Hi folks,

Late last year I started working on a change to the CPython CLI (*not* the shared library) to get it to coerce the legacy C locale to something based on UTF-8 when a suitable locale is available.

After a couple of rounds of iteration on linux-sig and python-ideas, I'm now bringing it to python-dev as a concrete proposal for Python 3.7.

For most folks, reading the Abstract plus the draft docs updates in the reference implementation will tell you everything you need to know (if the C.UTF-8, C.utf8 or UTF-8 locales are available, the CLI will automatically attempt to coerce the legacy C locale to one of those rather than persisting with the latter's default assumption of ASCII as the preferred text encoding).

However, the full PEP goes into a lot more detail on:

* exactly what's broken about CPython's behaviour in the legacy C locale
* why I'm in favour of this particular approach to fixing it (i.e. it integrates better with other C/C++ components, as well as being amenable to redistributor backports for 3.6, and environment based configuration for 3.5 and earlier)
* why I think implementing both this change *and* Victor's more comprehensive "PYTHONUTF8 mode" proposal in PEP 540 will be better than implementing just one or the other (in some situations, ignoring the platform locale subsystem entirely really is the right approach, and that's the aspect PEP 540 tackles, while this PEP tackles the situations where the C locale behaviour is broken, but you still need to be consistent with the platform settings).


PEP: 538
Title: Coercing the legacy C locale to a UTF-8 based locale
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>
Status: Draft
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)


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 C/C++ components in the same process and those invoked in 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 C and C++ components in the same process, as well as those
of components running in subprocesses that share the same environment.

It also requires changes to the internals of how CPython itself works, rather
than using 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 such that:

* unless the new ``PYTHONCOERCECLOCALE`` environment variable is set to ``0``,
  the standalone CPython binary will automatically attempt to coerce the ``C``
  locale to the first available locale out of ``C.UTF-8``, ``C.utf8``, or
* if the locale is successfully coerced, and PEP 540 is not accepted, then
  ``PYTHONIOENCODING`` (if not otherwise set) will be set to
* if the locale is successfully coerced, and PEP 540 *is* accepted, then
  ``PYTHONUTF8`` (if not otherwise set) will be set to ``1``
* if the subsequent runtime initialization process detects that the legacy
  ``C`` locale remains active (e.g. none of ``C.UTF-8``, ``C.utf8`` or ``UTF-8``
  are available, locale coercion is disabled, or the runtime is embedded in an
  application other than the main CPython binary), and the ``PYTHONUTF8``
  feature defined in PEP 540 is also disabled (or not implemented), it  will
  emit a warning on stderr that use of the legacy ``C`` locale's default ASCII
  text encoding may cause various Unicode compatibility issues

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 either the new ``PYTHONUTF8`` mode defined in PEP 540 is
used, or else a suitable locale other than the default ``C`` locale is
configured explicitly (e.g. `en_AU.UTF-8`, ``zh_CN.gb18030``).

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 when first introducing Python 3.6.0.


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 Apple platforms (including both Mac OS X and iOS), this is straightforward,
as Apple guarantees that these operations will always use UTF-8 to do the

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 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

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_ALL=C.UTF-8 LANG=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.

Mac OS X and other \*BSD systems have taken a different approach, and instead
of offering a ``C.UTF-8`` locale, instead 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``.

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 C/C++ components running in the same process
  and in subprocesses. This approach aims to make CPython behave less like a
  locale-aware C/C++ application, and more like C/C++ independent language
  runtimes like the JVM, .NET CLR, Go, Node.js, 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 C/C++ 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 components written
  in C and C++, while 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 C/C++ components,
and an approach more amenable to being backported to Python 3.6 by downstream

As a result, this PEP was amended to refer to PEP 540 as a complementary
solution that offered improved behaviour both when locale coercion triggered,
as well as when none of the standard UTF-8 based locales were available.

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 CPython instead relying solely on the proposed PYTHONUTF8 mode in such


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
Ubunty 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 LANG=C.UTF-8 fedora:25 python3 -c 'print("ℙƴ☂ℌøἤ")'
    $ docker run --rm -e LANG=C.UTF-8 ncoghlan/debian-python python3 -c 'print("ℙƴ☂ℌøἤ")'

    $ docker run --rm -e LANG=C.UTF-8 fedora:25 locale | grep -E 'LC_ALL|LC_CTYPE|LANG'
    $ docker run --rm -e LANG=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 as an on-disk locale the
way Debian and Fedora do. For these platforms, the mechanism proposed in
PEP 540 at least allows CPython itself to behave sensibly, albeit without any
mechanism to get other C/C++ components that decode binary streams as text to
do the same.

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
* 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
* any changes made will use *existing* configuration options

To minimize the negative impact on systems currently correctly configured to
use GB-18030 or another partially ASCII compatible universal encoding leads to
an additional 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.


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 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 at the point where the language runtime itself is initialized,
and the PEP 540 UTF-8 encoding override is also disabled, in order to warn
system and application integrators that they're running CPython in an
unsupported configuration.

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 C/C++ 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, 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)

For ``C.UTF-8`` and ``C.utf8``, the coercion will be implemented by actually
setting the ``LANG`` and ``LC_ALL`` environment variables to the candidate
locale name, such that future calls to ``setlocale()`` will see them, as will
other components looking for those settings (such as GUI development

For the platforms where it is defined, ``UTF-8`` is a partial locale that only
defines the ``LC_CTYPE`` category. Accordingly, only the ``LC_CTYPE``
environment variable would be set when using this fallback option.

To adjust automatically to future changes in locale availability, these checks
will be implemented at runtime on all platforms other than Mac OS X and Windows,
rather than attempting to determine which locales to try at compile time.

If the locale settings are changed successfully, and the ``PYTHONIOENCODING``
environment variable is currently unset, then it will be forced to

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_ALL & LANG set to C.UTF-8 (set another
    locale or PYTHONCOERCECLOCALE=0 to disable this locale coercion behaviour).

When falling back to the ``UTF-8`` locale, the message would be slightly

    Python detected LC_CTYPE=C, LC_CTYPE set to UTF-8 (set another locale
    or PYTHONCOERCECLOCALE=0 to disable this locale coercion behaviour).

In combination with PEP 540, this locale coercion will mean that the standard
Python binary *and* locale aware C/C++ extensions should once again "just work"
in the three main failure cases we're aware of (missing locale
settings, SSH forwarding of unknown locales, and developers explicitly
requesting ``LANG=C``), as long as the target platform provides at least one
of the candidate UTF-8 based environments.

If ``PYTHONCOERCECLOCALE=0`` is set, or none of the candidate locales is
successfully configured, then initialization will continue as usual 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.

The interpreter will always check for the ``PYTHONCOERCECLOCALE`` environment
variable (even when running under the ``-E`` or ``-I`` switches), as the locale
coercion check necessarily takes place before any command line argument

Changes to the runtime initialization process

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 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* the ``PYTHONUTF8``
feature from PEP 540 is disabled, 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.

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 would be conditionally compiled
based on the operating system (e.g. recommending ``LC_CTYPE=UTF-8`` on \*BSD

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.

On platforms where they would have no effect (e.g. Mac OS X, iOS, Android,
Windows) these preprocessor variables would always be undefined.

Platform Support Changes

A new "Legacy C Locale" section will be added to PEP 11 that states:

* as of CPython 3.7, the legacy C locale is only supported when operating in
  "UTF-8" mode. Any Unicode handling issues that occur only in that locale
  and cannot be reproduced in an appropriately configured non-ASCII locale will
  be closed as "won't fix"
* 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 with C/C++ extensions that occur
  only in that 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 underlying platform.

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
C/C++ 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 UTF-8 mode.

Rather than introducing yet another configuration option to address that,
this PEP proposes to use the existing ``PYTHONIOENCODING`` setting to ensure
that the ``surrogateescape`` handler is enabled when the interpreter is
required to make assumptions regarding the expected filesystem encoding.

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(sys.stdin.read())" \
      | iconv -f GB18030 -t UTF-8

Rather than reverting to the exception seen when a UTF-8 based locale is
explicitly configured::

    $ cat gb18030.txt \
      | python3 -c "import sys; print(sys.stdin.read())" \
      | iconv -f GB18030 -t UTF-8
    Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/usr/lib64/python3.5/codecs.py", 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

Note: an alternative to setting ``PYTHONIOENCODING`` as the PEP currently
proposes would be to instead *always* default to ``surrogateescape`` on the
standard streams, and require the use of ``PYTHONIOENCODING=:strict`` to request
text encoding validation during stream processing. Adopting such an approach
would bring Python 3 more into line with typical C/C++ tools that pass along
the raw bytes without checking them for conformance to their nominal encoding,
and would hence also make the last example display the desired output::

    $ cat gb18030.txt \
      | PYTHONIOENCODING=:surrogateescape python3 -c "import sys; print(sys.stdin.read())" \
      | iconv -f GB18030 -t UTF-8

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 ignore the nominal
C/C++ locale encoding entirely 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 need to do so can still opt-in to
running their Python code in the legacy C locale, it also makes clear that we
*don't* expect Python 3's Unicode handling to be reliable in that configuration,
and the recommended alternative is to use a more appropriate locale setting.

Providing implicit locale coercion only when running standalone

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.

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.

Setting both LANG & LC_ALL for C.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_ALL`` to ``C.UTF-8`` imposes a locale setting override on all
C/C++ components in the current process and in any subprocesses that inherit
the current environment. This 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).

Setting ``LANG`` to ``C.UTF-8`` ensures that even components that only check
the ``LANG`` fallback for their locale settings will still use ``C.UTF-8``.

Together, these should ensure that when the locale coercion is activated, the
switch to the C.UTF-8 locale will be applied consistently across the current
process and any subprocesses that inherit the current environment.

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.


A draft implementation of the change (including test cases and documentation)
is linked from issue 28180 [1_], which is an end user request that
``sys.getfilesystemencoding()`` default to ``utf-8`` rather than ``ascii``.

This patch is now being maintained as the ``pep538-coerce-c-locale`` feature
branch [18_] in Nick Coghlan's fork of the CPython repository on GitHub.

NOTE: As discussed in [1_], the currently posted draft implementation has some
known issues on Android.

Backporting to earlier Python 3 releases

Backporting to Python 3.6.0

If this PEP is accepted for Python 3.7, redistributors backporting the change
specifically to their initial Python 3.6.0 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.

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 http://click.pocoo.org/python3/ 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.

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
   ( https://www.gnu.org/software/libc/manual/html_node/Setting-the-Locale.html)

.. [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``


This document has been placed in the public domain under the terms of the
CC0 1.0 license: https://creativecommons.org/publicdomain/zero/1.0/

Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia