PEP: 489 Title: Redesigning extension module loading Version: $Revision$ Last-Modified: $Date$ Author: Petr Viktorin , Stefan Behnel , Nick Coghlan Discussions-To: import-sig@python.org Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 11-Aug-2013 Python-Version: 3.5 Post-History: 23-Aug-2013, 20-Feb-2015, 16-Apr-2015 Resolution: Abstract ======== This PEP proposes a redesign of the way in which extension modules interact with the import machinery. This was last revised for Python 3.0 in PEP 3121, but did not solve all problems at the time. The goal is to solve them by bringing extension modules closer to the way Python modules behave; specifically to hook into the ModuleSpec-based loading mechanism introduced in PEP 451. This proposal draws inspiration from PyType_Spec of PEP 384 to allow extension authors to only define features they need, and to allow future additions to extension module declarations. Extensions modules are created in a two-step process, fitting better into the ModuleSpec architecture, with parallels to __new__ and __init__ of classes. Extension modules can safely store arbitrary C-level per-module state in the module that is covered by normal garbage collection and supports reloading and sub-interpreters. Extension authors are encouraged to take these issues into account when using the new API. The proposal also allows extension modules with non-ASCII names. Motivation ========== Python modules and extension modules are not being set up in the same way. For Python modules, the module is created and set up first, then the module code is being executed (PEP 302). A ModuleSpec object (PEP 451) is used to hold information about the module, and passed to the relevant hooks. For extensions, i.e. shared libraries, the module init function is executed straight away and does both the creation and initialization. The initialization function is not passed the ModuleSpec, or any information it contains, such as the __file__ or fully-qualified name. This hinders relative imports and resource loading. In Py3, modules are also not being added to sys.modules, which means that a (potentially transitive) re-import of the module will really try to re-import it and thus run into an infinite loop when it executes the module init function again. Without the FQMN, it is not trivial to correctly add the module to sys.modules either. This is specifically a problem for Cython generated modules, for which it's not uncommon that the module init code has the same level of complexity as that of any 'regular' Python module. Also, the lack of __file__ and __name__ information hinders the compilation of "__init__.py" modules, i.e. packages, especially when relative imports are being used at module init time. Furthermore, the majority of currently existing extension modules has problems with sub-interpreter support and/or interpreter reloading, and, while it is possible with the current infrastructure to support these features, it is neither easy nor efficient. Addressing these issues was the goal of PEP 3121, but many extensions, including some in the standard library, took the least-effort approach to porting to Python 3, leaving these issues unresolved. This PEP keeps backwards compatibility, which should reduce pressure and give extension authors adequate time to consider these issues when porting. The current process =================== Currently, extension modules export an initialization function named "PyInit_modulename", named after the file name of the shared library. This function is executed by the import machinery and must return either NULL in the case of an exception, or a fully initialized module object. The function receives no arguments, so it has no way of knowing about its import context. During its execution, the module init function creates a module object based on a PyModuleDef struct. It then continues to initialize it by adding attributes to the module dict, creating types, etc. In the back, the shared library loader keeps a note of the fully qualified module name of the last module that it loaded, and when a module gets created that has a matching name, this global variable is used to determine the fully qualified name of the module object. This is not entirely safe as it relies on the module init function creating its own module object first, but this assumption usually holds in practice. The proposal ============ The current extension module initialization will be deprecated in favor of a new initialization scheme. Since the current scheme will continue to be available, existing code will continue to work unchanged, including binary compatibility. Extension modules that support the new initialization scheme must export the public symbol "PyModuleExport_", where "modulename" is the name of the module. (For modules with non-ASCII names the symbol name is slightly different, see "Export Hook Name" below.) If defined, this symbol must resolve to a C function with the following signature:: PyModuleDef* (*PyModuleExportFunction)(void) For cross-platform compatibility, the function should be declared as:: PyMODEXPORT_FUNC PyModuleExport_(void) The function must return a pointer to a PyModuleDef structure. This structure must be available for the lifetime of the module created from it – usually, it will be declared statically. Alternatively, this function can return NULL, in which case it is as if the symbol was not defined – see the "Legacy Init" section. The PyModuleDef structure will be changed to contain a list of slots, similarly to PEP 384's PyType_Spec for types. To keep binary compatibility, and avoid needing to introduce a new structure (which would introduce additional supporting functions and per-module storage), the currently unused m_reload pointer of PyModuleDef will be changed to hold the slots. The structures are defined as:: typedef struct { int slot; void *value; } PyModuleDef_Slot; typedef struct PyModuleDef { PyModuleDef_Base m_base; const char* m_name; const char* m_doc; Py_ssize_t m_size; PyMethodDef *m_methods; PyModuleDef_Slot *m_slots; /* changed from `inquiry m_reload;` */ traverseproc m_traverse; inquiry m_clear; freefunc m_free; } PyModuleDef; The *m_slots* member must be either NULL, or point to an array of PyModuleDef_Slot structures, terminated by a slot with id set to 0 (i.e. ``{0, NULL}``). To specify a slot, a unique slot ID must be provided. New Python versions may introduce new slot IDs, but slot IDs will never be recycled. Slots may get deprecated, but will continue to be supported throughout Python 3.x. A slot's value pointer may not be NULL, unless specified otherwise in the slot's documentation. The following slots are currently available, and described later: * Py_mod_create * Py_mod_exec Unknown slot IDs will cause the import to fail with SystemError. When using the new import mechanism, m_size must not be negative. Also, the *m_name* field of PyModuleDef will not be unused during importing; the module name will be taken from the ModuleSpec. Module Creation --------------- Module creation – that is, the implementation of ExecutionLoader.create_module – is governed by the Py_mod_create slot. The Py_mod_create slot ...................... The Py_mod_create slot is used to support custom module subclasses. The value pointer must point to a function with the following signature:: PyObject* (*PyModuleCreateFunction)(PyObject *spec, PyModuleDef *def) The function receives a ModuleSpec instance, as defined in PEP 451, and the PyModuleDef structure. It should return a new module object, or set an error and return NULL. This function is not responsible for setting import-related attributes specified in PEP 451 [#pep-0451-attributes]_ (such as ``__name__`` or ``__loader__``) on the new module. There is no requirement for the returned object to be an instance of types.ModuleType. Any type can be used, as long as it supports setting and getting attributes, including at least the import-related attributes. However, only ModuleType instances support module-specific functionality such as per-module state. Note that when this function is called, the module's entry in sys.modules is not populated yet. Attempting to import the same module again (possibly transitively), may lead to an infinite loop. Extension authors are advised to keep Py_mod_create minimal, an in particular to not call user code from it. Multiple Py_mod_create slots may not be specified. If they are, import will fail with SystemError. If Py_mod_create is not specified, the import machinery will create a normal module object by PyModule_New. The name is taken from *spec*. Post-creation steps ................... If the Py_mod_create function returns an instance of types.ModuleType (or subclass), or if a Py_mod_create slot is not present, the import machinery will do the following steps after the module is created: * If *m_size* is specified, per-module state is allocated and made accessible through PyModule_GetState * The PyModuleDef is associated with the module, making it accessible to PyModule_GetDef, and enabling the m_traverse, m_clear and m_free hooks. * The docstring is set from m_doc. * The module's functions are initialized from m_methods. If the Py_mod_create function does not return a module subclass, then m_size must be 0 or negative, and m_traverse, m_clear and m_free must all be NULL. Otherwise, SystemError is raised. Module Execution ---------------- Module execution -- that is, the implementation of ExecutionLoader.exec_module -- is governed by "execution slots". This PEP only adds one, Py_mod_exec, but others may be added in the future. Execution slots may be specified multiple times, and are processed in the order they appear in the slots array. When using the default import machinery, they are processed after import-related attributes specified in PEP 451 [#pep-0451-attributes]_ (such as ``__name__`` or ``__loader__``) are set and the module is added to sys.modules. The Py_mod_exec slot .................... The entry in this slot must point to a function with the following signature:: int (*PyModuleExecFunction)(PyObject* module) It will be called to initialize a module. Usually, this amounts to setting the module's initial attributes. The "module" argument receives the module object to initialize. If PyModuleExec replaces the module's entry in sys.modules, the new object will be used and returned by importlib machinery. (This mirrors the behavior of Python modules. Note that for extensions, implementing Py_mod_create is usually a better solution for the use cases this serves.) The function must return ``0`` on success, or, on error, set an exception and return ``-1``. Legacy Init ----------- If the PyModuleExport function is not defined, or if it returns NULL, the import machinery will try to initialize the module using the "PyInit_" hook, as described in PEP 3121. If the PyModuleExport function is defined, the PyInit function will be ignored. Modules requiring compatibility with previous versions of CPython may implement the PyInit function in addition to the new hook. Modules using the legacy init API will be initialized entirely in the Loader.create_module step; Loader.exec_module will be a no-op. A module that supports older CPython versions can be coded as:: #define Py_LIMITED_API #include static int spam_exec(PyObject *module) { PyModule_AddStringConstant(module, "food", "spam"); return 0; } static PyModuleDef_Slot spam_slots[] = { {Py_mod_exec, spam_exec}, {0, NULL} }; static PyModuleDef spam_def = { PyModuleDef_HEAD_INIT, /* m_base */ "spam", /* m_name */ PyDoc_STR("Utilities for cooking spam"), /* m_doc */ 0, /* m_size */ NULL, /* m_methods */ spam_slots, /* m_slots */ NULL, /* m_traverse */ NULL, /* m_clear */ NULL, /* m_free */ }; PyModuleDef* PyModuleExport_spam(void) { return &spam_def; } PyMODINIT_FUNC PyInit_spam(void) { PyObject *module; module = PyModule_Create(&spam_def); if (module == NULL) return NULL; if (spam_exec(module) != 0) { Py_DECREF(module); return NULL; } return module; } Note that this must be *compiled* on a new CPython version, but the resulting shared library will be backwards compatible. (Source-level compatibility is possible with preprocessor directives.) If a Py_mod_create slot is used, PyInit should call its function instead of PyModule_Create. Keep in mind that the ModuleSpec object is not available in the legacy init scheme. Subinterpreters and Interpreter Reloading ----------------------------------------- Extensions using the new initialization scheme are expected to support subinterpreters and multiple Py_Initialize/Py_Finalize cycles correctly. The mechanism is designed to make this easy, but care is still required on the part of the extension author. No user-defined functions, methods, or instances may leak to different interpreters. To achieve this, all module-level state should be kept in either the module dict, or in the module object's storage reachable by PyModule_GetState. A simple rule of thumb is: Do not define any static data, except built-in types with no mutable or user-settable class attributes. Behavior of existing module creation functions ---------------------------------------------- The PyModule_Create function will fail when used on a PyModuleDef structure with a non-NULL m_slots pointer. The function doesn't have access to the ModuleSpec object necessary for "new style" module creation. The PyState_FindModule function will return NULL, and PyState_AddModule and PyState_RemoveModule will fail with SystemError. PyState registration is disabled because multiple module objects may be created from the same PyModuleDef. Module state and C-level callbacks ---------------------------------- Due to the unavailability of PyState_FindModule, any function that needs access to module-level state (including functions, classes or exceptions defined at the module level) must receive a reference to the module object (or the particular object it needs), either directly or indirectly. This is currently difficult in two situations: * Methods of classes, which receive a reference to the class, but not to the class's module * Libraries with C-level callbacks, unless the callbacks can receive custom data set at cllback registration Fixing these cases is outside of the scope of this PEP, but will be needed for the new mechanism to be useful to all modules. Proper fixes have been discussed on the import-sig mailing list [#findmodule-discussion]_. As a rule of thumb, modules that rely on PyState_FindModule are, at the moment, not good candidates for porting to the new mechanism. New Functions ------------- A new function and macro will be added to implement module creation. These are similar to PyModule_Create and PyModule_Create2, except they take an additional ModuleSpec argument, and handle module definitions with non-NULL slots:: PyObject * PyModule_FromDefAndSpec(PyModuleDef *def, PyObject *spec) PyObject * PyModule_FromDefAndSpec2(PyModuleDef *def, PyObject *spec, int module_api_version) A new function will be added to run "execution slots" on a module:: PyAPI_FUNC(int) PyModule_ExecDef(PyObject *module, PyModuleDef *def) Additionally, two helpers will be added for setting the docstring and methods on a module:: int PyModule_SetDocString(PyObject *, const char *) int PyModule_AddFunctions(PyObject *, PyMethodDef *) Export Hook Name ---------------- As portable C identifiers are limited to ASCII, module names must be encoded to form the PyModuleExport hook name. For ASCII module names, the import hook is named PyModuleExport_, where is the name of the module. For module names containing non-ASCII characters, the import hook is named PyModuleExportU_, where the name is encoded using CPython's "punycode" encoding (Punycode [#rfc-3492]_ with a lowercase suffix), with hyphens ("-") replaced by underscores ("_"). In Python:: def export_hook_name(name): try: suffix = b'_' + name.encode('ascii') except UnicodeEncodeError: suffix = b'U_' + name.encode('punycode').replace(b'-', b'_') return b'PyModuleExport' + suffix Examples: ============= =========================== Module name Export hook name ============= =========================== spam PyModuleExport_spam lančmít PyModuleExportU_lanmt_2sa6t スパム PyModuleExportU_zck5b2b ============= =========================== Module Reloading ---------------- Reloading an extension module using importlib.reload() will continue to have no effect, except re-setting import-related attributes. Due to limitations in shared library loading (both dlopen on POSIX and LoadModuleEx on Windows), it is not generally possible to load a modified library after it has changed on disk. Use cases for reloading other than trying out a new version of the module are too rare to require all module authors to keep reloading in mind. If reload-like functionality is needed, authors can export a dedicated function for it. Multiple modules in one library ------------------------------- To support multiple Python modules in one shared library, the library can export additional PyModuleExport* symbols besides the one that corresponds to the library's filename. Note that this mechanism can currently only be used to *load* extra modules, not to *find* them. Given the filesystem location of a shared library and a module name, a module may be loaded with:: import importlib.machinery import importlib.util loader = importlib.machinery.ExtensionFileLoader(name, path) spec = importlib.util.spec_from_loader(name, loader) module = importlib.util.module_from_spec(spec) loader.exec_module(module) return module On platforms that support symbolic links, these may be used to install one library under multiple names, exposing all exported modules to normal import machinery. Testing and initial implementations ----------------------------------- For testing, a new built-in module ``_testmoduleexport`` will be created. The library will export several additional modules using the mechanism described in "Multiple modules in one library". The ``_testcapi`` module will be unchanged, and will use the old API indefinitely (or until the old API is removed). The ``array`` and ``xx*`` modules will be converted to the new API as part of the initial implementation. API Changes and Additions ------------------------- New functions: * PyModule_FromDefAndSpec (macro) * PyModule_FromDefAndSpec2 * PyModule_ExecDef * PyModule_SetDocString * PyModule_AddFunctions New macros: * PyMODEXPORT_FUNC * Py_mod_create * Py_mod_exec New structures: * PyModuleDef_Slot PyModuleDef.m_reload changes to PyModuleDef.m_slots. Possible Future Extensions ========================== The slots mechanism, inspired by PyType_Slot from PEP 384, allows later extensions. Some extension modules exports many constants; for example _ssl has a long list of calls in the form:: PyModule_AddIntConstant(m, "SSL_ERROR_ZERO_RETURN", PY_SSL_ERROR_ZERO_RETURN); Converting this to a declarative list, similar to PyMethodDef, would reduce boilerplate, and provide free error-checking which is often missing. String constants and types can be handled similarly. (Note that non-default bases for types cannot be portably specified statically; this case would need a Py_mod_exec function that runs before the slots are added. The free error-checking would still be beneficial, though.) Another possibility is providing a "main" function that would be run when the module is given to Python's -m switch. For this to work, the runpy module will need to be modified to take advantage of ModuleSpec-based loading introduced in PEP 451. Also, it will be necessary to add a mechanism for setting up a module according to slots it wasn't originally defined with. Implementation ============== Work-in-progress implementation is available in a Github repository [#gh-repo]_; a patchset is at [#gh-patch]_. Previous Approaches =================== Stefan Behnel's initial proto-PEP [#stefans_protopep]_ had a "PyInit_modulename" hook that would create a module class, whose ``__init__`` would be then called to create the module. This proposal did not correspond to the (then nonexistent) PEP 451, where module creation and initialization is broken into distinct steps. It also did not support loading an extension into pre-existing module objects. Nick Coghlan proposed "Create" and "Exec" hooks, and wrote a prototype implementation [#nicks-prototype]_. At this time PEP 451 was still not implemented, so the prototype does not use ModuleSpec. The original version of this PEP used Create and Exec hooks, and allowed loading into arbitrary pre-constructed objects with Exec hook. The proposal made extension module initialization closer to how Python modules are initialized, but it was later recognized that this isn't an important goal. The current PEP describes a simpler solution. References ========== .. [#lazy_import_concerns] https://mail.python.org/pipermail/python-dev/2013-August/128129.html .. [#pep-0451-attributes] https://www.python.org/dev/peps/pep-0451/#attributes .. [#stefans_protopep] https://mail.python.org/pipermail/python-dev/2013-August/128087.html .. [#nicks-prototype] https://mail.python.org/pipermail/python-dev/2013-August/128101.html .. [#rfc-3492] http://tools.ietf.org/html/rfc3492 .. [#gh-repo] https://github.com/encukou/cpython/commits/pep489 .. [#gh-patch] https://github.com/encukou/cpython/compare/master...encukou:pep489.patch .. [#findmodule-discussion] https://mail.python.org/pipermail/import-sig/2015-April/000959.html Copyright ========= This document has been placed in the public domain.