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#: howto/isolating-extensions.rst:5
msgid "Isolating Extension Modules"
msgstr ""

#: howto/isolating-extensions.rst:-1
msgid "Abstract"
msgstr ""

#: howto/isolating-extensions.rst:9
msgid ""
"Traditionally, state belonging to Python extension modules was kept in C "
"``static`` variables, which have process-wide scope. This document describes "
"problems of such per-process state and shows a safer way: per-module state."
msgstr ""

#: howto/isolating-extensions.rst:14
msgid ""
"The document also describes how to switch to per-module state where "
"possible. This transition involves allocating space for that state, "
"potentially switching from static types to heap types, and—perhaps most "
"importantly—accessing per-module state from code."
msgstr ""

#: howto/isolating-extensions.rst:21
msgid "Who should read this"
msgstr ""

#: howto/isolating-extensions.rst:23
msgid ""
"This guide is written for maintainers of :ref:`C-API <c-api-index>` "
"extensions who would like to make that extension safer to use in "
"applications where Python itself is used as a library."
msgstr ""

#: howto/isolating-extensions.rst:29
msgid "Background"
msgstr ""

#: howto/isolating-extensions.rst:31
msgid ""
"An *interpreter* is the context in which Python code runs. It contains "
"configuration (e.g. the import path) and runtime state (e.g. the set of "
"imported modules)."
msgstr ""

#: howto/isolating-extensions.rst:35
msgid ""
"Python supports running multiple interpreters in one process. There are two "
"cases to think about—users may run interpreters:"
msgstr ""

#: howto/isolating-extensions.rst:38
msgid ""
"in sequence, with several :c:func:`Py_InitializeEx`/:c:func:`Py_FinalizeEx` "
"cycles, and"
msgstr ""

#: howto/isolating-extensions.rst:40
msgid ""
"in parallel, managing \"sub-interpreters\" using :c:func:"
"`Py_NewInterpreter`/:c:func:`Py_EndInterpreter`."
msgstr ""

#: howto/isolating-extensions.rst:43
msgid ""
"Both cases (and combinations of them) would be most useful when embedding "
"Python within a library. Libraries generally shouldn't make assumptions "
"about the application that uses them, which include assuming a process-wide "
"\"main Python interpreter\"."
msgstr ""

#: howto/isolating-extensions.rst:48
msgid ""
"Historically, Python extension modules don't handle this use case well. Many "
"extension modules (and even some stdlib modules) use *per-process* global "
"state, because C ``static`` variables are extremely easy to use. Thus, data "
"that should be specific to an interpreter ends up being shared between "
"interpreters. Unless the extension developer is careful, it is very easy to "
"introduce edge cases that lead to crashes when a module is loaded in more "
"than one interpreter in the same process."
msgstr ""

#: howto/isolating-extensions.rst:56
msgid ""
"Unfortunately, *per-interpreter* state is not easy to achieve. Extension "
"authors tend to not keep multiple interpreters in mind when developing, and "
"it is currently cumbersome to test the behavior."
msgstr ""

#: howto/isolating-extensions.rst:61
msgid "Enter Per-Module State"
msgstr ""

#: howto/isolating-extensions.rst:63
msgid ""
"Instead of focusing on per-interpreter state, Python's C API is evolving to "
"better support the more granular *per-module* state. This means that C-level "
"data is be attached to a *module object*. Each interpreter creates its own "
"module object, keeping the data separate. For testing the isolation, "
"multiple module objects corresponding to a single extension can even be "
"loaded in a single interpreter."
msgstr ""

#: howto/isolating-extensions.rst:70
msgid ""
"Per-module state provides an easy way to think about lifetime and resource "
"ownership: the extension module will initialize when a module object is "
"created, and clean up when it's freed. In this regard, a module is just like "
"any other :c:expr:`PyObject *`; there are no \"on interpreter shutdown\" "
"hooks to think—or forget—about."
msgstr ""

#: howto/isolating-extensions.rst:76
msgid ""
"Note that there are use cases for different kinds of \"globals\": per-"
"process, per-interpreter, per-thread or per-task state. With per-module "
"state as the default, these are still possible, but you should treat them as "
"exceptional cases: if you need them, you should give them additional care "
"and testing. (Note that this guide does not cover them.)"
msgstr ""

#: howto/isolating-extensions.rst:85
msgid "Isolated Module Objects"
msgstr ""

#: howto/isolating-extensions.rst:87
msgid ""
"The key point to keep in mind when developing an extension module is that "
"several module objects can be created from a single shared library. For "
"example:"
msgstr ""

#: howto/isolating-extensions.rst:101
msgid ""
"As a rule of thumb, the two modules should be completely independent. All "
"objects and state specific to the module should be encapsulated within the "
"module object, not shared with other module objects, and cleaned up when the "
"module object is deallocated. Since this just is a rule of thumb, exceptions "
"are possible (see `Managing Global State`_), but they will need more thought "
"and attention to edge cases."
msgstr ""

#: howto/isolating-extensions.rst:109
msgid ""
"While some modules could do with less stringent restrictions, isolated "
"modules make it easier to set clear expectations and guidelines that work "
"across a variety of use cases."
msgstr ""

#: howto/isolating-extensions.rst:115
msgid "Surprising Edge Cases"
msgstr ""

#: howto/isolating-extensions.rst:117
msgid ""
"Note that isolated modules do create some surprising edge cases. Most "
"notably, each module object will typically not share its classes and "
"exceptions with other similar modules. Continuing from the `example above "
"<Isolated Module Objects_>`__, note that ``old_binascii.Error`` and "
"``binascii.Error`` are separate objects. In the following code, the "
"exception is *not* caught:"
msgstr ""

#: howto/isolating-extensions.rst:137
msgid ""
"This is expected. Notice that pure-Python modules behave the same way: it is "
"a part of how Python works."
msgstr ""

#: howto/isolating-extensions.rst:140
msgid ""
"The goal is to make extension modules safe at the C level, not to make hacks "
"behave intuitively. Mutating ``sys.modules`` \"manually\" counts as a hack."
msgstr ""

#: howto/isolating-extensions.rst:146
msgid "Making Modules Safe with Multiple Interpreters"
msgstr ""

#: howto/isolating-extensions.rst:150
msgid "Managing Global State"
msgstr ""

#: howto/isolating-extensions.rst:152
msgid ""
"Sometimes, the state associated with a Python module is not specific to that "
"module, but to the entire process (or something else \"more global\" than a "
"module). For example:"
msgstr ""

#: howto/isolating-extensions.rst:156
msgid "The ``readline`` module manages *the* terminal."
msgstr ""

#: howto/isolating-extensions.rst:157
msgid ""
"A module running on a circuit board wants to control *the* on-board LED."
msgstr ""

#: howto/isolating-extensions.rst:160
msgid ""
"In these cases, the Python module should provide *access* to the global "
"state, rather than *own* it. If possible, write the module so that multiple "
"copies of it can access the state independently (along with other libraries, "
"whether for Python or other languages). If that is not possible, consider "
"explicit locking."
msgstr ""

#: howto/isolating-extensions.rst:166
msgid ""
"If it is necessary to use process-global state, the simplest way to avoid "
"issues with multiple interpreters is to explicitly prevent a module from "
"being loaded more than once per process—see `Opt-Out: Limiting to One Module "
"Object per Process`_."
msgstr ""

#: howto/isolating-extensions.rst:173
msgid "Managing Per-Module State"
msgstr ""

#: howto/isolating-extensions.rst:175
msgid ""
"To use per-module state, use :ref:`multi-phase extension module "
"initialization <multi-phase-initialization>`. This signals that your module "
"supports multiple interpreters correctly."
msgstr ""

#: howto/isolating-extensions.rst:179
msgid ""
"Set ``PyModuleDef.m_size`` to a positive number to request that many bytes "
"of storage local to the module. Usually, this will be set to the size of "
"some module-specific ``struct``, which can store all of the module's C-level "
"state. In particular, it is where you should put pointers to classes "
"(including exceptions, but excluding static types) and settings (e.g. "
"``csv``'s :py:data:`~csv.field_size_limit`) which the C code needs to "
"function."
msgstr ""

#: howto/isolating-extensions.rst:188
msgid ""
"Another option is to store state in the module's ``__dict__``, but you must "
"avoid crashing when users modify ``__dict__`` from Python code. This usually "
"means error- and type-checking at the C level, which is easy to get wrong "
"and hard to test sufficiently."
msgstr ""

#: howto/isolating-extensions.rst:193
msgid ""
"However, if module state is not needed in C code, storing it in ``__dict__`` "
"only is a good idea."
msgstr ""

#: howto/isolating-extensions.rst:196
msgid ""
"If the module state includes ``PyObject`` pointers, the module object must "
"hold references to those objects and implement the module-level hooks "
"``m_traverse``, ``m_clear`` and ``m_free``. These work like ``tp_traverse``, "
"``tp_clear`` and ``tp_free`` of a class. Adding them will require some work "
"and make the code longer; this is the price for modules which can be "
"unloaded cleanly."
msgstr ""

#: howto/isolating-extensions.rst:203
msgid ""
"An example of a module with per-module state is currently available as "
"`xxlimited <https://github.com/python/cpython/blob/master/Modules/xxlimited."
"c>`__; example module initialization shown at the bottom of the file."
msgstr ""

#: howto/isolating-extensions.rst:209
msgid "Opt-Out: Limiting to One Module Object per Process"
msgstr ""

#: howto/isolating-extensions.rst:211
msgid ""
"A non-negative ``PyModuleDef.m_size`` signals that a module supports "
"multiple interpreters correctly. If this is not yet the case for your "
"module, you can explicitly make your module loadable only once per process. "
"For example::"
msgstr ""

#: howto/isolating-extensions.rst:232
msgid "Module State Access from Functions"
msgstr ""

#: howto/isolating-extensions.rst:234
msgid ""
"Accessing the state from module-level functions is straightforward. "
"Functions get the module object as their first argument; for extracting the "
"state, you can use ``PyModule_GetState``::"
msgstr ""

#: howto/isolating-extensions.rst:249
msgid ""
"``PyModule_GetState`` may return ``NULL`` without setting an exception if "
"there is no module state, i.e. ``PyModuleDef.m_size`` was zero. In your own "
"module, you're in control of ``m_size``, so this is easy to prevent."
msgstr ""

#: howto/isolating-extensions.rst:256
msgid "Heap Types"
msgstr ""

#: howto/isolating-extensions.rst:258
msgid ""
"Traditionally, types defined in C code are *static*; that is, ``static "
"PyTypeObject`` structures defined directly in code and initialized using "
"``PyType_Ready()``."
msgstr ""

#: howto/isolating-extensions.rst:262
msgid ""
"Such types are necessarily shared across the process. Sharing them between "
"module objects requires paying attention to any state they own or access. To "
"limit the possible issues, static types are immutable at the Python level: "
"for example, you can't set ``str.myattribute = 123``."
msgstr ""

#: howto/isolating-extensions.rst:268
msgid ""
"Sharing truly immutable objects between interpreters is fine, as long as "
"they don't provide access to mutable objects. However, in CPython, every "
"Python object has a mutable implementation detail: the reference count. "
"Changes to the refcount are guarded by the GIL. Thus, code that shares any "
"Python objects across interpreters implicitly depends on CPython's current, "
"process-wide GIL."
msgstr ""

#: howto/isolating-extensions.rst:275
msgid ""
"Because they are immutable and process-global, static types cannot access "
"\"their\" module state. If any method of such a type requires access to "
"module state, the type must be converted to a *heap-allocated type*, or "
"*heap type* for short. These correspond more closely to classes created by "
"Python's ``class`` statement."
msgstr ""

#: howto/isolating-extensions.rst:282
msgid "For new modules, using heap types by default is a good rule of thumb."
msgstr ""

#: howto/isolating-extensions.rst:286
msgid "Changing Static Types to Heap Types"
msgstr ""

#: howto/isolating-extensions.rst:288
msgid ""
"Static types can be converted to heap types, but note that the heap type API "
"was not designed for \"lossless\" conversion from static types—that is, "
"creating a type that works exactly like a given static type. So, when "
"rewriting the class definition in a new API, you are likely to "
"unintentionally change a few details (e.g. pickleability or inherited "
"slots). Always test the details that are important to you."
msgstr ""

#: howto/isolating-extensions.rst:297
msgid ""
"Watch out for the following two points in particular (but note that this is "
"not a comprehensive list):"
msgstr ""

#: howto/isolating-extensions.rst:300
msgid ""
"Unlike static types, heap type objects are mutable by default. Use the :c:"
"data:`Py_TPFLAGS_IMMUTABLETYPE` flag to prevent mutability."
msgstr ""

#: howto/isolating-extensions.rst:302
msgid ""
"Heap types inherit :c:member:`~PyTypeObject.tp_new` by default, so it may "
"become possible to instantiate them from Python code. You can prevent this "
"with the :c:data:`Py_TPFLAGS_DISALLOW_INSTANTIATION` flag."
msgstr ""

#: howto/isolating-extensions.rst:308
msgid "Defining Heap Types"
msgstr ""

#: howto/isolating-extensions.rst:310
msgid ""
"Heap types can be created by filling a :c:struct:`PyType_Spec` structure, a "
"description or \"blueprint\" of a class, and calling :c:func:"
"`PyType_FromModuleAndSpec` to construct a new class object."
msgstr ""

#: howto/isolating-extensions.rst:315
msgid ""
"Other functions, like :c:func:`PyType_FromSpec`, can also create heap types, "
"but :c:func:`PyType_FromModuleAndSpec` associates the module with the class, "
"allowing access to the module state from methods."
msgstr ""

#: howto/isolating-extensions.rst:319
msgid ""
"The class should generally be stored in *both* the module state (for safe "
"access from C) and the module's ``__dict__`` (for access from Python code)."
msgstr ""

#: howto/isolating-extensions.rst:325
msgid "Garbage-Collection Protocol"
msgstr ""

#: howto/isolating-extensions.rst:327
msgid ""
"Instances of heap types hold a reference to their type. This ensures that "
"the type isn't destroyed before all its instances are, but may result in "
"reference cycles that need to be broken by the garbage collector."
msgstr ""

#: howto/isolating-extensions.rst:332
msgid ""
"To avoid memory leaks, instances of heap types must implement the garbage "
"collection protocol. That is, heap types should:"
msgstr ""

#: howto/isolating-extensions.rst:336
msgid "Have the :c:data:`Py_TPFLAGS_HAVE_GC` flag."
msgstr ""

#: howto/isolating-extensions.rst:337
msgid ""
"Define a traverse function using ``Py_tp_traverse``, which visits the type "
"(e.g. using :c:expr:`Py_VISIT(Py_TYPE(self))`)."
msgstr ""

#: howto/isolating-extensions.rst:340
msgid ""
"Please refer to the :ref:`the documentation <type-structs>` of :c:data:"
"`Py_TPFLAGS_HAVE_GC` and :c:member:`~PyTypeObject.tp_traverse` for "
"additional considerations."
msgstr ""

#: howto/isolating-extensions.rst:344
msgid ""
"If your traverse function delegates to the ``tp_traverse`` of its base class "
"(or another type), ensure that ``Py_TYPE(self)`` is visited only once. Note "
"that only heap type are expected to visit the type in ``tp_traverse``."
msgstr ""

#: howto/isolating-extensions.rst:348
msgid "For example, if your traverse function includes::"
msgstr ""

#: howto/isolating-extensions.rst:352
msgid "...and ``base`` may be a static type, then it should also include::"
msgstr ""

#: howto/isolating-extensions.rst:360
msgid ""
"It is not necessary to handle the type's reference count in ``tp_new`` and "
"``tp_clear``."
msgstr ""

#: howto/isolating-extensions.rst:365
msgid "Module State Access from Classes"
msgstr ""

#: howto/isolating-extensions.rst:367
msgid ""
"If you have a type object defined with :c:func:`PyType_FromModuleAndSpec`, "
"you can call :c:func:`PyType_GetModule` to get the associated module, and "
"then :c:func:`PyModule_GetState` to get the module's state."
msgstr ""

#: howto/isolating-extensions.rst:371
msgid ""
"To save a some tedious error-handling boilerplate code, you can combine "
"these two steps with :c:func:`PyType_GetModuleState`, resulting in::"
msgstr ""

#: howto/isolating-extensions.rst:381
msgid "Module State Access from Regular Methods"
msgstr ""

#: howto/isolating-extensions.rst:383
msgid ""
"Accessing the module-level state from methods of a class is somewhat more "
"complicated, but is possible thanks to API introduced in Python 3.9. To get "
"the state, you need to first get the *defining class*, and then get the "
"module state from it."
msgstr ""

#: howto/isolating-extensions.rst:388
msgid ""
"The largest roadblock is getting *the class a method was defined in*, or "
"that method's \"defining class\" for short. The defining class can have a "
"reference to the module it is part of."
msgstr ""

#: howto/isolating-extensions.rst:392
msgid ""
"Do not confuse the defining class with :c:expr:`Py_TYPE(self)`. If the "
"method is called on a *subclass* of your type, ``Py_TYPE(self)`` will refer "
"to that subclass, which may be defined in different module than yours."
msgstr ""

#: howto/isolating-extensions.rst:397
msgid ""
"The following Python code can illustrate the concept. ``Base."
"get_defining_class`` returns ``Base`` even if ``type(self) == Sub``:"
msgstr ""

#: howto/isolating-extensions.rst:413
msgid ""
"For a method to get its \"defining class\", it must use the :data:"
"`METH_METHOD | METH_FASTCALL | METH_KEYWORDS` :c:type:`calling convention "
"<PyMethodDef>` and the corresponding :c:type:`PyCMethod` signature::"
msgstr ""

#: howto/isolating-extensions.rst:425
msgid ""
"Once you have the defining class, call :c:func:`PyType_GetModuleState` to "
"get the state of its associated module."
msgstr ""

#: howto/isolating-extensions.rst:428
msgid "For example::"
msgstr ""

#: howto/isolating-extensions.rst:456
msgid "Module State Access from Slot Methods, Getters and Setters"
msgstr ""

#: howto/isolating-extensions.rst:460
msgid "This is new in Python 3.11."
msgstr ""

#: howto/isolating-extensions.rst:468
msgid ""
"Slot methods—the fast C equivalents for special methods, such as :c:member:"
"`~PyNumberMethods.nb_add` for :py:attr:`~object.__add__` or :c:member:"
"`~PyType.tp_new` for initialization—have a very simple API that doesn't "
"allow passing in the defining class, unlike with :c:type:`PyCMethod`. The "
"same goes for getters and setters defined with :c:type:`PyGetSetDef`."
msgstr ""

#: howto/isolating-extensions.rst:475
msgid ""
"To access the module state in these cases, use the :c:func:"
"`PyType_GetModuleByDef` function, and pass in the module definition. Once "
"you have the module, call :c:func:`PyModule_GetState` to get the state::"
msgstr ""

#: howto/isolating-extensions.rst:486
msgid ""
"``PyType_GetModuleByDef`` works by searching the :term:`method resolution "
"order` (i.e. all superclasses) for the first superclass that has a "
"corresponding module."
msgstr ""

#: howto/isolating-extensions.rst:492
msgid ""
"In very exotic cases (inheritance chains spanning multiple modules created "
"from the same definition), ``PyType_GetModuleByDef`` might not return the "
"module of the true defining class. However, it will always return a module "
"with the same definition, ensuring a compatible C memory layout."
msgstr ""

#: howto/isolating-extensions.rst:500
msgid "Lifetime of the Module State"
msgstr ""

#: howto/isolating-extensions.rst:502
msgid ""
"When a module object is garbage-collected, its module state is freed. For "
"each pointer to (a part of) the module state, you must hold a reference to "
"the module object."
msgstr ""

#: howto/isolating-extensions.rst:506
msgid ""
"Usually this is not an issue, because types created with :c:func:"
"`PyType_FromModuleAndSpec`, and their instances, hold a reference to the "
"module. However, you must be careful in reference counting when you "
"reference module state from other places, such as callbacks for external "
"libraries."
msgstr ""

#: howto/isolating-extensions.rst:515
msgid "Open Issues"
msgstr ""

#: howto/isolating-extensions.rst:517
msgid "Several issues around per-module state and heap types are still open."
msgstr ""

#: howto/isolating-extensions.rst:519
msgid ""
"Discussions about improving the situation are best held on the `capi-sig "
"mailing list <https://mail.python.org/mailman3/lists/capi-sig.python.org/"
">`__."
msgstr ""

#: howto/isolating-extensions.rst:524
msgid "Per-Class Scope"
msgstr ""

#: howto/isolating-extensions.rst:526
msgid ""
"It is currently (as of Python 3.11) not possible to attach state to "
"individual *types* without relying on CPython implementation details (which "
"may change in the future—perhaps, ironically, to allow a proper solution for "
"per-class scope)."
msgstr ""

#: howto/isolating-extensions.rst:533
msgid "Lossless Conversion to Heap Types"
msgstr ""

#: howto/isolating-extensions.rst:535
msgid ""
"The heap type API was not designed for \"lossless\" conversion from static "
"types; that is, creating a type that works exactly like a given static type."
msgstr ""
