4 files changed, 32 insertions, 4 deletions
diff --git a/numpy/core/multiarray.py b/numpy/core/multiarray.py
index ec1294b85..d11283345 100644
--- a/numpy/core/multiarray.py
+++ b/numpy/core/multiarray.py
@@ -17,7 +17,7 @@ from ._multiarray_umath import (
     fastCopyAndTranspose, _flagdict, from_dlpack, _place, _reconstruct,
     _vec_string, _ARRAY_API, _monotonicity, _get_ndarray_c_version,
     _get_madvise_hugepage, _set_madvise_hugepage,
-    _get_promotion_state, _set_promotion_state,
+    _get_promotion_state, _set_promotion_state, _using_numpy2_behavior
     )
 
 __all__ = [
@@ -42,7 +42,7 @@ __all__ = [
     'set_legacy_print_mode', 'set_numeric_ops', 'set_string_function',
     'set_typeDict', 'shares_memory', 'tracemalloc_domain', 'typeinfo',
     'unpackbits', 'unravel_index', 'vdot', 'where', 'zeros',
-    '_get_promotion_state', '_set_promotion_state']
+    '_get_promotion_state', '_set_promotion_state', '_using_numpy2_behavior']
 
 # For backward compatibility, make sure pickle imports these functions from here
 _reconstruct.__module__ = 'numpy.core.multiarray'
@@ -72,6 +72,7 @@ seterrobj.__module__ = 'numpy'
 zeros.__module__ = 'numpy'
 _get_promotion_state.__module__ = 'numpy'
 _set_promotion_state.__module__ = 'numpy'
+_using_numpy2_behavior.__module__ = 'numpy'
 
 
 # We can't verify dispatcher signatures because NumPy's C functions don't
diff --git a/numpy/core/numeric.py b/numpy/core/numeric.py
index 864f47947..fbb284696 100644
--- a/numpy/core/numeric.py
+++ b/numpy/core/numeric.py
@@ -17,7 +17,8 @@ from .multiarray import (
     fromstring, inner, lexsort, matmul, may_share_memory,
     min_scalar_type, ndarray, nditer, nested_iters, promote_types,
     putmask, result_type, set_numeric_ops, shares_memory, vdot, where,
-    zeros, normalize_axis_index, _get_promotion_state, _set_promotion_state)
+    zeros, normalize_axis_index, _get_promotion_state, _set_promotion_state,
+    _using_numpy2_behavior)
 
 from . import overrides
 from . import umath
@@ -54,7 +55,8 @@ __all__ = [
     'False_', 'True_', 'bitwise_not', 'CLIP', 'RAISE', 'WRAP', 'MAXDIMS',
     'BUFSIZE', 'ALLOW_THREADS', 'full', 'full_like',
     'matmul', 'shares_memory', 'may_share_memory', 'MAY_SHARE_BOUNDS',
-    'MAY_SHARE_EXACT', '_get_promotion_state', '_set_promotion_state']
+    'MAY_SHARE_EXACT', '_get_promotion_state', '_set_promotion_state',
+    '_using_numpy2_behavior']
 
 
 def _zeros_like_dispatcher(a, dtype=None, order=None, subok=None, shape=None):
diff --git a/numpy/core/src/multiarray/multiarraymodule.c b/numpy/core/src/multiarray/multiarraymodule.c
index db7fda32d..4fa58c4df 100644
--- a/numpy/core/src/multiarray/multiarraymodule.c
+++ b/numpy/core/src/multiarray/multiarraymodule.c
@@ -98,6 +98,12 @@ _umath_strings_richcompare(
  * future.
  */
 int npy_legacy_print_mode = INT_MAX;
+/*
+ * Global variable to check whether NumPy 2.0 behavior is opted in.
+ * This flag is considered a runtime constant.
+ */
+int npy_numpy2_behavior = NPY_FALSE;
+
 
 static PyObject *
 set_legacy_print_mode(PyObject *NPY_UNUSED(self), PyObject *args)
@@ -4416,6 +4422,15 @@ _reload_guard(PyObject *NPY_UNUSED(self), PyObject *NPY_UNUSED(args)) {
     Py_RETURN_NONE;
 }
 
+
+static PyObject *
+_using_numpy2_behavior(
+        PyObject *NPY_UNUSED(self), PyObject *NPY_UNUSED(args))
+{
+    return PyBool_FromLong(npy_numpy2_behavior);
+}
+
+
 static struct PyMethodDef array_module_methods[] = {
     {"_get_implementing_args",
         (PyCFunction)array__get_implementing_args,
@@ -4641,6 +4656,8 @@ static struct PyMethodDef array_module_methods[] = {
     {"_reload_guard", (PyCFunction)_reload_guard,
         METH_NOARGS,
         "Give a warning on reload and big warning in sub-interpreters."},
+    {"_using_numpy2_behavior", (PyCFunction)_using_numpy2_behavior,
+        METH_NOARGS, NULL},
     {"from_dlpack", (PyCFunction)from_dlpack,
         METH_O, NULL},
     {NULL, NULL, 0, NULL}                /* sentinel */
@@ -4913,6 +4930,12 @@ initialize_static_globals(void)
         return -1;
     }
 
+    /* Initialize from certain environment variabels: */
+    char *env = getenv("NPY_NUMPY_2_BEHAVIOR");
+    if (env != NULL && strcmp(env, "0") != 0) {
+        npy_numpy2_behavior = NPY_TRUE;
+    }
+
     return 0;
 }
 
diff --git a/numpy/core/src/multiarray/multiarraymodule.h b/numpy/core/src/multiarray/multiarraymodule.h
index 809736cd2..992acd09f 100644
--- a/numpy/core/src/multiarray/multiarraymodule.h
+++ b/numpy/core/src/multiarray/multiarraymodule.h
@@ -1,6 +1,8 @@
 #ifndef NUMPY_CORE_SRC_MULTIARRAY_MULTIARRAYMODULE_H_
 #define NUMPY_CORE_SRC_MULTIARRAY_MULTIARRAYMODULE_H_
 
+NPY_VISIBILITY_HIDDEN extern int npy_numpy2_behavior;
+
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_current_allocator;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_array;
 NPY_VISIBILITY_HIDDEN extern PyObject * npy_ma_str_array_function;