android, passing ``""`` to ``setlocale`` is equivalent to passing ``"C"``
* the ``C.UTF-8`` locale is always available
Platform Support Changes
A new "Legacy C Locale" section will be added to PEP 11 that states:
* as of CPython 3.7, \*nix platforms are expected to provide at least one of
``C.UTF-8`` (full locale), ``C.utf8`` (full locale) or ``UTF-8`` (
``LC_CTYPE``-only locale) as an alternative to the legacy ``C`` locale.
Any Unicode related integration problems that occur only in the legacy ``C``
locale and cannot be reproduced in an appropriately configured non-ASCII
locale will be closed as "won't fix".
Improving the handling of the C locale
It has been clear for some time that the C locale's default encoding of
``ASCII`` is entirely the wrong choice for development of modern networked
services. Newer languages like Rust and Go have eschewed that default entirely,
and instead made it a deployment requirement that systems be configured to use
UTF-8 as the text encoding for operating system interfaces. Similarly, Node.js
assumes UTF-8 by default (a behaviour inherited from the V8 JavaScript engine)
and requires custom build settings to indicate it should use the system
locale settings for locale-aware operations. Both the JVM and the .NET CLR
use UTF-16-LE as their primary encoding for passing text between applications
and the application runtime (i.e. the JVM/CLR, not the host operating system).
The challenge for CPython has been the fact that in addition to being used for
network service development, it is also extensively used as an embedded
scripting language in larger applications, and as a desktop application
development language, where it is more important to be consistent with other
locale-aware components sharing the same process, as well as with the user's
desktop locale settings, than it is with the emergent conventions of modern
network service development.
The core premise of this PEP is that for *all* of these use cases, the
assumption of ASCII implied by the default "C" locale is the wrong choice,
and furthermore that the following assumptions are valid:
* in desktop application use cases, the process locale will *already* be
configured appropriately, and if it isn't, then that is an operating system
or embedding application level problem that needs to be reported to and
resolved by the operating system provider or application developer
* in network service development use cases (especially those based on Linux
containers), the process locale may not be configured *at all*, and if it
isn't, then the expectation is that components will impose their own default
encoding the way Rust, Go and Node.js do, rather than trusting the legacy C
default encoding of ASCII the way CPython currently does
Defaulting to "surrogateescape" error handling on the standard IO streams
By coercing the locale away from the legacy C default and its assumption of
ASCII as the preferred text encoding, this PEP also disables the implicit use
of the "surrogateescape" error handler on the standard IO streams that was
introduced in Python 3.5 ([15_]), as well as the automatic use of
``surrogateescape`` when operating in PEP 540's proposed UTF-8 mode.
Rather than introducing yet another configuration option to adjust that
behaviour, this PEP instead proposes to extend the "surrogateescape" default
for ``stdin`` and ``stderr`` error handling to also apply to the three
potential coercion target locales.
The aim of this behaviour is to attempt to ensure that operating system
provided text values are typically able to be transparently passed through a
Python 3 application even if it is incorrect in assuming that that text has
been encoded as UTF-8.
In particular, GB 18030 [12_] is a Chinese national text encoding standard
that handles all Unicode code points, that is formally incompatible with both
ASCII and UTF-8, but will nevertheless often tolerate processing as surrogate
escaped data - the points where GB 18030 reuses ASCII byte values in an
incompatible way are likely to be invalid in UTF-8, and will therefore be
escaped and opaque to string processing operations that split on or search for
the relevant ASCII code points. Operations that don't involve splitting on or
searching for particular ASCII or Unicode code point values are almost
certain to work correctly.
Similarly, Shift-JIS [13_] and ISO-2022-JP [14_] remain in widespread use in
Japan, and are incompatible with both ASCII and UTF-8, but will tolerate text
processing operations that don't involve splitting on or searching for
particular ASCII or Unicode code point values.
As an example, consider two files, one encoded with UTF-8 (the default encoding
for ``en_AU.UTF-8``), and one encoded with GB-18030 (the default encoding for
$ python3 -c 'open("utf8.txt", "wb").write("ℙƴ☂ℌøἤ\n".encode("utf-8"))'
$ python3 -c 'open("gb18030.txt", "wb").write("ℙƴ☂ℌøἤ\n".encode("gb18030"))'
On disk, we can see that these are two very different files::
$ python3 -c 'print("UTF-8: ", open("utf8.txt", "rb").read().strip()); \
print("GB18030:", open("gb18030.txt", "rb").read().strip())'
UTF-8: b'\xe2\x84\x99\xc6\xb4\xe2\x98\x82\xe2\x84\x8c\xc3\xb8\xe1\xbc\xa4\n'
GB18030: b'\x816\xbd6\x810\x9d0\x817\xa29\x816\xbc4\x810\x8b3\x816\x8d6\n'
That nevertheless can both be rendered correctly to the terminal as long as
they're decoded prior to printing::
$ python3 -c 'print("UTF-8: ", open("utf8.txt", "r", encoding="utf-8").read().strip()); \
print("GB18030:", open("gb18030.txt", "r", encoding="gb18030").read().strip())'
UTF-8: ℙƴ☂ℌøἤ
GB18030: ℙƴ☂ℌøἤ
By contrast, if we just pass along the raw bytes, as ``cat`` and similar C/C++
utilities will tend to do::
$ LANG=en_AU.UTF-8 cat utf8.txt gb18030.txt
Even setting a specifically Chinese locale won't help in getting the
GB-18030 encoded file rendered correctly::
$ LANG=zh_CN.gb18030 cat utf8.txt gb18030.txt
The problem is that the *terminal* encoding setting remains UTF-8, regardless
of the nominal locale. A GB18030 terminal can be emulated using the ``iconv``
$ cat utf8.txt gb18030.txt | iconv -f GB18030 -t UTF-8
This reverses the problem, such that the GB18030 file is rendered correctly,
but the UTF-8 file has been converted to unrelated hanzi characters, rather than
the expected rendering of "Python" as non-ASCII characters.
With the emulated GB18030 terminal encoding, assuming UTF-8 in Python results
in *both* files being displayed incorrectly::
$ python3 -c 'print("UTF-8: ", open("utf8.txt", "r", encoding="utf-8").read().strip()); \
print("GB18030:", open("gb18030.txt", "r", encoding="gb18030").read().strip())' \
| iconv -f GB18030 -t UTF-8
UTF-8: 鈩櫰粹槀鈩屆羔激
GB18030: 鈩櫰粹槀鈩屆羔激
However, setting the locale correctly means that the emulated GB18030 terminal
now displays both files as originally intended::
$ LANG=zh_CN.gb18030 \
python3 -c 'print("UTF-8: ", open("utf8.txt", "r", encoding="utf-8").read().strip()); \
print("GB18030:", open("gb18030.txt", "r", encoding="gb18030").read().strip())' \
| iconv -f GB18030 -t UTF-8
UTF-8: ℙƴ☂ℌøἤ
GB18030: ℙƴ☂ℌøἤ
The rationale for retaining ``surrogateescape`` as the default IO encoding is
that it will preserve the following helpful behaviour in the ``C`` locale::
$ cat gb18030.txt \
| LANG=C python3 -c "import sys; print(" \
| iconv -f GB18030 -t UTF-8
Rather than reverting to the exception currently seen when a UTF-8 based locale is
explicitly configured::
$ cat gb18030.txt \
| python3 -c "import sys; print(" \
| iconv -f GB18030 -t UTF-8
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/usr/lib64/python3.5/", line 321, in decode
(result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x81 in position 0: invalid start byte
As an added benefit, environments explicitly configured to use one of the
coercion target locales will implicitly gain the encoding transparency behaviour
currently enabled by default in the ``C`` locale.
Avoiding setting PYTHONIOENCODING during UTF-8 locale coercion
Rather than changing the default handling of the standard streams during
interpreter initialization, earlier versions of this PEP proposed setting
``PYTHONIOENCODING`` to ``utf-8:surrogateescape``. This turned out to create
a significant compatibility problem: since the ``surrogateescape`` handler
only exists in Python 3.1+, running Python 2.7 processes in subprocesses could
potentially break in a confusing way with that configuration.
The current design means that earlier Python versions will instead retain their
default ``strict`` error handling on the standard streams, while Python 3.7+
will consistently use the more permissive ``surrogateescape`` handler even
when these locales are explicitly configured (rather than being reached through
locale coercion).
Dropping official support for ASCII based text handling in the legacy C locale
We've been trying to get strict bytes/text separation to work reliably in the
legacy C locale for over a decade at this point. Not only haven't we been able
to get it to work, neither has anyone else - the only viable alternatives
identified have been to pass the bytes along verbatim without eagerly decoding
them to text (C/C++, Python 2.x, Ruby, etc), or else to largely ignore the
nominal C/C++ locale encoding and assume the use of either UTF-8 (PEP 540,
Rust, Go, Node.js, etc) or UTF-16-LE (JVM, .NET CLR).
While this PEP ensures that developers that genuinely need to do so can still
opt-in to running their Python code in the legacy C locale (by setting
``LC_ALL=C``, ``PYTHONCOERCECLOCALE=0``, or running a custom build that sets
``--without-c-locale-coercion``), it also makes it clear that we *don't*
expect Python 3's Unicode handling to be completely reliable in that
configuration, and the recommended alternative is to use a more appropriate
locale setting (potentially in combination with PEP 540's UTF-8 mode, if that
is available).
Providing implicit locale coercion only when running standalone
The major downside of the proposed design in this PEP is that it introduces a
potential discrepancy between the behaviour of the CPython runtime when it is
run as a standalone application and when it is run as an embedded component
inside a larger system (e.g. ``mod_wsgi`` running inside Apache ``httpd``).
Over the course of Python 3.x development, multiple attempts have been made
to improve the handling of incorrect locale settings at the point where the
Python interpreter is initialised. The problem that emerged is that this is
ultimately *too late* in the interpreter startup process - data such as command
line arguments and the contents of environment variables may have already been
retrieved from the operating system and processed under the incorrect ASCII
text encoding assumption well before ``Py_Initialize`` is called.
The problems created by those inconsistencies were then even harder to diagnose
and debug than those created by believing the operating system's claim that
ASCII was a suitable encoding to use for operating system interfaces. This was
the case even for the default CPython binary, let alone larger C/C++
applications that embed CPython as a scripting engine.
The approach proposed in this PEP handles that problem by moving the locale
coercion as early as possible in the interpreter startup sequence when running
standalone: it takes place directly in the C-level ``main()`` function, even
before calling in to the ``Py_Main()`` library function that implements the
features of the CPython interpreter CLI.
The ``Py_Initialize`` API then only gains an explicit warning (emitted on
``stderr``) when it detects use of the ``C`` locale, and relies on the
embedding application to specify something more reasonable.
That said, the reference implementation for this PEP adds most of the
functionality to the shared library, with the CLI being updated to
unconditionally call two new private APIs::
if (_Py_LegacyLocaleDetected()) {
These are similar to other "pre-configuration" APIs intended for embedding
applications: they're designed to be called *before* ``Py_Initialize``, and
hence change the way the interpreter gets initialized.
If these were made public (either as part of this PEP or in a subsequent RFE),
then it would be straightforward for other embedding applications to recreate
the same behaviour as is proposed for the CPython CLI.
Allowing restoration of the legacy behaviour
The CPython command line interpreter is often used to investigate faults that
occur in other applications that embed CPython, and those applications may still
be using the C locale even after this PEP is implemented.
Providing a simple on/off switch for the locale coercion behaviour makes it
much easier to reproduce the behaviour of such applications for debugging
purposes, as well as making it easier to reproduce the behaviour of older 3.x
runtimes even when running a version with this change applied.
Querying LC_CTYPE for C locale detection
``LC_CTYPE`` is the actual locale category that CPython relies on to drive the
implicit decoding of environment variables, command line arguments, and other
text values received from the operating system.
As such, it makes sense to check it specifically when attempting to determine
whether or not the current locale configuration is likely to cause Unicode
handling problems.
Explicitly setting LC_CTYPE for UTF-8 locale coercion
Python is often used as a glue language, integrating other C/C++ ABI compatible
components in the current process, and components written in arbitrary
languages in subprocesses.
Setting ``LC_CTYPE`` to ``C.UTF-8`` is important to handle cases where the
problem has arisen from a setting like ``LC_CTYPE=UTF-8`` being provided on a
system where no ``UTF-8`` locale is defined (e.g. when a Mac OS X ssh client is
configured to forward locale settings, and the user logs into a Linux server).
This should be sufficient to ensure that when the locale coercion is activated,
the switch to the UTF-8 based locale will be applied consistently across the
current process and any subprocesses that inherit the current environment.
Avoiding setting LANG for UTF-8 locale coercion
Earlier versions of this PEP proposed setting the ``LANG`` category indepdent
default locale, in addition to setting ``LC_CTYPE``.
This was later removed on the grounds that setting only ``LC_CTYPE`` is
sufficient to handle all of the problematic scenarios that the PEP aimed
to resolve, while setting ``LANG`` as well would break cases where ``LANG``
was set correctly, and the locale problems were solely due to an incorrect
``LC_CTYPE`` setting ([22_]).
For example, consider a Python application that called the Linux ``date``
utility in a subprocess rather than doing its own date formatting::
$ LANG=ja_JP.UTF-8 LC_CTYPE=C date
2017年 5月 23日 火曜日 17:31:03 JST
$ LANG=ja_JP.UTF-8 LC_CTYPE=C.UTF-8 date # Coercing only LC_CTYPE
2017年 5月 23日 火曜日 17:32:58 JST
$ LANG=C.UTF-8 LC_CTYPE=C.UTF-8 date # Coercing both of LC_CTYPE and LANG
Tue May 23 17:31:10 JST 2017
With only ``LC_CTYPE`` updated in the Python process, the subprocess would
continue to behave as expected. However, if ``LANG`` was updated as well,
that would effectively override the ``LC_TIME`` setting and use the wrong
date formatting conventions.
Avoiding setting LC_ALL for UTF-8 locale coercion
Earlier versions of this PEP proposed setting the ``LC_ALL`` locale override,
in addition to setting ``LC_CTYPE``.
This was changed after it was determined that just setting ``LC_CTYPE`` and
``LANG`` should be sufficient to handle all the scenarios the PEP aims to
cover, as it avoids causing any problems in cases like the following::
$ LANG=C LC_MONETARY=ja_JP.utf8 ./python -c \
"from locale import setlocale, LC_ALL, currency; setlocale(LC_ALL, ''); print(currency(1e6))"
Skipping locale coercion if LC_ALL is set in the current environment
With locale coercion now only setting ``LC_CTYPE`` and ``LANG``, it will have
no effect if ``LC_ALL`` is also set. To avoid emitting a spurious locale
coercion notice in that case, coercion is instead skipped entirely.
Considering locale coercion independently of "UTF-8 mode"
With both this PEP's locale coercion and PEP 540's UTF-8 mode under
consideration for Python 3.7, it makes sense to ask whether or not we can
limit ourselves to only doing one or the other, rather than making both
The UTF-8 mode proposed in PEP 540 has two major limitations that make it a
potential complement to this PEP rather than a potential replacement.
First, unlike this PEP, PEP 540's UTF-8 mode makes it possible to change default
behaviours that are not currently configurable at all. While that's exactly
what makes the proposal interesting, it's also what makes it an entirely
unproven approach. By contrast, the approach proposed in this PEP builds
directly atop existing configuration settings for the C locale system (
``LC_CTYPE``, ``LANG``) and Python's standard streams (``PYTHONIOENCODING``)
that have already been in use for years to handle the kinds of compatibility
problems discussed in this PEP.
Secondly, one of the things we know based on that experience is that the
proposed locale coercion can resolve problems not only in CPython itself,
but also in extension modules that interact with the standard streams, like
GNU readline. As an example, consider the following interactive session
from a PEP 538 enabled CPython build, where each line after the first is
executed by doing "up-arrow, left-arrow x4, delete, enter"::
$ LANG=C ./python
Python 3.7.0a0 (heads/pep538-coerce-c-locale:188e780, May 7 2017, 00:21:13)
[GCC 6.3.1 20161221 (Red Hat 6.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("ℙƴ☂ℌøἤ")
>>> print("ℙƴ☂ℌἤ")
>>> print("ℙƴ☂ἤ")
>>> print("ℙƴἤ")
>>> print("ℙἤ")
>>> print("ἤ")
This is exactly what we'd expect from a well-behaved command history editor.
By contrast, the following is what currently happens on an older release if
you only change the Python level stream encoding settings without updating the
locale settings::
$ LANG=C PYTHONIOENCODING=utf-8:surrogateescape python3
Python 3.5.3 (default, Apr 24 2017, 13:32:13)
[GCC 6.3.1 20161221 (Red Hat 6.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("ℙƴ☂ℌøἤ")
>>> print("ℙƴ☂ℌ�")
File "<stdin>", line 0
SyntaxError: 'utf-8' codec can't decode bytes in position 20-21:
invalid continuation byte
That particular misbehaviour is coming from GNU readline, *not* CPython -
because the command history editing wasn't UTF-8 aware, it corrupted the history
buffer and fed such nonsense to stdin that even the surrogateescape error
handler was bypassed. While PEP 540's UTF-8 mode could technically be updated
to also reconfigure readline, that's just *one* extension module that might
be interacting with the standard streams without going through the CPython
C API, and any change made by CPython would only apply when readline is running
directly as part of Python 3.7 rather than in a separate subprocess.
However, if we actually change the configured locale, GNU readline starts
behaving itself, without requiring any changes to the embedding application::
$ LANG=C.UTF-8 python3
Python 3.5.3 (default, Apr 24 2017, 13:32:13)
[GCC 6.3.1 20161221 (Red Hat 6.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("ℙƴ☂ℌøἤ")
>>> print("ℙƴ☂ℌἤ")
>>> print("ℙƴ☂ἤ")
>>> print("ℙƴἤ")
>>> print("ℙἤ")
>>> print("ἤ")
$ LC_CTYPE=C.UTF-8 python3
Python 3.5.3 (default, Apr 24 2017, 13:32:13)
[GCC 6.3.1 20161221 (Red Hat 6.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("ℙƴ☂ℌøἤ")
>>> print("ℙƴ☂ℌἤ")
>>> print("ℙƴ☂ἤ")
>>> print("ℙƴἤ")
>>> print("ℙἤ")
>>> print("ἤ")
Enabling C locale coercion and warnings on Mac OS X, iOS and Android
On Mac OS X, iOS, and Android, CPython already assumes the use of UTF-8 for
system interfaces, and we expect most other locale-aware components to do the
Accordingly, this PEP originally proposed to disable locale coercion and
warnings at build time for these platforms, on the assumption that it would
be entirely redundant.
However, that assumption turned out to be incorrect, as subsequent
investigations showed that if you explicitly configure ``LANG=C`` on
these platforms, extension modules like GNU readline will misbehave in much the
same way as they do on other \*nix systems. [21_]
In addition, Mac OS X is also frequently used as a development and testing
platform for Python software intended for deployment to other \*nix environments
(such as Linux or Android), and Linux is similarly often used as a development
and testing platform for mobile and Mac OS X applications.
Accordingly, this PEP enables the locale coercion and warning features by
default on all platforms that use CPython's ``autotools`` based build toolchain
(i.e. everywhere other than Windows).
The reference implementation is being developed in the
``pep538-coerce-c-locale`` feature branch [18_] in Nick Coghlan's fork of the
CPython repository on GitHub. A work-in-progress PR is available at [20_].
This reference implementation covers not only the enhancement request in
issue 28180 [1_], but also the Android compatibility fixes needed to resolve
issue 28997 [16_].
Backporting to earlier Python 3 releases
Backporting to Python 3.6.x
If this PEP is accepted for Python 3.7, redistributors backporting the change
specifically to their initial Python 3.6.x release will be both allowed and
encouraged. However, such backports should only be undertaken either in
conjunction with the changes needed to also provide a suitable locale by
default, or else specifically for platforms where such a locale is already
consistently available.
At least the Fedora project is planning to pursue this approach for the
upcoming Fedora 26 release [19_].
Backporting to other 3.x releases
While the proposed behavioural change is seen primarily as a bug fix addressing
Python 3's current misbehaviour in the default ASCII-based C locale, it still
represents a reasonably significant change in the way CPython interacts with
the C locale system. As such, while some redistributors may still choose to
backport it to even earlier Python 3.x releases based on the needs and
interests of their particular user base, this wouldn't be encouraged as a
general practice.
However, configuring Python 3 *environments* (such as base container
images) to use these configuration settings by default is both allowed
and recommended.
The locale coercion approach proposed in this PEP is inspired directly by
Armin Ronacher's handling of this problem in the ``click`` command line
utility development framework [2_]::
$ LANG=C python3 -c 'import click; cli = click.command()(lambda:None); cli()'
Traceback (most recent call last):
RuntimeError: Click will abort further execution because Python 3 was
configured to use ASCII as encoding for the environment. Either run this
under Python 2 or consult for mitigation
This system supports the C.UTF-8 locale which is recommended.
You might be able to resolve your issue by exporting the
following environment variables:
export LC_ALL=C.UTF-8
export LANG=C.UTF-8
The change was originally proposed as a downstream patch for Fedora's
system Python 3.6 package [3_], and then reformulated as a PEP for Python 3.7
with a section allowing for backports to earlier versions by redistributors.
In parallel with the development of the upstream patch, Charalampos Stratakis
has been working on the Fedora 26 backport and providing feedback on the
practical viability of the proposed changes.
The initial draft was posted to the Python Linux SIG for discussion [10_] and
then amended based on both that discussion and Victor Stinner's work in
PEP 540 [11_].
The "ℙƴ☂ℌøἤ" string used in the Unicode handling examples throughout this PEP
is taken from Ned Batchelder's excellent "Pragmatic Unicode" presentation [9_].
Stephen Turnbull has long provided valuable insight into the text encoding
handling challenges he regularly encounters at the University of Tsukuba
.. [1] CPython: sys.getfilesystemencoding() should default to utf-8
.. [2] Locale configuration required for click applications under Python 3
.. [3] Fedora: force C.UTF-8 when Python 3 is run under the C locale
.. [4] GNU C: How Programs Set the Locale
.. [5] GNU C: Locale Categories
.. [6] glibc C.UTF-8 locale proposal
.. [7] GNOME Flatpak
.. [8] Ubuntu Snappy
.. [9] Pragmatic Unicode
.. [10] linux-sig discussion of initial PEP draft
.. [11] Feedback notes from linux-sig discussion and PEP 540
.. [12] GB 18030
.. [13] Shift-JIS
.. [14] ISO-2022
.. [15] Use "surrogateescape" error handler for sys.stdin and sys.stdout on UNIX for the C locale
.. [16] test_readline.test_nonascii fails on Android
.. [17] UTF-8 locale discussion on "locale.getdefaultlocale() fails on Mac OS X with default language set to English"
.. [18] GitHub branch diff for ``ncoghlan:pep538-coerce-c-locale``
.. [19] Fedora 26 change proposal for locale coercion backport
.. [20] GitHub pull request for the reference implementation
.. [21] GNU readline misbehaviour on Mac OS X with ``LANG=C``
.. [22] Potential problems when setting LANG in addition to setting LC_CTYPE
This document has been placed in the public domain under the terms of the
CC0 1.0 license:
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