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bpo-32604: Add support for a "default" arg in channel_recv(). (GH-19770)
This allows the caller to avoid creation of an exception when the channel is empty (just like `dict.get()` works). `ChannelEmptyError` is still raised if no default is provided. Automerge-Triggered-By: @ericsnowcurrently
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2 files changed

+46
-11
lines changed

2 files changed

+46
-11
lines changed

Lib/test/test__xxsubinterpreters.py

Lines changed: 21 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1302,6 +1302,27 @@ def test_recv_empty(self):
13021302
with self.assertRaises(interpreters.ChannelEmptyError):
13031303
interpreters.channel_recv(cid)
13041304

1305+
def test_recv_default(self):
1306+
default = object()
1307+
cid = interpreters.channel_create()
1308+
obj1 = interpreters.channel_recv(cid, default)
1309+
interpreters.channel_send(cid, None)
1310+
interpreters.channel_send(cid, 1)
1311+
interpreters.channel_send(cid, b'spam')
1312+
interpreters.channel_send(cid, b'eggs')
1313+
obj2 = interpreters.channel_recv(cid, default)
1314+
obj3 = interpreters.channel_recv(cid, default)
1315+
obj4 = interpreters.channel_recv(cid)
1316+
obj5 = interpreters.channel_recv(cid, default)
1317+
obj6 = interpreters.channel_recv(cid, default)
1318+
1319+
self.assertIs(obj1, default)
1320+
self.assertIs(obj2, None)
1321+
self.assertEqual(obj3, 1)
1322+
self.assertEqual(obj4, b'spam')
1323+
self.assertEqual(obj5, b'eggs')
1324+
self.assertIs(obj6, default)
1325+
13051326
def test_run_string_arg_unresolved(self):
13061327
cid = interpreters.channel_create()
13071328
interp = interpreters.create()

Modules/_xxsubinterpretersmodule.c

Lines changed: 25 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -1350,19 +1350,16 @@ _channel_recv(_channels *channels, int64_t id)
13501350
_PyCrossInterpreterData *data = _channel_next(chan, PyInterpreterState_GetID(interp));
13511351
PyThread_release_lock(mutex);
13521352
if (data == NULL) {
1353-
if (!PyErr_Occurred()) {
1354-
PyErr_Format(ChannelEmptyError, "channel %" PRId64 " is empty", id);
1355-
}
13561353
return NULL;
13571354
}
13581355

13591356
// Convert the data back to an object.
13601357
PyObject *obj = _PyCrossInterpreterData_NewObject(data);
1358+
_PyCrossInterpreterData_Release(data);
1359+
PyMem_Free(data);
13611360
if (obj == NULL) {
13621361
return NULL;
13631362
}
1364-
_PyCrossInterpreterData_Release(data);
1365-
PyMem_Free(data);
13661363

13671364
return obj;
13681365
}
@@ -2351,20 +2348,37 @@ Add the object's data to the channel's queue.");
23512348
static PyObject *
23522349
channel_recv(PyObject *self, PyObject *args, PyObject *kwds)
23532350
{
2354-
static char *kwlist[] = {"cid", NULL};
2351+
static char *kwlist[] = {"cid", "default", NULL};
23552352
int64_t cid;
2356-
if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:channel_recv", kwlist,
2357-
channel_id_converter, &cid)) {
2353+
PyObject *dflt = NULL;
2354+
if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&|O:channel_recv", kwlist,
2355+
channel_id_converter, &cid, &dflt)) {
23582356
return NULL;
23592357
}
2358+
Py_XINCREF(dflt);
23602359

2361-
return _channel_recv(&_globals.channels, cid);
2360+
PyObject *obj = _channel_recv(&_globals.channels, cid);
2361+
if (obj != NULL) {
2362+
Py_XDECREF(dflt);
2363+
return obj;
2364+
} else if (PyErr_Occurred()) {
2365+
Py_XDECREF(dflt);
2366+
return NULL;
2367+
} else if (dflt != NULL) {
2368+
return dflt;
2369+
} else {
2370+
PyErr_Format(ChannelEmptyError, "channel %" PRId64 " is empty", cid);
2371+
return NULL;
2372+
}
23622373
}
23632374

23642375
PyDoc_STRVAR(channel_recv_doc,
2365-
"channel_recv(cid) -> obj\n\
2376+
"channel_recv(cid, [default]) -> obj\n\
2377+
\n\
2378+
Return a new object from the data at the front of the channel's queue.\n\
23662379
\n\
2367-
Return a new object from the data at the from of the channel's queue.");
2380+
If there is nothing to receive then raise ChannelEmptyError, unless\n\
2381+
a default value is provided. In that case return it.");
23682382

23692383
static PyObject *
23702384
channel_close(PyObject *self, PyObject *args, PyObject *kwds)

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