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bpo-44653: Support typing.Union in parameter substitution of the union type #27232

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bpo-44653: Support typing.Union in parameter substitution of the union type #27232

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15 changes: 15 additions & 0 deletions Lib/test/test_types.py
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Original file line number Diff line number Diff line change
Expand Up @@ -772,6 +772,21 @@ def test_union_parameter_chaining(self):
self.assertEqual((list[T] | list[S])[int, T], list[int] | list[T])
self.assertEqual((list[T] | list[S])[int, int], list[int])

def test_union_parameter_substitution_union(self):
T = typing.TypeVar("T")
res = (int | T)[str | list]
self.assertEqual(res, int | str | list)
self.assertIsInstance(res, types.Union)
res = (int | T)[str | int]
self.assertEqual(res, int | str)
self.assertIsInstance(res, types.Union)
res = (int | T)[typing.Union[str, list]]
self.assertEqual(res, int | str | list)
self.assertIsInstance(res, types.Union)
res = (int | T)[typing.Union[str, int]]
self.assertEqual(res, int | str)
self.assertIsInstance(res, types.Union)

def test_union_parameter_substitution_errors(self):
T = typing.TypeVar("T")
x = int | T
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2021-07-18-19-32-38.bpo-44653.lu99mE.rst
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@@ -0,0 +1 @@
Support ``typing.Union`` in parameter substitution of the union type.
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Maybe this is more specific:

Suggested change
Support ``typing.Union`` in parameter substitution of the union type.
Flatten ``typing.Union`` during parameter substitution of the union type.

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Is it better? Before this PR typing.Union was not supported at all.

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Hmm oh right I forgot. I think it's good to also mention that the arguments are flattened though, since none of the other types are treated like that. Some people might expect (int | T)[Union[str, int]] to become int | Union[str, int].

Suggested change
Support ``typing.Union`` in parameter substitution of the union type.
Support ``typing.Union`` in parameter substitution of the union type.
Arguments in ``typing.Union`` are flattened and merged into the
union type.

45 changes: 41 additions & 4 deletions Objects/unionobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -273,6 +273,12 @@ is_new_type(PyObject *obj)
return is_typing_module(obj);
}

static int
is_typing_union(PyObject *obj)
{
return is_typing_name(obj, "_UnionGenericAlias");
}

// Emulates short-circuiting behavior of the ``||`` operator
// while also checking negative values.
#define CHECK_RES(res) { \
Expand Down Expand Up @@ -429,6 +435,19 @@ static PyMethodDef union_methods[] = {
{0}};


static PyObject *
from_typing_union(PyObject *obj)
{
_Py_IDENTIFIER(__args__);
PyObject *args = _PyObject_GetAttrId(obj, &PyId___args__);
if (args == NULL) {
return NULL;
}
PyObject *result = make_union(args);
Py_DECREF(args);
return result;
}

static PyObject *
union_getitem(PyObject *self, PyObject *item)
{
Expand All @@ -447,16 +466,34 @@ union_getitem(PyObject *self, PyObject *item)
}

// Check arguments are unionable.
assert(Py_REFCNT(newargs) == 1);
Py_ssize_t nargs = PyTuple_GET_SIZE(newargs);
for (Py_ssize_t iarg = 0; iarg < nargs; iarg++) {
PyObject *arg = PyTuple_GET_ITEM(newargs, iarg);
int is_arg_unionable = is_unionable(arg);
if (is_arg_unionable <= 0) {
Py_DECREF(newargs);
if (is_arg_unionable == 0) {
int r = is_unionable(arg);
if (r == 0) {
r = is_typing_union(arg);
if (r == 0) {
PyErr_Format(PyExc_TypeError,
"Each union argument must be a type, got %.100R", arg);
Py_DECREF(newargs);
return NULL;
}
if (r > 0) {
// Replace typing.Union with types.Union.
PyObject *newarg = from_typing_union(arg);
if (newarg == NULL) {
Py_DECREF(newargs);
return NULL;
}
assert(Py_REFCNT(newargs) == 1);
assert(PyTuple_GET_ITEM(newargs, iarg) == arg);
PyTuple_SET_ITEM(newargs, iarg, newarg);
Py_DECREF(arg);
}
}
if (r < 0) {
Py_DECREF(newargs);
return NULL;
}
}
Expand Down