[Python-3000] PEP 3121: Module Initialization and finalization (original) (raw)

Brett Cannon brett at python.org
Fri Apr 27 20:23:49 CEST 2007

On 4/27/07, "Martin v. Löwis" <martin at v.loewis.de> wrote:

Continuing a discussion from last April, I added PEP 3121, included below for convenience. Please comment.

Regards, Martin PEP: 3121 Title: Module Initialization and finalization Version: Revision:54998Revision: 54998 Revision:54998 Last-Modified: Date:2007−04−2710:31:58+0200(Fr,27Apr2007)Date: 2007-04-27 10:31:58 +0200 (Fr, 27 Apr 2007) Date:2007−04−2710:31:58+0200(Fr,27Apr2007) Author: Martin v. Löwis <martin at v.loewis.de> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 27-Apr-2007 Python-Version: 3.0 Post-History: Abstract ======== Module initialization currently has a few deficiencies. There is no cleanup for modules, the entry point name might give naming conflicts, the entry functions don't follow the usual calling convention, and multiple interpreters are not supported well. This PEP addresses these issues.

Thanks for trying to solve this, Martin!

Module Finalization ===================

Currently, C modules are initialized usually once and then "live" forever. The only exception is when PyFinalize() is called: then the initialization routine is invoked a second time. This is bad from a resource management point of view: memory and other resources might get allocated each time initialization is called, but there is no way to reclaim them. As a result, there is currently no way to completely release all resources Python has allocated. Entry point name conflicts ========================== The entry point is currently called init. This might conflict with other symbols also called init. In particular, initsocket is known to have conflicted in the past (this specific problem got resolved as a side effect of renaming the module to socket). Entry point signature ===================== The entry point is currently a procedure (returning void). This deviates from the usual calling conventions; callers can find out whether there was an error during initialization only by checking PyErrOccurred. The entry point should return a PyObject*, which will be the module created, or NULL in case of an exception. Multiple Interpreters ===================== Currently, extension modules share their state across all interpreters. This allows for undesirable information leakage across interpreters: one script could permanently corrupt objects in an extension module, possibly breaking all scripts in other interpreters.

After the intro and up to here, it would seem like changing each section into a sub-section of a "Problems" section of some sort would make the document more organized and easier to read.

Specification =============

The module initialization routines change their signature to:: PyObject *PyInit() The initialization routine will be invoked once per interpreter, when the module is imported. It should return a new module object each time. In order to store per-module state in C variables, each module object will contain a block of memory that is interpreted only by the module. The amount of memory used for the module is specified at the point of creation of the module. In addition to the initialization function, a module may implement a number of additional callback function, which are invoked when the module's tptraverse, tpclear, and tpfree functions are invoked, and when the module is reloaded. The entire module definition is combined in a struct PyModuleDef:: struct PyModuleDef{ PyModuleDefBase mbase; /* To be filled out by the interpreter */ Pyssizet msize; /* Size of per-module data */ PyMethodDef *mmethods; inquiry mreload; traverseproc mtraverse; inquiry mclear; freefunc mfree; }; Creation of a module is changed to expect an optional PyModuleDef*. The module state will be null-initialized. Each module method with be passed the module object

I think you meant "will", not "with".

as the first parameter. To access the module data, a function::

void* PyModuleGetData(PyObject*); will be provided. In addition, to lookup a module more efficiently than going through sys.modules, a function:: PyObject* PyStateFindModule(struct PyModuleDef*); will be provided. This lookup function will use an index located in the mbase field, to find the module by index, not by name. As all Python objects should be controlled through the Python memory management, usage of "static" type objects is discouraged, unless the type object itself has no memory-managed state.

Ooh, I like this side-effect of the proposal!

To simplify definition of heap types, a new method::

PyTypeObject* PyTypeCopy(PyTypeObject*); is added. Example ======= xxmodule.c would be changed to remove the initxx function, and add the following code instead:: struct xxstate{ PyObject *ErrorObject; PyObject *XxoType; }; #define xxstate(o) ((struct xxstate*)PyModuleGetState(o)) static int xxtraverse(PyObject *m, visitproc v, void *arg) { PyVISIT(xxstate(m)->ErrorObject); PyVISIT(xxstate(m)->XxoType); return 0; } static int xxclear(PyObject *m) { PyCLEAR(xxstate(m)->ErrorObject); PyCLEAR(xxstate(m)->XxoType); return 0; } static struct PyModuleDef xxmodule = { {}, /* mbase */ sizeof(struct xxstate), &xxmethods, 0, /* mreload */ xxtraverse, xxclear, 0, /* mfree - not needed, since all is done in mclear */ } PyObject* PyInitxx() { PyObject *res = PyModuleNew("xx", &xxmodule); if (!res) return NULL; xxstate(res)->ErrorObject = PyErrNewException("xx.error, NULL, NULL); if (!xxstate(res)->ErrorObject) { PyDECREF(res); return NULL; } xxstate(res)->XxoType = PyTypeCopy(&XxoType); if (!xxstate(res)->XxoType) { PyDECREF(res); return NULL; } return res; }

How would I raise xx.error in C code now? I am guessing like this::

PyObject* module = PyState_FindModule(&xxmodule); if (!module) return NULL; PyObject* xx_error = xxstate(module)->ErrorObject; if (!xx_error) { PyErr_SetString(PyExc_SystemError, "xx.error missing"); return NULL; } PyErr_SetString(xx_error, "oops"); return NULL;

Since most objects will move to being memory-managed, one needs to worry about checking that the object still exists. I assume I didn't go overboard with the error checking here, right? I guess people are going to end up writing helper functions to access the various data fields as the above would get rather tedious if you had to write it more than twice.

Discussion ========== Tim Peters reports in [1] that PythonLabs considered such a feature at one point, and lists the following additional hooks which aren't currently supported in this PEP: * when the module object is deleted from sys.modules * when PyFinalize is called * when Python exits

Wouldn't the above be covered by the deallocation of the module?

Overall I like the idea. I think people will need to get used to the idea of writing more accessor functions for the data field, though, if using static variables to hold things like exceptions becomes discouraged.

-Brett

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