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bpo-32379: Faster MRO computation for single inheritance #4932

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Merged
pitrou merged 5 commits into from
Dec 20, 2017
Merged

bpo-32379: Faster MRO computation for single inheritance #4932

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4249fbe
bpo-32379: Faster MRO computation for single inheritance
pitrou Dec 19, 2017
eeae6e4
Add NEWS blurb
pitrou Dec 19, 2017
0ea23a5
Ensure mro() returns a list.
pitrou Dec 20, 2017
f3b5d4a
Remove unrelated change
pitrou Dec 20, 2017
8f345a3
Address review comments
pitrou Dec 20, 2017
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6 changes: 6 additions & 0 deletions Lib/test/test_descr.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1783,6 +1783,12 @@ class C(A):
def f(self): return "C"
class D(B, C):
pass
self.assertEqual(A.mro(), [A, object])
self.assertEqual(A.__mro__, (A, object))
self.assertEqual(B.mro(), [B, A, object])
self.assertEqual(B.__mro__, (B, A, object))
self.assertEqual(C.mro(), [C, A, object])
self.assertEqual(C.__mro__, (C, A, object))
self.assertEqual(D.mro(), [D, B, C, A, object])
self.assertEqual(D.__mro__, (D, B, C, A, object))
self.assertEqual(D().f(), "C")
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2017-12-19-21-14-41.bpo-32379.B7mOmI.rst
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@@ -0,0 +1 @@
Make MRO computation faster when a class inherits from a single base.
40 changes: 36 additions & 4 deletions Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1761,6 +1761,36 @@ mro_implementation(PyTypeObject *type)
return NULL;
}

bases = type->tp_bases;
n = PyTuple_GET_SIZE(bases);
if (n == 1) {
/* Fast path: if there is a single base, constructing the MRO
* is trivial.
*/
PyTypeObject *base = (PyTypeObject *)PyTuple_GET_ITEM(bases, 0);
Py_ssize_t k;

if (base->tp_mro == NULL) {
PyErr_Format(PyExc_TypeError,
"Cannot extend an incomplete type '%.100s'",
base->tp_name);
return NULL;
}
k = PyTuple_GET_SIZE(base->tp_mro);
result = PyTuple_New(k + 1);

@serhiy-storchaka serhiy-storchaka Dec 20, 2017

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Shouldn't the result be a tuple?

@pitrou pitrou Dec 20, 2017

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You mean a list? The caller calls PySequence_Tuple on the result, so it's faster if we create a tuple upfront.

if (result == NULL) {
return NULL;
}
Py_INCREF(type);
PyTuple_SET_ITEM(result, 0, (PyObject *) type);
for (i = 0; i < k; i++) {
PyObject *cls = PyTuple_GET_ITEM(base->tp_mro, i);
Py_INCREF(cls);
PyTuple_SET_ITEM(result, i + 1, cls);
}
return result;
}

/* Find a superclass linearization that honors the constraints
of the explicit lists of bases and the constraints implied by
each base class.
Expand All @@ -1770,9 +1800,6 @@ mro_implementation(PyTypeObject *type)
to_merge is the declared list of bases.
*/

bases = type->tp_bases;
n = PyTuple_GET_SIZE(bases);

to_merge = PyList_New(n+1);
if (to_merge == NULL)
return NULL;
Expand Down Expand Up @@ -1830,7 +1857,12 @@ static PyObject *
type_mro_impl(PyTypeObject *self)
/*[clinic end generated code: output=bffc4a39b5b57027 input=28414f4e156db28d]*/
{
return mro_implementation(self);
PyObject *seq;
seq = mro_implementation(self);
if (seq != NULL && !PyList_Check(seq)) {
Py_SETREF(seq, PySequence_List(seq));
}
return seq;
}

static int
Expand Down