API: Add environment variable for behavior planned in a 2.0

The idea of the flag is not to allow to change it right now, since
there may be some things where that is hard to do in general, and
it doesn't seem relevant:  nobody is supposed to use it besides for
testing.

3 files changed, 27 insertions, 2 deletions

diff --git a/numpy/__init__.py b/numpy/__init__.py
index 052ba7327..976062d13 100644
--- a/numpy/__init__.py
+++ b/numpy/__init__.py
@@ -122,6 +122,10 @@ except NameError:
 if __NUMPY_SETUP__:
     sys.stderr.write('Running from numpy source directory.\n')
 else:
+    # Make variable available during multiarray/C initialization
+    import os
+    _numpy2_behavior = os.environ.get("NPY_NUMPY_2_BEHAVIOR", "0") != "0"
+
     try:
         from numpy.__config__ import show as show_config
     except ImportError as e:
@@ -392,7 +396,6 @@ else:
     # is slow and thus better avoided.
     # Specifically kernel version 4.6 had a bug fix which probably fixed this:
     # https://github.com/torvalds/linux/commit/7cf91a98e607c2f935dbcc177d70011e95b8faff
-    import os
     use_hugepage = os.environ.get("NUMPY_MADVISE_HUGEPAGE", None)
     if sys.platform == "linux" and use_hugepage is None:
         # If there is an issue with parsing the kernel version,
@@ -415,13 +418,16 @@ else:
 
     # Note that this will currently only make a difference on Linux
     core.multiarray._set_madvise_hugepage(use_hugepage)
+    del use_hugepage
 
     # Give a warning if NumPy is reloaded or imported on a sub-interpreter
     # We do this from python, since the C-module may not be reloaded and
     # it is tidier organized.
     core.multiarray._multiarray_umath._reload_guard()
 
-    core._set_promotion_state(os.environ.get("NPY_PROMOTION_STATE", "legacy"))
+    # default to "weak" promotion for "NumPy 2".
+    core._set_promotion_state(os.environ.get(
+            "NPY_PROMOTION_STATE", "weak" if _numpy2_behavior else "legacy"))
 
     # Tell PyInstaller where to find hook-numpy.py
     def _pyinstaller_hooks_dir():
diff --git a/numpy/core/src/multiarray/multiarraymodule.c b/numpy/core/src/multiarray/multiarraymodule.c
index db7fda32d..86869c8e4 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)
@@ -4913,6 +4919,17 @@ initialize_static_globals(void)
         return -1;
     }
 
+    PyObject *_is_numpy2 = NULL;
+    npy_cache_import("numpy", "_numpy2_behavior", &_is_numpy2);
+    if (_is_numpy2 == NULL) {
+        return -1;
+    }
+    npy_numpy2_behavior = PyObject_IsTrue(_is_numpy2);
+    Py_DECREF(_is_numpy2);
+    if (npy_numpy2_behavior < 0) {
+        return -1;
+    }
+
     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;