I've updated the PEP heavily based on the previous thread and miscellanous comments in response to checkins. Latest version is at http://www.python.org/dev/peps/pep-0432/ and inline below. The biggest change in the new version is moving from a Python dictionary to a C struct as the storage for the full low level interpreter configuration as Antoine suggested. The individual settings are now either C integers for the various flag values (defaulting to -1 to indicate "figure this out"), or pointers to the appropriate specific Python type (defaulting to NULL to indicate "figure this out"). I'm happy enough with the design now that I think it's worth starting to implement it before I tinker with the PEP any further. Cheers, Nick. ================================ PEP: 432 Title: Simplifying the CPython startup sequence Version: $Revision$ Last-Modified: $Date$ Author: Nick Coghlan <ncoghlan@gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 28-Dec-2012 Python-Version: 3.4 Post-History: 28-Dec-2012, 2-Jan-2013 Abstract ======== This PEP proposes a mechanism for simplifying 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. 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 CPython move to an explicit multi-phase initialization process, where a preliminary interpreter is put in place with limited OS interaction capabilities early in the startup sequence. This essential core remains in place while all of the configuration settings are determined, until a final configuration call takes those settings and finishes bootstrapping the interpreter immediately before locating and executing the main module. In the new design, the interpreter will move through the following well-defined phases during the startup sequence: * Pre-Initialization - no interpreter available * Initialization - interpreter partially available * Initialized - full interpreter available, __main__ related metadata incomplete * Main Execution - optional state, __main__ related metadata populated, bytecode executing in the __main__ module namespace 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 also proposes the creation and distribution of a separate system Python (``spython``) executable which, by default, ignores user site directories and environment variables, and does not implicitly set ``sys.path[0]`` based on the current directory or the script being executed. To keep the implementation complexity under control, this PEP does *not* propose wholesale changes to the way the interpreter state is accessed at runtime, nor does it propose changes to the way subinterpreters are created after the main interpreter has already been initialized. 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 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 as well as bootstrapping a pure Python implementation of the import system). Much of this complexity is 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 (easily adding additional directories on the command line in a cross-platform fashion, as well as controlling the configuration of ``sys.path[0]``), easier configuration of utilities like coverage tracing when launching Python subprocesses, and easier control of the encoding used for the standard IO streams when embedding CPython in a larger application. Rather than attempting to bolt such behaviour onto an already complicated system, this PEP proposes to instead simplify the status quo *first*, with the aim of making these further feature requests easier to implement. Key Concerns ============ There are a couple of key concerns that any change to the startup sequence needs to take into account. Maintainability --------------- The current CPython startup sequence is difficult to understand, and even more difficult to modify. It is not clear what state the interpreter is in while much of the initialization code executes, 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 features are not available in the core bootstrapping state, as the vast majority of the configuration process will now take place in that state. 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. 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:: python3 -m timeit -s "from subprocess import call" "call(['./python', '-c', 'pass'])" Current numbers on my system for 2.7, 3.2 and 3.3 (using the 3.3 subprocess and timeit modules to execute the check, all with non-debug builds):: # Python 2.7 $ py33/python -m timeit -s "from subprocess import call" "call(['py27/python', '-c', 'pass'])" 100 loops, best of 3: 17.8 msec per loop # Python 3.2 $ py33/python -m timeit -s "from subprocess import call" "call(['py32/python', '-c', 'pass'])" 10 loops, best of 3: 39 msec per loop # Python 3.3 $ py33/python -m timeit -s "from subprocess import call" "call(['py33/python', '-c', 'pass'])" 10 loops, best of 3: 25.3 msec per loop Improvements in the import system and the Unicode support already resulted in a more than 30% improvement in startup time in Python 3.3 relative to 3.2. Python 3.3 is still slightly slower to start than Python 2.7 due to the additional infrastructure that needs to be put in place to support the Unicode based text model. 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 =============================== A comprehensive configuration scheme requires that an embedding application be able to control the following aspects of the final interpreter state: * Whether or not to use randomised hashes (and if used, potentially specify a specific random seed) * The "Where is Python located?" elements in the ``sys`` module: * ``sys.executable`` * ``sys.base_exec_prefix`` * ``sys.base_prefix`` * ``sys.exec_prefix`` * ``sys.prefix`` * The path searched for imports from the filesystem (and other path hooks): * ``sys.path`` * The command line arguments seen by the interpeter: * ``sys.argv`` * The filesystem encoding used by: * ``sys.getfsencoding`` * ``os.fsencode`` * ``os.fsdecode`` * The IO encoding (if any) and the buffering used by: * ``sys.stdin`` * ``sys.stdout`` * ``sys.stderr`` * The initial warning system state: * ``sys.warnoptions`` * Arbitrary extended options (e.g. to automatically enable ``faulthandler``): * ``sys._xoptions`` * Whether or not to implicitly cache bytecode files: * ``sys.dont_write_bytecode`` * 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 * What code (if any) should be executed as ``__main__``: * Nothing (just create an empty module) * A filesystem path referring to a Python script (source or bytecode) * A filesystem path referring to a valid ``sys.path`` entry (typically a directory or zipfile) * A given string (equivalent to the "-c" option) * A module or package (equivalent to the "-m" option) * Standard input as a script (i.e. a non-interactive stream) * Standard input as an interactive interpreter session <TBD: Did I miss anything?> Note that this just covers settings that are currently configurable in some manner when using the main CPython executable. While this PEP aims to make adding additional configuration settings easier in the future, it deliberately avoids adding any new settings of its own. The Status Quo ============== 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. (Note: some of the info below could probably be cleaned up and added to the C API documentation - 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 420). 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 420) 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: * 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_] Design Details ============== (Note: details here are still very much in flux, but preliminary feedback is appreciated anyway) The main theme of this proposal is to create the 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. In the following, the term "embedding application" also covers the standard CPython command line application. Interpreter Initialization Phases --------------------------------- Four distinct phases are proposed: * Pre-Initialization: * no interpreter is available. * ``Py_IsInitializing()`` returns ``0`` * ``Py_IsInitialized()`` returns ``0`` * ``Py_IsRunningMain()`` returns ``0`` * The embedding application determines the settings required to create the main interpreter and moves to the next phase by calling ``Py_BeginInitialization``. * Initialization: * the main interpreter is available, but only partially configured. * ``Py_IsInitializing()`` returns ``1`` * ``Py_IsInitialized()`` returns ``0`` * ``Py_RunningMain()`` returns ``0`` * The embedding application determines and applies the settings required to complete the initialization process by calling ``Py_ReadConfiguration`` and ``Py_EndInitialization``. * Initialized: * the main interpreter is available and fully operational, but ``__main__`` related metadata is incomplete. * ``Py_IsInitializing()`` returns ``0`` * ``Py_IsInitialized()`` returns ``1`` * ``Py_IsRunningMain()`` returns ``0`` * Optionally, the embedding application may identify and begin executing code in the ``__main__`` module namespace by calling ``Py_RunPathAsMain``, ``Py_RunModuleAsMain`` or ``Py_RunStreamAsMain``. * Main Execution: * bytecode is being executed in the ``__main__`` namespace * ``Py_IsInitializing()`` returns ``0`` * ``Py_IsInitialized()`` returns ``1`` * ``Py_IsRunningMain()`` returns ``1`` As indicated by the phase reporting functions, main module execution is an optional subphase of Initialized rather than a completely distinct phase. All 4 phases will be used by the standard CPython interpreter and the proposed System Python interpreter. Other embedding applications may choose to skip the step of executing code in the ``__main__`` namespace. An embedding application may still continue to leave initialization almost entirely under CPython's control by using the existing ``Py_Initialize`` API. 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:: /* Phase 1: Pre-Initialization */ Py_CoreConfig core_config = Py_CoreConfig_INIT; Py_Config config = Py_Config_INIT; /* Easily control the core configuration */ core_config.ignore_environment = 1; /* Ignore environment variables */ core_config.use_hash_seed = 0; /* Full hash randomisation */ Py_BeginInitialization(&core_config); /* Phase 2: Initialization */ /* Optionally preconfigure some settings here - they will then be * used to derive other settings */ Py_ReadConfiguration(&config); /* Can completely override derived settings here */ Py_EndInitialization(&config); /* Phase 3: Initialized */ /* If an embedding application has no real concept of a main module * it can leave the interpreter in this state indefinitely. * Otherwise, it can launch __main__ via the Py_Run*AsMain functions. */ Pre-Initialization Phase ------------------------ The pre-initialization phase is where an embedding application determines the settings which are absolutely required before the interpreter can be initialized at all. Currently, the only 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 specific 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. The proposed API for this step in the startup sequence is:: void Py_BeginInitialization(const Py_CoreConfig *config); Like Py_Initialize, this part of the new API treats initialization failures as fatal errors. While that's still not particularly embedding friendly, the operations in this step *really* shouldn't be failing, and changing them to return error codes instead of aborting would be an even larger task than the one already being proposed. The new ``Py_CoreConfig`` struct holds the settings required for preliminary configuration:: /* Note: if changing anything in Py_CoreConfig, also update * Py_CoreConfig_INIT */ typedef struct { int ignore_environment; /* -E switch */ int use_hash_seed; /* PYTHONHASHSEED */ unsigned long hash_seed; /* PYTHONHASHSEED */ } Py_CoreConfig; #define Py_CoreConfig_INIT {0, -1, 0} The core configuration settings pointer may be ``NULL``, in which case the default values are ``ignore_environment = 0`` and ``use_hash_seed = -1``. The ``Py_CoreConfig_INIT`` macro is designed to allow easy initialization of a struct instance with sensible defaults:: Py_CoreConfig core_config = Py_CoreConfig_INIT; ``ignore_environment`` controls the processing of all Python related environment variables. If the flag is zero, then environment variables are 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 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 ``ignore_environment`` is zero), then CPython will inspect the ``PYTHONHASHSEED`` environment variable. If it 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 it 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 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 go in the config dict passed to ``Py_EndInitialization()`` rather than in the core configuration. A new query API will allow code to determine if the interpreter is in the bootstrapping state between the creation of the interpreter state and the completion of the bulk of the initialization process:: int Py_IsInitializing(); Attempting to call ``Py_BeginInitialization()`` again when ``Py_IsInitializing()`` or ``Py_IsInitialized()`` is true is a fatal error. While in the initializing state, the interpreter 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.flags`` * ``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 ``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 datatypes, 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, allow any further configuration data to be stored on the interpreter object rather than in C process globals. Any call to ``Py_BeginInitialization()`` must have a matching call to ``Py_Finalize()``. It is acceptable to skip calling Py_EndInitialization() in between (e.g. if attempting to read the configuration settings fails) Determining the remaining configuration settings ------------------------------------------------ The next step in the initialization sequence is to determine the full settings needed to complete the process. No changes are made to the interpreter state at this point. The core API for this step is:: int Py_ReadConfiguration(PyConfig *config); The config argument should be a pointer to a Python dictionary. For any supported configuration setting already in the dictionary, CPython will sanity check the supplied value, but otherwise accept it as correct. Unlike ``Py_Initialize`` and ``Py_BeginInitialization``, 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. Any supported configuration setting which is not already set will be populated appropriately. The default configuration can be overridden entirely by setting the value *before* calling ``Py_ReadConfiguration``. The provided value will then also be used in calculating any settings derived from that value. Alternatively, settings may be overridden *after* the ``Py_ReadConfiguration`` 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]``). Supported configuration settings -------------------------------- The new ``Py_Config`` struct holds the settings required to complete the interpreter configuration. All fields are either pointers to Python data types (not set == ``NULL``) or numeric flags (not set == ``-1``):: /* Note: if changing anything in Py_Config, also update Py_Config_INIT */ typedef struct { /* Argument processing */ PyList *raw_argv; PyList *argv; PyList *warnoptions; /* -W switch, PYTHONWARNINGS */ PyDict *xoptions; /* -X switch */ /* Filesystem locations */ PyUnicode *program_name; PyUnicode *executable; PyUnicode *prefix; /* PYTHONHOME */ PyUnicode *exec_prefix; /* PYTHONHOME */ PyUnicode *base_prefix; /* pyvenv.cfg */ PyUnicode *base_exec_prefix; /* pyvenv.cfg */ /* Site module */ int no_site; /* -S switch */ int no_user_site; /* -s switch, PYTHONNOUSERSITE */ /* Import configuration */ int dont_write_bytecode; /* -B switch, PYTHONDONTWRITEBYTECODE */ int ignore_module_case; /* PYTHONCASEOK */ PyList *import_path; /* PYTHONPATH (etc) */ /* Standard streams */ int use_unbuffered_io; /* -u switch, PYTHONUNBUFFEREDIO */ PyUnicode *stdin_encoding; /* PYTHONIOENCODING */ PyUnicode *stdin_errors; /* PYTHONIOENCODING */ PyUnicode *stdout_encoding; /* PYTHONIOENCODING */ PyUnicode *stdout_errors; /* PYTHONIOENCODING */ PyUnicode *stderr_encoding; /* PYTHONIOENCODING */ PyUnicode *stderr_errors; /* PYTHONIOENCODING */ /* Filesystem access */ PyUnicode *fs_encoding; /* Interactive interpreter */ int stdin_is_interactive; /* Force interactive behaviour */ int inspect_main; /* -i switch, PYTHONINSPECT */ PyUnicode *startup_file; /* PYTHONSTARTUP */ /* Debugging output */ int debug_parser; /* -d switch, PYTHONDEBUG */ int verbosity; /* -v switch */ int suppress_banner; /* -q switch */ /* Code generation */ int bytes_warnings; /* -b switch */ int optimize; /* -O switch */ /* Signal handling */ int install_sig_handlers; } Py_Config; /* Struct initialization is pretty ugly in C89. Avoiding this mess would * be the most attractive aspect of using a PyDict* instead... */ #define _Py_ArgConfig_INIT NULL, NULL, NULL, NULL #define _Py_LocationConfig_INIT NULL, NULL, NULL, NULL, NULL, NULL #define _Py_SiteConfig_INIT -1, -1 #define _Py_ImportConfig_INIT -1, -1, NULL #define _Py_StreamConfig_INIT -1, NULL, NULL, NULL, NULL, NULL, NULL #define _Py_FilesystemConfig_INIT NULL #define _Py_InteractiveConfig_INIT -1, -1, NULL #define _Py_DebuggingConfig_INIT -1, -1, -1 #define _Py_CodeGenConfig_INIT -1, -1 #define _Py_SignalConfig_INIT -1 #define Py_Config_INIT {_Py_ArgConfig_INIT, _Py_LocationConfig_INIT, _Py_SiteConfig_INIT, _Py_ImportConfig_INIT, _Py_StreamConfig_INIT, _Py_FilesystemConfig_INIT, _Py_InteractiveConfig_INIT, _Py_DebuggingConfig_INIT, _Py_CodeGenConfig_INIT, _Py_SignalConfig_INIT} <TBD: did I miss anything?> Completing the interpreter initialization ----------------------------------------- The final step in the initialization process is to actually put the configuration settings into effect and finish bootstrapping the interpreter up to full operation:: int Py_EndInitialization(const PyConfig *config); Like Py_ReadConfiguration, 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. All configuration settings are required - the configuration struct should always be passed through ``Py_ReadConfiguration()`` to ensure it is fully populated. After a successful call, ``Py_IsInitializing()`` will be false, while ``Py_IsInitialized()`` will become true. The caveats described above for the interpreter during the initialization phase will no longer hold. 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) * the metadata in the ``__main__`` module will still indicate it is a builtin module Executing the main module ------------------------- <TBD> Initial thought is that hiding the various options behind a single API would make that API too complicated, so 3 separate APIs is more likely:: Py_RunPathAsMain Py_RunModuleAsMain Py_RunStreamAsMain Query API to indicate that ``sys.argv[0]`` is fully populated:: Py_IsRunningMain() 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 an embedded copy of the ``Py_CoreConfig`` and ``Py_Config`` structs. For debugging purposes, the configuration settings will be exposed as a ``sys._configuration`` simple namespace (similar to ``sys.flags`` and ``sys.implementation``. Field names will match those in the configuration structs, exception for ``hash_seed``, which will be deliberately excluded. These are *snapshots* of the initial configuration settings. They are not consulted by the interpreter during runtime. Stable ABI ---------- All 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. Backwards Compatibility ----------------------- Backwards compatibility will be preserved primarily by ensuring that Py_ReadConfiguration() interrogates all the previously defined configuration settings stored in global variables and environment variables, and that Py_EndInitialization() writes affected settings back to the relevant locations. One acknowledged incompatiblity is that some environment variables which are currently read lazily may instead be read once during interpreter initialization. As the PEP 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. 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_BeginInitialization()``. To minimise unnecessary code churn, and to ensure the backwards compatibility is well tested, the main CPython executable may continue to use some elements of the old style initialization API. (very much TBC) Open Questions ============== * Is ``Py_IsRunningMain()`` worth keeping? * Should the answers to ``Py_IsInitialized()`` and ``Py_RunningMain()`` be exposed via the ``sys`` module? * Is the ``Py_Config`` struct too unwieldy to be practical? Would a Python dictionary be a better choice? * Would it be better to manage the flag variables in ``Py_Config`` as Python integers so the struct can be initialized with a simple ``memset(&config, 0, sizeof(*config))``? 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. 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 Implementation ============== None as yet. Once I have a reasonably solid plan of attack, I intend to work on a reference implementation as a feature branch in my BitBucket sandbox [2_] References ========== .. [1] CPython interpreter initialization notes (http://wiki.python.org/moin/CPythonInterpreterInitialization) .. [2] BitBucket Sandbox (https://bitbucket.org/ncoghlan/cpython_sandbox) .. [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) Copyright =========== This document has been placed in the public domain. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia