Argument Clinic: make it possible to clone __init__ functions

[erlend-aasland]

See #93057 (comment)

In sqlite3, both sqlite3.connect and sqlite3.Connection.__init__ have the same param spec. This has led to the far-from-optimal status quo:

cpython/Modules/_sqlite/module.c

Lines 51 to 69 in d93b4ac

	// NB: This needs to be in sync with the Connection.__init__ docstring.
	PyDoc_STRVAR(module_connect_doc,
	"connect($module, /, database, timeout=5.0, detect_types=0,\n"
	" isolation_level='', check_same_thread=True,\n"
	" factory=ConnectionType, cached_statements=128, uri=False, *,\n"
	" autocommit=sqlite3.LEGACY_TRANSACTION_CONTROL)\n"
	"--\n"
	"\n"
	"Opens a connection to the SQLite database file database.\n"
	"\n"
	"You can use \":memory:\" to open a database connection to a database that resides\n"
	"in RAM instead of on disk.");
	#define PYSQLITE_CONNECT_METHODDEF \
	{"connect", _PyCFunction_CAST(module_connect), METH_FASTCALL|METH_KEYWORDS, module_connect_doc},
	static PyObject *
	module_connect(PyObject *module, PyObject *const *args, Py_ssize_t nargsf,
	PyObject *kwnames)

cpython/Modules/_sqlite/connection.c

Lines 215 to 239 in d93b4ac

	// NB: This needs to be in sync with the sqlite3.connect docstring
	/*[clinic input]
	_sqlite3.Connection.__init__ as pysqlite_connection_init
	database: object
	timeout: double = 5.0
	detect_types: int = 0
	isolation_level: IsolationLevel = ""
	check_same_thread: bool = True
	factory: object(c_default='(PyObject*)clinic_state()->ConnectionType') = ConnectionType
	cached_statements as cache_size: int = 128
	uri: bool = False
	*
	autocommit: Autocommit(c_default='LEGACY_TRANSACTION_CONTROL') = sqlite3.LEGACY_TRANSACTION_CONTROL
	[clinic start generated code]*/
	static int
	pysqlite_connection_init_impl(pysqlite_Connection *self, PyObject *database,
	double timeout, int detect_types,
	const char *isolation_level,
	int check_same_thread, PyObject *factory,
	int cache_size, int uri,
	enum autocommit_mode autocommit)
	/*[clinic end generated code: output=cba057313ea7712f input=9b0ab6c12f674fa3]*/
	{

Instead, we want to be able to do this in connection.c:

/*[clinic input]
sqlite3.Connection.__init__ as pysqlite_connection_init
    database: object
    timeout: double = 5.0
    detect_types: int = 0
    isolation_level: IsolationLevel = ""
    check_same_thread: bool = True
    factory: object(c_default='(PyObject*)clinic_state()->ConnectionType') = ConnectionType
    cached_statements as cache_size: int = 128
    uri: bool = False
    *
    autocommit: Autocommit(c_default='LEGACY_TRANSACTION_CONTROL') = sqlite3.LEGACY_TRANSACTION_CONTROL
[clinic start generated code]*/

static int
pysqlite_connection_init_impl(pysqlite_Connection *self, PyObject *database,
                              double timeout, int detect_types,
                              const char *isolation_level,
                              int check_same_thread, PyObject *factory,
                              int cache_size, int uri,
                              enum autocommit_mode autocommit)
/*[clinic end generated code: output=cba057313ea7712f input=a0949fb85339104d]*/
{
    // __init__ function is here; fast forward ...
}

/*[clinic input]
# Save the clinic output config.
output push

# Create a new destination 'connect' for the docstring and methoddef only.
destination connect new file '{dirname}/clinic/_sqlite3.connect.c.h'
output everything suppress
output docstring_definition connect
output methoddef_define connect

# Now, define the connect function.
sqlite3.connect as pysqlite_connect = sqlite3.Connection.__init__

[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=7913cd0b3bfc1b4a]*/

/*[clinic input]
# Restore the clinic output config.
output pop
[clinic start generated code]*/

The methoddef and docstring for sqlite3.connect could then be included in module.c.

However, to achieve this, we need to teach Argument Clinic how to clone __init__ functions.

Linked PRs

• gh-107880: Teach Argument Clinic to clone __init__ and __new__ methods #107885
• gh-107880: Argument Clinic: Fix regression in gh-107885 #107974

[erlend-aasland]

#107885 introduced a regression; it is now possible to clone from a normal method to either __new__ or __init__ bypassing the required checks. The following code should fail:

/*[clinic input]
class C "void *" ""
C.meth
  a: int
[clinic start generated code]*/
/*[clinic input]
# This should not be allowed!
C.__new__ = C.meth
[clinic start generated code]*/
/*[clinic input]
# Neither should this
@classmethod
C.__init__ = C.meth
[clinic start generated code]*/