Align __post_init__ behaviour after copy.copy / copy.deepcopy / copy.replace with dataclasses

src/msgspec/_core.c

@@ -519,6 +519,7 @@ typedef struct {
 #endif
     PyObject *astimezone;
     PyObject *re_compile;
+    PyObject *copy_deepcopy;
     uint8_t gc_cycle;
 } MsgspecState;
 
@@ -7938,6 +7939,66 @@ Struct_copy(PyObject *self, PyObject *args)
     return NULL;
 }
 
+
+static PyObject* get_deepcopy_func() {
+    // lazily copy.deepcopy and cache in global state
+    PyObject *copy_mod, *deepcopy_func;
+    MsgspecState* st = msgspec_get_global_state();
+    deepcopy_func = st->copy_deepcopy;
+    if (deepcopy_func == NULL) {
+        copy_mod = PyImport_ImportModule("copy");
+        if (copy_mod == NULL) return NULL;
+        deepcopy_func = PyObject_GetAttrString(copy_mod, "deepcopy");
+        st->copy_deepcopy = deepcopy_func;
+        Py_DECREF(copy_mod);
+        if (st->copy_deepcopy == NULL) return NULL;
+    }
+
+    return deepcopy_func;
+}
+
+static PyObject *
+Struct_deepcopy(PyObject *self, PyObject *args)
+{
+    PyObject *memo;
+    PyObject *val = NULL, *res = NULL, *dc_val = NULL;
+    PyObject *deepcopy_func;
+    Py_ssize_t i, nfields;
+
+    if (!PyArg_ParseTuple(args, "O!:__deepcopy__", &PyDict_Type, &memo))
+        return NULL;
+
+    deepcopy_func = get_deepcopy_func();
+
+    res = Struct_alloc(Py_TYPE(self));
+    if (res == NULL)
+        return NULL;
+
+    nfields = StructMeta_GET_NFIELDS(Py_TYPE(self));
+    for (i = 0; i < nfields; i++) {
+        val = Struct_get_index(self, i);
+        if (val == NULL)
+            goto error;
+
+        dc_val = PyObject_CallFunctionObjArgs(deepcopy_func, val, memo, NULL);
+        if (dc_val == NULL)
+            goto error;
+
+        Struct_set_index(res, i, dc_val);
+    }
+
+    /* If self is tracked, then copy is tracked */
+    if (MS_OBJECT_IS_GC(self) && MS_IS_TRACKED(self))
+        PyObject_GC_Track(res);
+
+    return res;
+
+error:
+    Py_DECREF(res);
+    return NULL;
+}
+
+
 static PyObject *
 Struct_replace(
     PyObject *self,
@@ -8002,6 +8063,8 @@ Struct_replace(
         }
     }
 
+    if (Struct_post_init(struct_type, out) < 0) goto error;
+
     if (is_gc && !should_untrack) {
         PyObject_GC_Track(out);
     }
@@ -8358,6 +8421,7 @@ StructMixin_config(StructMetaObject *self, void *closure) {
 
 static PyMethodDef Struct_methods[] = {
     {"__copy__", Struct_copy, METH_NOARGS, "copy a struct"},
+    {"__deepcopy__", Struct_deepcopy, METH_VARARGS, "deepcopy a struct"},
     {"__replace__", (PyCFunction) Struct_replace, METH_FASTCALL | METH_KEYWORDS, "create a new struct with replacements" },
     {"__reduce__", Struct_reduce, METH_NOARGS, "reduce a struct"},
     {"__rich_repr__", Struct_rich_repr, METH_NOARGS, "rich repr"},
@@ -22306,6 +22370,7 @@ msgspec_clear(PyObject *m)
 #endif
     Py_CLEAR(st->astimezone);
     Py_CLEAR(st->re_compile);
+    Py_CLEAR(st->copy_deepcopy);
     return 0;
 }
 
@@ -22380,6 +22445,7 @@ msgspec_traverse(PyObject *m, visitproc visit, void *arg)
 #endif
     Py_VISIT(st->astimezone);
     Py_VISIT(st->re_compile);
+    Py_VISIT(st->copy_deepcopy);
     return 0;
 }
 
@@ -22674,6 +22740,9 @@ PyInit__core(void)
     Py_DECREF(temp_module);
     if (st->re_compile == NULL) return NULL;
 
+    // cache for 'copy.deepcopy'. to access this function, use 'get_deepcopy_func'
+    st->copy_deepcopy = NULL;
+
     /* Initialize cached constant strings */
 #define CACHED_STRING(attr, str) \
     if ((st->attr = PyUnicode_InternFromString(str)) == NULL) return NULL

tests/unit/test_struct.py

@@ -810,12 +810,65 @@ class Test(Struct):
         b: int
         a: int
 
-    x = copy.copy(Test(1, 2))
+    o = Test(1, 2)
+    x = copy.copy(o)
     assert type(x) is Test
+    assert x is not o
     assert x.b == 1
     assert x.a == 2
 
 
+def test_struct_deepcopy():
+    o = Struct()
+    x = copy.deepcopy(Struct())
+    assert type(x) is Struct
+    assert x is not o
+
+    class Sub(Struct):
+        one: str
+        two: list[int]
+
+    class Test(Struct):
+        a: int
+        b: int
+        c: list[str]
+        sub: Sub
+
+    o = Test(
+        a=1,
+        b=2,
+        c=["1", "2"],
+        sub=Sub(one="hello", two=[3]),
+    )
+    x = copy.deepcopy(o)
+    assert type(x) is Test
+    assert x.a == 1
+    assert x.b == 2
+    assert x.c == ["1", "2"]
+    assert x.c is not o.c
+    assert x.sub is not o.sub
+    assert x.sub.one == "hello"
+    assert x.sub.two == [3]
+    assert x.sub.two is not o.sub.two
+
+
+def test_struct_deepcopy_custom_impl():
+    # ensure we respect custom __deepcopy__ methods
+    class CustomThing:
+        def __init__(self, value):
+            self.value = value
+
+        def __deepcopy__(self, memo):
+            return CustomThing(value=self.value + 1)
+
+    class TestWithCustom(Struct):
+        custom: CustomThing
+
+    t = TestWithCustom(CustomThing(1))
+    tc = copy.deepcopy(t)
+    assert tc.custom.value == 2
+
+
 class FrozenPoint(Struct, frozen=True):
     x: int
     y: int
@@ -2664,7 +2717,7 @@ def __post_init__(self):
         assert x1 == x2
         assert count == 1
 
-    def test_post_init_not_called_on_replace(self):
+    def test_post_init_not_called_on_deepcopy(self):
         count = 0
 
         class Ex(Struct):
@@ -2674,6 +2727,20 @@ def __post_init__(self):
 
         x1 = Ex()
         assert count == 1
-        x2 = msgspec.structs.replace(x1)
+        x2 = copy.deepcopy(x1)
         assert x1 == x2
         assert count == 1
+
+    def test_post_init_called_on_replace(self, replace):
+        count = 0
+
+        class Ex(Struct):
+            def __post_init__(self):
+                nonlocal count
+                count += 1
+
+        x1 = Ex()
+        assert count == 1
+        x2 = replace(x1)
+        assert x1 == x2
+        assert count == 2