PEP: 432
Title: Restructuring the CPython startup sequence
Version: $Revision$
Last-Modified: $Date$
Author: Alyssa Coghlan <ncoghlan@gmail.com>,
        Victor Stinner <vstinner@python.org>,
        Eric Snow <ericsnowcurrently@gmail.com>
Discussions-To: capi-sig@python.org
Status: Withdrawn
Type: Standards Track
Content-Type: text/x-rst
Requires: 587
Created: 28-Dec-2012
Post-History: 28-Dec-2012, 02-Jan-2013, 30-Mar-2019, 28-Jun-2020

.. highlight:: c

PEP Withdrawal
==============

From late 2012 to mid 2020, this PEP provided general background and specific
concrete proposals for making the CPython startup sequence easier to maintain
and the CPython runtime easier to embed as part of a larger application.

For most of that time, the changes were maintained either in a separate feature
branch, or else as underscore-prefixed private APIs in the main CPython repo.

In 2019, :pep:`587` migrated a subset of those API changes to the public CPython
API for Python 3.8+ (specifically, the PEP updated the interpreter runtime to
offer an explicitly multi-stage struct-based configuration interface).

In June 2020, in response to a query from the Steering Council, the PEP authors
decided that it made sense to withdraw the original PEP, as enough has changed
since :pep:`432` was first written that we think any further changes to the
startup sequence and embedding API would be best formulated as a new PEP (or
PEPs) that take into account not only the not-yet-implemented ideas from :pep:`432`
that weren't considered sufficiently well validated to make their way into
:pep:`587`, but also any feedback on the public :pep:`587` API, and any other lessons
that have been learned while adjusting the CPython implementation to be more
embedding and subinterpreter friendly.

In particular, PEPs proposing the following changes, and any further
infrastructure changes needed to enable them, would likely still be worth
exploring:

* shipping an alternate Python executable that ignores all user level
  settings and runs in isolated mode by default, and would hence be more
  suitable for execution of system level Python applications than the default
  interpreter
* enhancing the zipapp module to support the creation of single-file executables
  from pure Python scripts (and potentially even Python extension modules, given
  the introduction of multi-phase extension module initialisation)
* migrating the complex sys.path initialisation logic from C to Python in order
  to improve test suite coverage and the general maintainability of that code


Abstract
========

This PEP proposes a mechanism for restructuring the startup sequence for
CPython, making it easier to modify the initialization behaviour of the
reference interpreter executable, as well as making it easier to control
CPython's startup behaviour when creating an alternate executable or
embedding it as a Python execution engine inside a larger application.

When implementation of this proposal is completed, interpreter startup will
consist of three clearly distinct and independently configurable phases:

* Python core runtime preinitialization

  * setting up memory management
  * determining the encodings used for system interfaces (including settings
    passed in for later configuration phase)

* Python core runtime initialization

  * ensuring C API is ready for use
  * ensuring builtin and frozen modules are accessible

* Main interpreter configuration

  * ensuring external modules are accessible
  * (Note: the name of this phase is quite likely to change)

Changes are also proposed that impact main module execution and subinterpreter
initialization.

Note: TBC = To Be Confirmed, TBD = To Be Determined. The appropriate
resolution for most of these should become clearer as the reference
implementation is developed.


Proposal
========

This PEP proposes that initialization of the CPython runtime be split into
three clearly distinct phases:

* core runtime preinitialization
* core runtime initialization
* main interpreter configuration

(Earlier versions proposed only two phases, but experience with attempting to
implement the PEP as an internal CPython refactoring showed that at least 3
phases are needed to get clear separation of concerns)

The proposed design also has significant implications for:

* main module execution
* subinterpreter initialization

In the new design, the interpreter will move through the following
well-defined phases during the initialization sequence:

* Uninitialized - haven't even started the pre-initialization phase yet
* Pre-Initialization - no interpreter available
* Runtime Initialized - main interpreter partially available,
  subinterpreter creation not yet available
* Initialized - main interpreter fully available, subinterpreter creation
  available

:pep:`587` is a more detailed proposal that covers separating out the
Pre-Initialization phase from the last two phases, but doesn't allow embedding
applications to run arbitrary code while in the "Runtime Initialized" state
(instead, initializing the core runtime will also always fully initialize the
main interpreter, as that's the way the native CPython CLI still works in
Python 3.8).

As a concrete use case to help guide any design changes, and to solve a known
problem where the appropriate defaults for system utilities differ from those
for running user scripts, this PEP proposes the creation and
distribution of a separate system Python (``system-python``) executable
which, by default, operates in "isolated mode" (as selected by the CPython
``-I`` switch), as well as the creation of an example stub binary that just
runs an appended zip archive (permitting single-file pure Python executables)
rather than going through the normal CPython startup sequence.

To keep the implementation complexity under control, this PEP does *not*
propose wholesale changes to the way the interpreter state is accessed at
runtime. Changing the order in which the existing initialization steps
occur in order to make the startup sequence easier to maintain is already a
substantial change, and attempting to make those other changes at the same time
will make the change significantly more invasive and much harder to review.
However, such proposals may be suitable topics for follow-on PEPs or patches
- one key benefit of this PEP and its related subproposals is decreasing the
coupling between the internal storage model and the configuration interface,
so such changes should be easier once this PEP has been implemented.


Background
==========

Over time, CPython's initialization sequence has become progressively more
complicated, offering more options, as well as performing more complex tasks
(such as configuring the Unicode settings for OS interfaces in Python 3 [10]_,
bootstrapping a pure Python implementation of the import system, and
implementing an isolated mode more suitable for system applications that run
with elevated privileges [6]_).

Much of this complexity is formally accessible only through the ``Py_Main``
and ``Py_Initialize`` APIs, offering embedding applications little
opportunity for customisation. This creeping complexity also makes life
difficult for maintainers, as much of the configuration needs to take
place prior to the ``Py_Initialize`` call, meaning much of the Python C
API cannot be used safely.

A number of proposals are on the table for even *more* sophisticated
startup behaviour, such as better control over ``sys.path``
initialization (e.g. easily adding additional directories on the command line
in a cross-platform fashion [7]_, controlling the configuration of
``sys.path[0]`` [8]_), easier configuration of utilities like coverage
tracing when launching Python subprocesses [9]_).

Rather than continuing to bolt such behaviour onto an already complicated
system indefinitely, this PEP proposes to start simplifying the status quo by
introducing a more structured startup sequence, with the aim of making these
further feature requests easier to implement.

Originally the entire proposal was maintained in this one PEP, but that proved
impractical, so as parts of the proposed design stabilised, they are now split
out into their own PEPs, allowing progress to be made, even while the details
of the overall design are still evolving.


Key Concerns
============

There are a few key concerns that any change to the startup sequence
needs to take into account.


Maintainability
---------------

The CPython startup sequence as of Python 3.6 was difficult to understand, and
even more difficult to modify. It was not clear what state the interpreter was
in while much of the initialization code executed, leading to behaviour such
as lists, dictionaries and Unicode values being created prior to the call
to ``Py_Initialize`` when the ``-X`` or ``-W`` options are used [1]_.

By moving to an explicitly multi-phase startup sequence, developers should
only need to understand:

* which APIs and features are available prior to pre-configuration (essentially
  none, except for the pre-configuration API itself)
* which APIs and features are available prior to core runtime configuration, and
  will implicitly run the pre-configuration with default settings that match the
  behaviour of Python 3.6 if the pre-configuration hasn't been run explicitly
* which APIs and features are only available after the main interpreter has been
  fully configured (which will hopefully be a relatively small subset of the
  full C API)

The first two aspects of that are covered by :pep:`587`, while the details of the
latter distinction are still being considered.

By basing the new design on a combination of C structures and Python
data types, it should also be easier to modify the system in the
future to add new configuration options.


Testability
-----------

One of the problems with the complexity of the CPython startup sequence is the
combinatorial explosion of possible interactions between different configuration
settings.

This concern impacts both the design of the new initialisation system, and
the proposed approach for getting there.


Performance
-----------

CPython is used heavily to run short scripts where the runtime is dominated
by the interpreter initialization time. Any changes to the startup sequence
should minimise their impact on the startup overhead.

Experience with the importlib migration suggests that the startup time is
dominated by IO operations. However, to monitor the impact of any changes,
a simple benchmark can be used to check how long it takes to start and then
tear down the interpreter:

.. code-block:: bash

   python3 -m timeit -s "from subprocess import call" "call(['./python', '-Sc', 'pass'])"

Current numbers on my system for Python 3.7 (as built by the Fedora project):

.. code-block:: console

    $ python3 -m timeit -s "from subprocess import call" "call(['python3', '-Sc', 'pass'])"
    50 loops, best of 5: 6.48 msec per loop

(TODO: run this microbenchmark with perf rather than the stdlib timeit)

This PEP is not expected to have any significant effect on the startup time,
as it is aimed primarily at *reordering* the existing initialization
sequence, without making substantial changes to the individual steps.

However, if this simple check suggests that the proposed changes to the
initialization sequence may pose a performance problem, then a more
sophisticated microbenchmark will be developed to assist in investigation.


Required Configuration Settings
===============================

See :pep:`587` for a detailed listing of CPython interpreter configuration settings
and the various means available for setting them.


Implementation Strategy
=======================

An initial attempt was made at implementing an earlier version of this PEP for
Python 3.4 [2]_, with one of the significant problems encountered being merge
conflicts after the initial structural changes were put in place to start the
refactoring process. Unlike some other previous major changes, such as the
switch to an AST-based compiler in Python 2.5, or the switch to the importlib
implementation of the import system in Python 3.3, there is no clear way to
structure a draft implementation that won't be prone to the kinds of merge
conflicts that afflicted the original attempt.

Accordingly, the implementation strategy was revised to instead first implement
this refactoring as a private API for CPython 3.7, and then review the viability
of exposing the new functions and structures as public API elements in CPython
3.8.

After the initial merge, Victor Stinner then proceeded to actually migrate
settings to the new structure in order to successfully implement the :pep:`540`
UTF-8 mode changes (which required the ability to track all settings that had
previously been decoded with the locale encoding, and decode them again using
UTF-8 instead). Eric Snow also migrated a number of internal subsystems over as
part of making the subinterpreter feature more robust.

That work showed that the detailed design originally proposed in this PEP had a
range of practical issues, so Victor designed and implemented an improved
private API (inspired by an earlier iteration of this PEP), which :pep:`587`
proposes to promote to a public API in Python 3.8.


Design Details
==============

.. note::

    The API details here are still very much in flux. The header files that show
    the current state of the private API are mainly:

    * https://github.com/python/cpython/blob/master/Include/cpython/coreconfig.h
    * https://github.com/python/cpython/blob/master/Include/cpython/pystate.h
    * https://github.com/python/cpython/blob/master/Include/cpython/pylifecycle.h

    :pep:`587` covers the aspects of the API that are considered potentially stable
    enough to make public. Where a proposed API is covered by that PEP,
    "(see PEP 587)" is added to the text below.

The main theme of this proposal is to initialize the core language runtime
and create a partially initialized interpreter state for the main interpreter
*much* earlier in the startup process. This will allow most of the CPython API
to be used during the remainder of the initialization process, potentially
simplifying a number of operations that currently need to rely on basic C
functionality rather than being able to use the richer data structures provided
by the CPython C API.

:pep:`587` covers a subset of that task, which is splitting out the components that
even the existing "May be called before ``Py_Initialize``" interfaces need (like
memory allocators and operating system interface encoding details) into a
separate pre-configuration step.

In the following, the term "embedding application" also covers the standard
CPython command line application.


Interpreter Initialization Phases
---------------------------------

The following distinct interpreter initialisation phases are proposed:

* Uninitialized:

  * Not really a phase, but the absence of a phase
  * ``Py_IsInitializing()`` returns ``0``
  * ``Py_IsRuntimeInitialized()`` returns ``0``
  * ``Py_IsInitialized()`` returns ``0``
  * The embedding application determines which memory allocator to use, and
    which encoding to use to access operating system interfaces (or chooses
    to delegate those decisions to the Python runtime)
  * Application starts the initialization process by calling one of the
    ``Py_PreInitialize`` APIs (see :pep:`587`)

* Runtime Pre-Initialization:

  * no interpreter is available
  * ``Py_IsInitializing()`` returns ``1``
  * ``Py_IsRuntimeInitialized()`` returns ``0``
  * ``Py_IsInitialized()`` returns ``0``
  * The embedding application determines the settings required to initialize
    the core CPython runtime and create the main interpreter and moves to the
    next phase by calling ``Py_InitializeRuntime``
  * Note: as of :pep:`587`, the embedding application instead calls ``Py_Main()``,
    ``Py_UnixMain``, or one of the ``Py_Initialize`` APIs, and hence jumps
    directly to the Initialized state.

* Main Interpreter Initialization:

  * the builtin data types and other core runtime services are available
  * the main interpreter is available, but only partially configured
  * ``Py_IsInitializing()`` returns ``1``
  * ``Py_IsRuntimeInitialized()`` returns ``1``
  * ``Py_IsInitialized()`` returns ``0``
  * The embedding application determines and applies the settings
    required to complete the initialization process by calling
    ``Py_InitializeMainInterpreter``
  * Note: as of :pep:`587`, this state is not reachable via any public API, it
    only exists as an implicit internal state while one of the ``Py_Initialize``
    functions is running

* Initialized:

  * the main interpreter is available and fully operational, but
    ``__main__`` related metadata is incomplete
  * ``Py_IsInitializing()`` returns ``0``
  * ``Py_IsRuntimeInitialized()`` returns ``1``
  * ``Py_IsInitialized()`` returns ``1``


Invocation of Phases
--------------------

All listed phases will be used by the standard CPython interpreter and the
proposed System Python interpreter.

An embedding application may still continue to leave initialization almost
entirely under CPython's control by using the existing ``Py_Initialize``
or ``Py_Main()`` APIs - backwards compatibility will be preserved.

Alternatively, if an embedding application wants greater control
over CPython's initial state, it will be able to use the new, finer
grained API, which allows the embedding application greater control
over the initialization process.

:pep:`587` covers an initial iteration of that API, separating out the
pre-initialization phase without attempting to separate core runtime
initialization from main interpreter initialization.


Uninitialized State
-------------------

The uninitialized state is where an embedding application determines the settings
which are required in order to be able to correctly pass configurations settings
to the embedded Python runtime.

This covers telling Python which memory allocator to use, as well as which text
encoding to use when processing provided settings.

:pep:`587` defines the settings needed to exit this state in its ``PyPreConfig``
struct.

A new query API will allow code to determine if the interpreter hasn't even
started the initialization process::

    int Py_IsInitializing();

The query for a completely uninitialized environment would then be
``!(Py_Initialized() || Py_Initializing())``.


Runtime Pre-Initialization Phase
--------------------------------

.. note:: In :pep:`587`, the settings for this phase are not yet separated out,
   and are instead only available through the combined ``PyConfig`` struct

The pre-initialization phase is where an embedding application determines
the settings which are absolutely required before the CPython runtime can be
initialized at all. Currently, the primary configuration settings in this
category are those related to the randomised hash algorithm - the hash
algorithms must be consistent for the lifetime of the process, and so they
must be in place before the core interpreter is created.

The essential settings needed are a flag indicating whether or not to use a
specific seed value for the randomised hashes, and if so, the specific value
for the seed (a seed value of zero disables randomised hashing). In addition,
due to the possible use of ``PYTHONHASHSEED`` in configuring the hash
randomisation, the question of whether or not to consider environment
variables must also be addressed early. Finally, to support the CPython
build process, an option is offered to completely disable the import
system.

The proposed APIs for this step in the startup sequence are::

    PyInitError Py_InitializeRuntime(
        const PyRuntimeConfig *config
    );

    PyInitError Py_InitializeRuntimeFromArgs(
        const PyRuntimeConfig *config, int argc, char **argv
    );

    PyInitError Py_InitializeRuntimeFromWideArgs(
        const PyRuntimeConfig *config, int argc, wchar_t **argv
    );

If ``Py_IsInitializing()`` is false, the ``Py_InitializeRuntime`` functions will
implicitly call the corresponding ``Py_PreInitialize`` function. The
``use_environment`` setting will be passed down, while other settings will be
processed according to their defaults, as described in :pep:`587`.

The ``PyInitError`` return type is defined in :pep:`587`, and allows an embedding
application to gracefully handle Python runtime initialization failures,
rather than having the entire process abruptly terminated by ``Py_FatalError``.

The new ``PyRuntimeConfig`` struct holds the settings required for preliminary
configuration of the core runtime and creation of the main interpreter::

    /* Note: if changing anything in PyRuntimeConfig, also update
     * PyRuntimeConfig_INIT */
    typedef struct {
        bool use_environment;     /* as in PyPreConfig, PyConfig from PEP 587 */
        int use_hash_seed;        /* PYTHONHASHSEED, as in PyConfig from PEP 587 */
        unsigned long hash_seed;  /* PYTHONHASHSEED, as in PyConfig from PEP 587 */
        bool _install_importlib;  /* Needed by freeze_importlib */
    } PyRuntimeConfig;

    /* Rely on the "designated initializer" feature of C99 */
    #define PyRuntimeConfig_INIT {.use_hash_seed=-1}

The core configuration settings pointer may be ``NULL``, in which case the
default values are as specified in ``PyRuntimeConfig_INIT``.

The ``PyRuntimeConfig_INIT`` macro is designed to allow easy initialization
of a struct instance with sensible defaults::

    PyRuntimeConfig runtime_config = PyRuntimeConfig_INIT;

``use_environment`` controls the processing of all Python related
environment variables. If the flag is true, then ``PYTHONHASHSEED`` is
processed normally. Otherwise, all Python-specific environment variables
are considered undefined (exceptions may be made for some OS specific
environment variables, such as those used on Mac OS X to communicate
between the App bundle and the main Python binary).

``use_hash_seed`` controls the configuration of the randomised hash
algorithm. If it is zero, then randomised hashes with a random seed will
be used. It is positive, then the value in ``hash_seed`` will be used
to seed the random number generator. If the ``hash_seed`` is zero in this
case, then the randomised hashing is disabled completely.

If ``use_hash_seed`` is negative (and ``use_environment`` is true),
then CPython will inspect the ``PYTHONHASHSEED`` environment variable. If the
environment variable is not set, is set to the empty string, or to the value
``"random"``, then randomised hashes with a random seed will be used. If the
environment variable is set to the string ``"0"`` the randomised hashing will
be disabled. Otherwise, the hash seed is expected to be a string
representation of an integer in the range ``[0; 4294967295]``.

To make it easier for embedding applications to use the ``PYTHONHASHSEED``
processing with a different data source, the following helper function
will be added to the C API::

    int Py_ReadHashSeed(char *seed_text,
                        int *use_hash_seed,
                        unsigned long *hash_seed);

This function accepts a seed string in ``seed_text`` and converts it to
the appropriate flag and seed values. If ``seed_text`` is ``NULL``,
the empty string or the value ``"random"``, both ``use_hash_seed`` and
``hash_seed`` will be set to zero. Otherwise, ``use_hash_seed`` will be set to
``1`` and the seed text will be interpreted as an integer and reported as
``hash_seed``. On success the function will return zero. A non-zero return
value indicates an error (most likely in the conversion to an integer).

The ``_install_importlib`` setting is used as part of the CPython build
process to create an interpreter with no import capability at all. It is
considered private to the CPython development team (hence the leading
underscore), as the only currently supported use case is to permit compiler
changes that invalidate the previously frozen bytecode for
``importlib._bootstrap`` without breaking the build process.

The aim is to keep this initial level of configuration as small as possible
in order to keep the bootstrapping environment consistent across
different embedding applications. If we can create a valid interpreter state
without the setting, then the setting should appear solely in the comprehensive
``PyConfig`` struct rather than in the core runtime configuration.

A new query API will allow code to determine if the interpreter is in the
bootstrapping state between the core runtime initialization and the creation of
the main interpreter state and the completion of the bulk of the main
interpreter initialization process::

    int Py_IsRuntimeInitialized();

Attempting to call ``Py_InitializeRuntime()`` again when
``Py_IsRuntimeInitialized()`` is already true is reported as a user
configuration error. (TBC, as existing public initialisation APIs support being
called multiple times without error, and simply ignore changes to any
write-once settings. It may make sense to keep that behaviour rather than trying
to make the new API stricter than the old one)

As frozen bytecode may now be legitimately run in an interpreter which is not
yet fully initialized, ``sys.flags`` will gain a new ``initialized`` flag.

With the core runtime initialised, the main interpreter and most of the CPython
C API should be fully functional except that:

* compilation is not allowed (as the parser and compiler are not yet
  configured properly)
* creation of subinterpreters is not allowed
* creation of additional thread states is not allowed
* The following attributes in the ``sys`` module are all either missing or
  ``None``:
  * ``sys.path``
  * ``sys.argv``
  * ``sys.executable``
  * ``sys.base_exec_prefix``
  * ``sys.base_prefix``
  * ``sys.exec_prefix``
  * ``sys.prefix``
  * ``sys.warnoptions``
  * ``sys.dont_write_bytecode``
  * ``sys.stdin``
  * ``sys.stdout``
* The filesystem encoding is not yet defined
* The IO encoding is not yet defined
* CPython signal handlers are not yet installed
* Only builtin and frozen modules may be imported (due to above limitations)
* ``sys.stderr`` is set to a temporary IO object using unbuffered binary
  mode
* The ``sys.flags`` attribute exists, but the individual flags may not yet
  have their final values.
* The ``sys.flags.initialized`` attribute is set to ``0``
* The ``warnings`` module is not yet initialized
* The ``__main__`` module does not yet exist

<TBD: identify any other notable missing functionality>

The main things made available by this step will be the core Python
data types, in particular dictionaries, lists and strings. This allows them
to be used safely for all of the remaining configuration steps (unlike the
status quo).

In addition, the current thread will possess a valid Python thread state,
allowing any further configuration data to be stored on the main interpreter
object rather than in C process globals.

Any call to ``Py_InitializeRuntime()`` must have a matching call to
``Py_Finalize()``. It is acceptable to skip calling
``Py_InitializeMainInterpreter()`` in between (e.g. if attempting to build the
main interpreter configuration settings fails).


Determining the remaining configuration settings
------------------------------------------------

The next step in the initialization sequence is to determine the remaining
settings needed to complete the process. No changes are made to the
interpreter state at this point. The core APIs for this step are::

    int Py_BuildPythonConfig(
        PyConfigAsObjects *py_config, const PyConfig *c_config
    );

    int Py_BuildPythonConfigFromArgs(
        PyConfigAsObjects *py_config, const PyConfig *c_config, int argc, char **argv
    );

    int Py_BuildPythonConfigFromWideArgs(
        PyConfigAsObjects *py_config, const PyConfig *c_config, int argc, wchar_t **argv
    );

The ``py_config`` argument should be a pointer to a PyConfigAsObjects struct
(which may be a temporary one stored on the C stack). For any already configured
value (i.e. any non-NULL pointer), CPython will sanity check the supplied value,
but otherwise accept it as correct.

A struct is used rather than a Python dictionary as the struct is easier
to work with from C, the list of supported fields is fixed for a given
CPython version and only a read-only view needs to be exposed to Python
code (which is relatively straightforward, thanks to the infrastructure
already put in place to expose ``sys.implementation``).

Unlike ``Py_InitializeRuntime``, this call will raise a Python exception and
report an error return rather than returning a Python initialization specific
C struct if a problem is found with the config data.

Any supported configuration setting which is not already set will be
populated appropriately in the supplied configuration struct. The default
configuration can be overridden entirely by setting the value *before*
calling ``Py_BuildPythonConfig``. The provided value will then also be
used in calculating any other settings derived from that value.

Alternatively, settings may be overridden *after* the
``Py_BuildPythonConfig`` call (this can be useful if an embedding
application wants to adjust a setting rather than replace it completely,
such as removing ``sys.path[0]``).

The ``c_config`` argument is an optional pointer to a ``PyConfig`` structure,
as defined in :pep:`587`. If provided, it is used in preference to reading settings
directly from the environment or process global state.

Merely reading the configuration has no effect on the interpreter state: it
only modifies the passed in configuration struct. The settings are not
applied to the running interpreter until the ``Py_InitializeMainInterpreter``
call (see below).


Supported configuration settings
--------------------------------

The interpreter configuration is split into two parts: settings which are
either relevant only to the main interpreter or must be identical across the
main interpreter and all subinterpreters, and settings which may vary across
subinterpreters.

NOTE: For initial implementation purposes, only the flag indicating whether
or not the interpreter is the main interpreter will be configured on a per
interpreter basis. Other fields will be reviewed for whether or not they can
feasibly be made interpreter specific over the course of the implementation.

.. note:: The list of config fields below is currently out of sync with :pep:`587`.
   Where they differ, :pep:`587` takes precedence.

The ``PyConfigAsObjects`` struct mirrors the ``PyConfig`` struct from :pep:`587`,
but uses full Python objects to store values, rather than C level data types.
It adds ``raw_argv`` and ``argv`` list fields, so later initialisation steps
don't need to accept those separately.

Fields are always pointers to Python data types, with unset values indicated by
``NULL``::

    typedef struct {
        /* Argument processing */
        PyListObject *raw_argv;
        PyListObject *argv;
        PyListObject *warnoptions; /* -W switch, PYTHONWARNINGS */
        PyDictObject *xoptions;    /* -X switch */

        /* Filesystem locations */
        PyUnicodeObject *program_name;
        PyUnicodeObject *executable;
        PyUnicodeObject *prefix;           /* PYTHONHOME */
        PyUnicodeObject *exec_prefix;      /* PYTHONHOME */
        PyUnicodeObject *base_prefix;      /* pyvenv.cfg */
        PyUnicodeObject *base_exec_prefix; /* pyvenv.cfg */

        /* Site module */
        PyBoolObject *enable_site_config;  /* -S switch (inverted) */
        PyBoolObject *no_user_site;        /* -s switch, PYTHONNOUSERSITE */

        /* Import configuration */
        PyBoolObject *dont_write_bytecode; /* -B switch, PYTHONDONTWRITEBYTECODE */
        PyBoolObject *ignore_module_case;  /* PYTHONCASEOK */
        PyListObject *import_path;        /* PYTHONPATH (etc) */

        /* Standard streams */
        PyBoolObject    *use_unbuffered_io; /* -u switch, PYTHONUNBUFFEREDIO */
        PyUnicodeObject *stdin_encoding;    /* PYTHONIOENCODING */
        PyUnicodeObject *stdin_errors;      /* PYTHONIOENCODING */
        PyUnicodeObject *stdout_encoding;   /* PYTHONIOENCODING */
        PyUnicodeObject *stdout_errors;     /* PYTHONIOENCODING */
        PyUnicodeObject *stderr_encoding;   /* PYTHONIOENCODING */
        PyUnicodeObject *stderr_errors;     /* PYTHONIOENCODING */

        /* Filesystem access */
        PyUnicodeObject *fs_encoding;

        /* Debugging output */
        PyBoolObject *debug_parser;    /* -d switch, PYTHONDEBUG */
        PyLongObject *verbosity;       /* -v switch */

        /* Code generation */
        PyLongObject *bytes_warnings;  /* -b switch */
        PyLongObject *optimize;        /* -O switch */

        /* Signal handling */
        PyBoolObject *install_signal_handlers;

        /* Implicit execution */
        PyUnicodeObject *startup_file;  /* PYTHONSTARTUP */

        /* Main module
         *
         * If prepare_main is set, at most one of the main_* settings should
         * be set before calling PyRun_PrepareMain (Py_ReadMainInterpreterConfig
         * will set one of them based on the command line arguments if
         * prepare_main is non-zero when that API is called).
        PyBoolObject    *prepare_main;
        PyUnicodeObject *main_source; /* -c switch */
        PyUnicodeObject *main_path;   /* filesystem path */
        PyUnicodeObject *main_module; /* -m switch */
        PyCodeObject    *main_code;   /* Run directly from a code object */
        PyObject        *main_stream; /* Run from stream */
        PyBoolObject    *run_implicit_code; /* Run implicit code during prep */

        /* Interactive main
         *
         * Note: Settings related to interactive mode are very much in flux.
         */
        PyObject *prompt_stream;      /* Output interactive prompt */
        PyBoolObject *show_banner;    /* -q switch (inverted) */
        PyBoolObject *inspect_main;   /* -i switch, PYTHONINSPECT */

    } PyConfigAsObjects;

The ``PyInterpreterConfig`` struct holds the settings that may vary between
the main interpreter and subinterpreters. For the main interpreter, these
settings are automatically populated by ``Py_InitializeMainInterpreter()``.

::

    typedef struct {
        PyBoolObject *is_main_interpreter;    /* Easily check for subinterpreters */
    } PyInterpreterConfig;

As these structs consist solely of object pointers, no explicit initializer
definitions are needed - C99's default initialization of struct memory to zero
is sufficient.


Completing the main interpreter initialization
----------------------------------------------

The final step in the initialization process is to actually put the
configuration settings into effect and finish bootstrapping the main
interpreter up to full operation::

    int Py_InitializeMainInterpreter(const PyConfigAsObjects *config);

Like ``Py_BuildPythonConfig``, this call will raise an exception and
report an error return rather than exhibiting fatal errors if a problem is
found with the config data. (TBC, as existing public initialisation APIs support
being called multiple times without error, and simply ignore changes to any
write-once settings. It may make sense to keep that behaviour rather than trying
to make the new API stricter than the old one)

All configuration settings are required - the configuration struct
should always be passed through ``Py_BuildPythonConfig`` to ensure it
is fully populated.

After a successful call ``Py_IsInitialized()`` will become true and
``Py_IsInitializing()`` will become false. The caveats described above for the
interpreter during the phase where only the core runtime is initialized will
no longer hold.

Attempting to call ``Py_InitializeMainInterpreter()`` again when
``Py_IsInitialized()`` is true is an error.

However, some metadata related to the ``__main__`` module may still be
incomplete:

* ``sys.argv[0]`` may not yet have its final value

  * it will be ``-m`` when executing a module or package with CPython
  * it will be the same as ``sys.path[0]`` rather than the location of
    the ``__main__`` module when executing a valid ``sys.path`` entry
    (typically a zipfile or directory)
  * otherwise, it will be accurate:

    * the script name if running an ordinary script
    * ``-c`` if executing a supplied string
    * ``-`` or the empty string if running from stdin

* the metadata in the ``__main__`` module will still indicate it is a
  builtin module

This function will normally implicitly import site as its final operation
(after ``Py_IsInitialized()`` is already set). Setting the
"enable_site_config" flag to ``Py_False`` in the configuration settings will
disable this behaviour, as well as eliminating any side effects on global
state if ``import site`` is later explicitly executed in the process.


Preparing the main module
-------------------------

.. note:: In :pep:`587`, ``PyRun_PrepareMain`` and ``PyRun_ExecMain`` are not
   exposed separately, and are instead accessed through a ``Py_RunMain`` API
   that both prepares and executes main, and then finalizes the Python
   interpreter.

This subphase completes the population of the ``__main__`` module
related metadata, without actually starting execution of the ``__main__``
module code.

It is handled by calling the following API::

    int PyRun_PrepareMain();

This operation is only permitted for the main interpreter, and will raise
``RuntimeError`` when invoked from a thread where the current thread state
belongs to a subinterpreter.

The actual processing is driven by the main related settings stored in
the interpreter state as part of the configuration struct.

If ``prepare_main`` is zero, this call does nothing.

If all of ``main_source``, ``main_path``, ``main_module``,
``main_stream`` and ``main_code`` are NULL, this call does nothing.

If more than one of ``main_source``, ``main_path``, ``main_module``,
``main_stream`` or ``main_code`` are set, ``RuntimeError`` will be reported.

If ``main_code`` is already set, then this call does nothing.

If ``main_stream`` is set, and ``run_implicit_code`` is also set, then
the file identified in ``startup_file`` will be read, compiled and
executed in the ``__main__`` namespace.

If ``main_source``, ``main_path`` or ``main_module`` are set, then this
call will take whatever steps are needed to populate ``main_code``:

* For ``main_source``, the supplied string will be compiled and saved to
  ``main_code``.

* For ``main_path``:

  * if the supplied path is recognised as a valid ``sys.path`` entry, it
    is inserted as ``sys.path[0]``, ``main_module`` is set
    to ``__main__`` and processing continues as for ``main_module`` below.
  * otherwise, path is read as a CPython bytecode file
  * if that fails, it is read as a Python source file and compiled
  * in the latter two cases, the code object is saved to ``main_code``
    and ``__main__.__file__`` is set appropriately

* For ``main_module``:

  * any parent package is imported
  * the loader for the module is determined
  * if the loader indicates the module is a package, add ``.__main__`` to
    the end of ``main_module`` and try again (if the final name segment
    is already ``.__main__`` then fail immediately)
  * once the module source code is located, save the compiled module code
    as ``main_code`` and populate the following attributes in ``__main__``
    appropriately: ``__name__``, ``__loader__``, ``__file__``,
    ``__cached__``, ``__package__``.


(Note: the behaviour described in this section isn't new, it's a write-up
of the current behaviour of the CPython interpreter adjusted for the new
configuration system)


Executing the main module
-------------------------

.. note:: In :pep:`587`, ``PyRun_PrepareMain`` and ``PyRun_ExecMain`` are not
   exposed separately, and are instead accessed through a ``Py_RunMain`` API
   that both prepares and executes main, and then finalizes the Python
   interpreter.


This subphase covers the execution of the actual ``__main__`` module code.

It is handled by calling the following API::

    int PyRun_ExecMain();

This operation is only permitted for the main interpreter, and will raise
``RuntimeError`` when invoked from a thread where the current thread state
belongs to a subinterpreter.

The actual processing is driven by the main related settings stored in
the interpreter state as part of the configuration struct.

If both ``main_stream`` and ``main_code`` are NULL, this call does nothing.

If both ``main_stream`` and ``main_code`` are set, ``RuntimeError`` will
be reported.

If ``main_stream`` and ``prompt_stream`` are both set, main execution will
be delegated to a new internal API::

    int _PyRun_InteractiveMain(PyObject *input, PyObject* output);

If ``main_stream`` is set and ``prompt_stream`` is NULL, main execution will
be delegated to a new internal API::

    int _PyRun_StreamInMain(PyObject *input);

If ``main_code`` is set, main execution will be delegated to a new internal
API::

    int _PyRun_CodeInMain(PyCodeObject *code);

After execution of main completes, if ``inspect_main`` is set, or
the ``PYTHONINSPECT`` environment variable has been set, then
``PyRun_ExecMain`` will invoke
``_PyRun_InteractiveMain(sys.__stdin__, sys.__stdout__)``.


Internal Storage of Configuration Data
--------------------------------------

The interpreter state will be updated to include details of the configuration
settings supplied during initialization by extending the interpreter state
object with at least an embedded copy of the ``PyConfigAsObjects`` and
``PyInterpreterConfig`` structs.

For debugging purposes, the configuration settings will be exposed as
a ``sys._configuration`` simple namespace (similar to ``sys.flags`` and
``sys.implementation``. The attributes will be themselves by simple namespaces
corresponding to the two levels of configuration setting:

* ``all_interpreters``
* ``active_interpreter``

Field names will match those in the configuration structs, except for
``hash_seed``, which will be deliberately excluded.

An underscored attribute is chosen deliberately, as these configuration
settings are part of the CPython implementation, rather than part of the
Python language definition. If new settings are needed to support
cross-implementation compatibility in the standard library, then those
should be agreed with the other implementations and exposed as new required
attributes on ``sys.implementation``, as described in :pep:`421`.

These are *snapshots* of the initial configuration settings. They are not
modified by the interpreter during runtime (except as noted above).


Creating and Configuring Subinterpreters
----------------------------------------

As the new configuration settings are stored in the interpreter state, they
need to be initialised when a new subinterpreter is created. This turns out
to be trickier than one might expect due to ``PyThreadState_Swap(NULL);``
(which is fortunately exercised by CPython's own embedding tests, allowing
this problem to be detected during development).

To provide a straightforward solution for this case, the PEP proposes to
add a new API::

    Py_InterpreterState *Py_InterpreterState_Main();

This will be a counterpart to ``Py_InterpreterState_Head()``, only reporting the
oldest currently existing interpreter rather than the newest. If
``Py_NewInterpreter()`` is called from a thread with an existing thread
state, then the interpreter configuration for that thread will be
used when initialising the new subinterpreter. If there is no current
thread state, the configuration from ``Py_InterpreterState_Main()``
will be used.

While the existing ``Py_InterpreterState_Head()`` API could be used instead,
that reference changes as subinterpreters are created and destroyed, while
``PyInterpreterState_Main()`` will always refer to the initial interpreter
state created in ``Py_InitializeRuntime()``.

A new constraint is also added to the embedding API: attempting to delete
the main interpreter while subinterpreters still exist will now be a fatal
error.


Stable ABI
----------

Most of the APIs proposed in this PEP are excluded from the stable ABI, as
embedding a Python interpreter involves a much higher degree of coupling
than merely writing an extension module.

The only newly exposed APIs that will be part of the stable ABI are the
``Py_IsInitializing()`` and ``Py_IsRuntimeInitialized()`` queries.


Build time configuration
------------------------

This PEP makes no changes to the handling of build time configuration
settings, and thus has no effect on the contents of ``sys.implementation``
or the result of ``sysconfig.get_config_vars()``.


Backwards Compatibility
-----------------------

Backwards compatibility will be preserved primarily by ensuring that
``Py_BuildPythonConfig()`` interrogates all the previously defined
configuration settings stored in global variables and environment variables,
and that ``Py_InitializeMainInterpreter()`` writes affected settings back to
the relevant locations.

One acknowledged incompatibility is that some environment variables which
are currently read lazily may instead be read once during interpreter
initialization. As the reference implementation matures, these will be
discussed in more detail on a case-by-case basis. The environment variables
which are currently known to be looked up dynamically are:

* ``PYTHONCASEOK``: writing to ``os.environ['PYTHONCASEOK']`` will no longer
  dynamically alter the interpreter's handling of filename case differences
  on import (TBC)
* ``PYTHONINSPECT``: ``os.environ['PYTHONINSPECT']`` will still be checked
  after execution of the ``__main__`` module terminates

The ``Py_Initialize()`` style of initialization will continue to be
supported. It will use (at least some elements of) the new API
internally, but will continue to exhibit the same behaviour as it
does today, ensuring that ``sys.argv`` is not populated until a subsequent
``PySys_SetArgv`` call (TBC). All APIs that currently support being called
prior to ``Py_Initialize()`` will
continue to do so, and will also support being called prior to
``Py_InitializeRuntime()``.


A System Python Executable
==========================

When executing system utilities with administrative access to a system, many
of the default behaviours of CPython are undesirable, as they may allow
untrusted code to execute with elevated privileges. The most problematic
aspects are the fact that user site directories are enabled,
environment variables are trusted and that the directory containing the
executed file is placed at the beginning of the import path.

Issue 16499 [6]_ added a ``-I`` option to change the behaviour of
the normal CPython executable, but this is a hard to discover solution (and
adds yet another option to an already complex CLI). This PEP proposes to
instead add a separate ``system-python`` executable

Currently, providing a separate executable with different default behaviour
would be prohibitively hard to maintain. One of the goals of this PEP is to
make it possible to replace much of the hard to maintain bootstrapping code
with more normal CPython code, as well as making it easier for a separate
application to make use of key components of ``Py_Main``. Including this
change in the PEP is designed to help avoid acceptance of a design that
sounds good in theory but proves to be problematic in practice.

Cleanly supporting this kind of "alternate CLI" is the main reason for the
proposed changes to better expose the core logic for deciding between the
different execution modes supported by CPython:

* script execution
* directory/zipfile execution
* command execution ("-c" switch)
* module or package execution ("-m" switch)
* execution from stdin (non-interactive)
* interactive stdin

Actually implementing this may also reveal the need for some better
argument parsing infrastructure for use during the initializing phase.


Open Questions
==============

* Error details for ``Py_BuildPythonConfig`` and
  ``Py_InitializeMainInterpreter`` (these should become clearer as the
  implementation progresses)


Implementation
==============

The reference implementation is being developed as a private API refactoring
within the CPython reference interpreter (as attempting to maintain it as an
independent project proved impractical).

:pep:`587` extracts a subset of the proposal that is considered sufficiently stable
to be worth proposing as a public API for Python 3.8.


The Status Quo (as of Python 3.6)
=================================

The current mechanisms for configuring the interpreter have accumulated in
a fairly ad hoc fashion over the past 20+ years, leading to a rather
inconsistent interface with varying levels of documentation.

Also see :pep:`587` for further discussion of the existing settings and their
handling.

(Note: some of the info below could probably be cleaned up and added to the
C API documentation for 3.x - it's all CPython specific, so it
doesn't belong in the language reference)


Ignoring Environment Variables
------------------------------

The ``-E`` command line option allows all environment variables to be
ignored when initializing the Python interpreter. An embedding application
can enable this behaviour by setting ``Py_IgnoreEnvironmentFlag`` before
calling ``Py_Initialize()``.

In the CPython source code, the ``Py_GETENV`` macro implicitly checks this
flag, and always produces ``NULL`` if it is set.

<TBD: I believe PYTHONCASEOK is checked regardless of this setting >
<TBD: Does -E also ignore Windows registry keys? >


Randomised Hashing
------------------

The randomised hashing is controlled via the ``-R`` command line option (in
releases prior to 3.3), as well as the ``PYTHONHASHSEED`` environment
variable.

In Python 3.3, only the environment variable remains relevant. It can be
used to disable randomised hashing (by using a seed value of 0) or else
to force a specific hash value (e.g. for repeatability of testing, or
to share hash values between processes)

However, embedding applications must use the ``Py_HashRandomizationFlag``
to explicitly request hash randomisation (CPython sets it in ``Py_Main()``
rather than in ``Py_Initialize()``).

The new configuration API should make it straightforward for an
embedding application to reuse the ``PYTHONHASHSEED`` processing with
a text based configuration setting provided by other means (e.g. a
config file or separate environment variable).


Locating Python and the standard library
----------------------------------------

The location of the Python binary and the standard library is influenced
by several elements. The algorithm used to perform the calculation is
not documented anywhere other than in the source code [3]_, [4]_. Even that
description is incomplete, as it failed to be updated for the virtual
environment support added in Python 3.3 (detailed in :pep:`405`).

These calculations are affected by the following function calls (made
prior to calling ``Py_Initialize()``) and environment variables:

* ``Py_SetProgramName()``
* ``Py_SetPythonHome()``
* ``PYTHONHOME``

The filesystem is also inspected for ``pyvenv.cfg`` files (see :pep:`405`) or,
failing that, a ``lib/os.py`` (Windows) or ``lib/python$VERSION/os.py``
file.

The build time settings for ``PREFIX`` and ``EXEC_PREFIX`` are also relevant,
as are some registry settings on Windows. The hardcoded fallbacks are
based on the layout of the CPython source tree and build output when
working in a source checkout.


Configuring ``sys.path``
------------------------

An embedding application may call ``Py_SetPath()`` prior to
``Py_Initialize()`` to completely override the calculation of
``sys.path``. It is not straightforward to only allow *some* of the
calculations, as modifying ``sys.path`` after initialization is
already complete means those modifications will not be in effect
when standard library modules are imported during the startup sequence.

If ``Py_SetPath()`` is not used prior to the first call to ``Py_GetPath()``
(implicit in ``Py_Initialize()``), then it builds on the location data
calculations above to calculate suitable path entries, along with
the ``PYTHONPATH`` environment variable.

<TBD: On Windows, there's also a bunch of stuff to do with the registry>

The ``site`` module, which is implicitly imported at startup (unless
disabled via the ``-S`` option) adds additional paths to this initial
set of paths, as described in its documentation [5]_.

The ``-s`` command line option can be used to exclude the user site
directory from the list of directories added. Embedding applications
can control this by setting the ``Py_NoUserSiteDirectory`` global variable.

The following commands can be used to check the default path configurations
for a given Python executable on a given system:

* ``./python -c "import sys, pprint; pprint.pprint(sys.path)"``
  - standard configuration
* ``./python -s -c "import sys, pprint; pprint.pprint(sys.path)"``
  - user site directory disabled
* ``./python -S -c "import sys, pprint; pprint.pprint(sys.path)"``
  - all site path modifications disabled

(Note: you can see similar information using ``-m site`` instead of ``-c``,
but this is slightly misleading as it calls ``os.abspath`` on all of the
path entries, making relative path entries look absolute. Using the ``site``
module also causes problems in the last case, as on Python versions prior to
3.3, explicitly importing site will carry out the path modifications ``-S``
avoids, while on 3.3+ combining ``-m site`` with ``-S`` currently fails)

The calculation of ``sys.path[0]`` is comparatively straightforward:

* For an ordinary script (Python source or compiled bytecode),
  ``sys.path[0]`` will be the directory containing the script.
* For a valid ``sys.path`` entry (typically a zipfile or directory),
  ``sys.path[0]`` will be that path
* For an interactive session, running from stdin or when using the ``-c`` or
  ``-m`` switches, ``sys.path[0]`` will be the empty string, which the import
  system interprets as allowing imports from the current directory


Configuring ``sys.argv``
------------------------

Unlike most other settings discussed in this PEP, ``sys.argv`` is not
set implicitly by ``Py_Initialize()``. Instead, it must be set via an
explicitly call to ``Py_SetArgv()``.

CPython calls this in ``Py_Main()`` after calling ``Py_Initialize()``. The
calculation of ``sys.argv[1:]`` is straightforward: they're the command line
arguments passed after the script name or the argument to the ``-c`` or
``-m`` options.

The calculation of ``sys.argv[0]`` is a little more complicated:

* For an ordinary script (source or bytecode), it will be the script name
* For a ``sys.path`` entry (typically a zipfile or directory) it will
  initially be the zipfile or directory name, but will later be changed by
  the ``runpy`` module to the full path to the imported ``__main__`` module.
* For a module specified with the ``-m`` switch, it will initially be the
  string ``"-m"``, but will later be changed by the ``runpy`` module to the
  full path to the executed module.
* For a package specified with the ``-m`` switch, it will initially be the
  string ``"-m"``, but will later be changed by the ``runpy`` module to the
  full path to the executed ``__main__`` submodule of the package.
* For a command executed with ``-c``, it will be the string ``"-c"``
* For explicitly requested input from stdin, it will be the string ``"-"``
* Otherwise, it will be the empty string

Embedding applications must call Py_SetArgv themselves. The CPython logic
for doing so is part of ``Py_Main()`` and is not exposed separately.
However, the ``runpy`` module does provide roughly equivalent logic in
``runpy.run_module`` and ``runpy.run_path``.



Other configuration settings
----------------------------

TBD: Cover the initialization of the following in more detail:

* Completely disabling the import system
* The initial warning system state:

  * ``sys.warnoptions``
  * (-W option, PYTHONWARNINGS)

* Arbitrary extended options (e.g. to automatically enable ``faulthandler``):

  * ``sys._xoptions``
  * (-X option)

* The filesystem encoding used by:

  * ``sys.getfsencoding``
  * ``os.fsencode``
  * ``os.fsdecode``

* The IO encoding and buffering used by:

  * ``sys.stdin``
  * ``sys.stdout``
  * ``sys.stderr``
  * (-u option, PYTHONIOENCODING, PYTHONUNBUFFEREDIO)

* Whether or not to implicitly cache bytecode files:

  * ``sys.dont_write_bytecode``
  * (-B option, PYTHONDONTWRITEBYTECODE)

* Whether or not to enforce correct case in filenames on case-insensitive
  platforms

  * ``os.environ["PYTHONCASEOK"]``

* The other settings exposed to Python code in ``sys.flags``:

  * ``debug`` (Enable debugging output in the pgen parser)
  * ``inspect`` (Enter interactive interpreter after __main__ terminates)
  * ``interactive`` (Treat stdin as a tty)
  * ``optimize`` (__debug__ status, write .pyc or .pyo, strip doc strings)
  * ``no_user_site`` (don't add the user site directory to sys.path)
  * ``no_site`` (don't implicitly import site during startup)
  * ``ignore_environment`` (whether environment vars are used during config)
  * ``verbose`` (enable all sorts of random output)
  * ``bytes_warning`` (warnings/errors for implicit str/bytes interaction)
  * ``quiet`` (disable banner output even if verbose is also enabled or
    stdin is a tty and the interpreter is launched in interactive mode)

* Whether or not CPython's signal handlers should be installed

Much of the configuration of CPython is currently handled through C level
global variables::

    Py_BytesWarningFlag (-b)
    Py_DebugFlag (-d option)
    Py_InspectFlag (-i option, PYTHONINSPECT)
    Py_InteractiveFlag (property of stdin, cannot be overridden)
    Py_OptimizeFlag (-O option, PYTHONOPTIMIZE)
    Py_DontWriteBytecodeFlag (-B option, PYTHONDONTWRITEBYTECODE)
    Py_NoUserSiteDirectory (-s option, PYTHONNOUSERSITE)
    Py_NoSiteFlag (-S option)
    Py_UnbufferedStdioFlag (-u, PYTHONUNBUFFEREDIO)
    Py_VerboseFlag (-v option, PYTHONVERBOSE)

For the above variables, the conversion of command line options and
environment variables to C global variables is handled by ``Py_Main``,
so each embedding application must set those appropriately in order to
change them from their defaults.

Some configuration can only be provided as OS level environment variables::

    PYTHONSTARTUP
    PYTHONCASEOK
    PYTHONIOENCODING

The ``Py_InitializeEx()`` API also accepts a boolean flag to indicate
whether or not CPython's signal handlers should be installed.

Finally, some interactive behaviour (such as printing the introductory
banner) is triggered only when standard input is reported as a terminal
connection by the operating system.

TBD: Document how the "-x" option is handled (skips processing of the
first comment line in the main script)

Also see detailed sequence of operations notes at [1]_.


References
==========

.. [1] CPython interpreter initialization notes
   (http://wiki.python.org/moin/CPythonInterpreterInitialization)

.. [2] BitBucket Sandbox
   (https://bitbucket.org/ncoghlan/cpython_sandbox/compare/pep432_modular_bootstrap..default#commits)

.. [3] \*nix getpath implementation
   (http://hg.python.org/cpython/file/default/Modules/getpath.c)

.. [4] Windows getpath implementation
   (http://hg.python.org/cpython/file/default/PC/getpathp.c)

.. [5] Site module documentation
   (http://docs.python.org/3/library/site.html)

.. [6] Proposed CLI option for isolated mode
   (http://bugs.python.org/issue16499)

.. [7] Adding to sys.path on the command line
   (https://mail.python.org/pipermail/python-ideas/2010-October/008299.html)
   (https://mail.python.org/pipermail/python-ideas/2012-September/016128.html)

.. [8] Control sys.path[0] initialisation
   (http://bugs.python.org/issue13475)

.. [9] Enabling code coverage in subprocesses when testing
   (http://bugs.python.org/issue14803)

.. [10] Problems with PYTHONIOENCODING in Blender
   (http://bugs.python.org/issue16129)



Copyright
===========
This document has been placed in the public domain.
