From recent discussions around “what should have a PEP”, it’s clear that this should have been a PEP long ago. Better late than never, I guess!
We submit this PEP to explain the changes, seek consensus on whether they are good, propose the remaining changes, and set best practices for new modules.
Please head to the PEP page to read it. The initial version is below, so you can quote it, and so that if the PEP is changed future readers can make sense of the discussion.
Text of the PEP:
Title: Isolating modules in the standard library
Author: Erlend Egeberg Aasland firstname.lastname@example.org, Petr Viktorin email@example.com
Type: Standards Track
Requires: 489, 573, 630
Extensions in the standard library will be converted to multi-phase
489) and where possible, all state will be
stored on module objects rather than in process-global variables.
Much of this proposal has already been implemented.
We submit this PEP to explain the changes, seek consensus on
whether they are good, propose the remaining changes,
and set best practices for new modules.
The informational :pep:
630 describes the background, motivation, rationale,
implications and implementation notes of the proposed changes as they apply
generally to any extension module (not just the standard library).
It is an integral part of this proposal. Read it first.
This PEP discusses specifics of the standard library.
The body of :pep:
630 will be converted to a HOWTO in the Python
documentation, and that PEP will be retired (marked Final).
All extension modules in the standard library will be converted to multi-phase
initialization introduced in :pep:
All stdlib extension modules will be isolated. That is:
Types, functions and other objects defined by the module will either be
immutable, or not shared with other module instances.
State specific to the module will not be shared with other module instances,
unless it represents global state.
_csv.field_size_limitwill get/set a module-specific
number. On the other hand, functions like
os.getpidwill continue to work with state that is process-global
(external to the module, and possibly shared across other libraries, including
Types whose methods need access to their module instance will be converted
to heap types following :pep:
630, with the following considerations:
All standard library types that used to be static types should remain
immutable. Heap types must be defined with the
flag to retain immutability.
Tests should ensure
TypeErroris raised when trying to create a new
attribute of an immutable type.
A static type with
tp_new = NULLdoes not have a public constructor, but
heap types inherit the constructor from the base class. Make sure types that
previously were impossible to instantiate retain that feature; use
Py_TPFLAGS_DISALLOW_INSTANTIATION. Add tests using
Converted heap types may unintentionally become serializable
pickle-able). Test that calling
pickle.dumpshas the same result
before and after conversion, and if the test fails, add a
method that raises
for an example.
These issues will be added to the Devguide to help any future conversions.
If another kind of issue is found, the module in question should be unchanged
until a solution is found and added to the Devguide, and already
converted modules are checked and fixed.
Static types that do not need module state access, and have no other reason to
be converted, should stay static.
The following process should be added to the Devguide, and remain until
all modules are converted.
Any new findings should be documented there or in the general HOWTO.
Part 1: Preparation
- Open a discussion, either on the bug tracker or on Discourse. Involve the
module maintainer and/or code owner. Explain the reason and rationale for
- Identify global state performance bottlenecks.
Create a proof-of-concept implementation, and measure the performance impact.
pyperfis a good tool for benchmarking.
- Create an implementation plan. For small modules with few types, a single PR
may do the job. For larger modules with lots of types, and possibly also
external library callbacks, multiple PR’s will be needed.
Part 2: Implementation
Note: this is a suggested implementation plan for a complex module, based on
lessons learned with other modules. Feel free to simplify it for
- Add Argument Clinic where possible; it enables you to easily use the
defining class to fetch module state from type methods.
- Prepare for module state; establish a module state
struct, add an instance
as a static global variable, and create helper stubs for fetching the module
- Add relevant global variables to the module state
struct, and modify code
that accesses the global state to use the module state helpers instead. This
step may be broken into several PR’s.
- Where necessary, convert heap types to static types.
- Convert the global module state struct to true module state.
- Implement multi-phase initialisation.
Steps 4 through 6 should preferably land in a single alpha development phase.
Extension modules in the standard library will now be loadable more than once.
For example, deleting such a module from
sys.modules and re-importing it
will result in a fresh module instance, isolated from any previously loaded
This may affect code that expected the previous behavior: globals of
extension modules were shallowly copied from the first loaded module.
A large part of this proposal is a HOWTO aimed at experienced users,
which will be moved to the documentation.
Beginners should not be affected.
Most of the changes are now in the main branch, as commits for these issues:
bpo-40077, Convert static types to heap types: use PyType_FromSpec() <https://bugs.python.org/issue40077>_
bpo-46417, Clear static types in Py_Finalize() for embedded Python <https://bugs.python.org/issue46417>_
bpo-1635741, Py_Finalize() doesn't clear all Python objects at exit <https://bugs.python.org/issue1635741>_
As an example, changes and fix-ups done in the
_csv module are:
GH-23224, Remove static state from the _csv module <https://github.com/python/cpython/pull/23224>_
GH-26008, Allow subclassing of csv.Error <https://github.com/python/cpython/pull/26008>_
GH-26074, Add GC support to _csv heap types <https://github.com/python/cpython/pull/26074>_
GH-26351, Make heap types converted during 3.10 alpha immutable <https://github.com/python/cpython/pull/26351>_
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.