BUG: Show allocatable arrays in dir() for F2PY modules

Add a __dir__ method to PyFortranObject that includes variable names
from the defs array (which contains allocatable arrays) alongside the
regular dict keys. Previously dir() only showed dict contents, hiding
allocatable variables that are handled dynamically via getattr.

Closes numpy#27696

Author: HaoZeke

doc/release/upcoming_changes/30965.improvement.rst

@@ -0,0 +1,4 @@
+``f2py`` modules now show allocatable arrays in ``dir()``
+---------------------------------------------------------
+Allocatable module variables wrapped by ``f2py`` now appear in ``dir()``
+output, matching their accessibility by name.

numpy/f2py/src/fortranobject.c

@@ -576,13 +576,55 @@ fortran_repr(PyFortranObject *fp)
     return repr;
 }
 
+static PyObject *
+fortran_dir(PyFortranObject *fp, PyObject *Py_UNUSED(args))
+{
+    int i;
+    PyObject *dir_list = PyDict_Keys(fp->dict);
+    if (dir_list == NULL) {
+        return NULL;
+    }
+    for (i = 0; i < fp->len; i++) {
+        PyObject *name = PyUnicode_FromString(fp->defs[i].name);
+        if (name == NULL) {
+            Py_DECREF(dir_list);
+            return NULL;
+        }
+        int contains = PySequence_Contains(dir_list, name);
+        if (contains == -1) {
+            Py_DECREF(name);
+            Py_DECREF(dir_list);
+            return NULL;
+        }
+        if (!contains) {
+            if (PyList_Append(dir_list, name) < 0) {
+                Py_DECREF(name);
+                Py_DECREF(dir_list);
+                return NULL;
+            }
+        }
+        Py_DECREF(name);
+    }
+    if (PyList_Sort(dir_list) < 0) {
+        Py_DECREF(dir_list);
+        return NULL;
+    }
+    return dir_list;
+}
+
+static PyMethodDef fortran_methods[] = {
+        {"__dir__", (PyCFunction)fortran_dir, METH_NOARGS, NULL},
+        {NULL, NULL, 0, NULL}
+};
+
 PyTypeObject PyFortran_Type = {
         PyVarObject_HEAD_INIT(NULL, 0).tp_name = "fortran",
         .tp_basicsize = sizeof(PyFortranObject),
         .tp_dealloc = (destructor)fortran_dealloc,
         .tp_getattr = (getattrfunc)fortran_getattr,
         .tp_setattr = (setattrfunc)fortran_setattr,
         .tp_repr = (reprfunc)fortran_repr,
+        .tp_methods = fortran_methods,
         .tp_call = (ternaryfunc)fortran_call,
 };
 

numpy/f2py/tests/test_modules.py

@@ -58,12 +58,19 @@ class TestModuleAndSubroutine(util.F2PyTest):
     sources = [
         util.getpath("tests", "src", "modules", "gh25337", "data.f90"),
         util.getpath("tests", "src", "modules", "gh25337", "use_data.f90"),
+        util.getpath("tests", "src", "regression", "datonly.f90"),
     ]
 
     def test_gh25337(self):
         self.module.data.set_shift(3)
         assert "data" in dir(self.module)
 
+    def test_allocatable_in_dir(self):
+        # gh-27696: allocatable arrays should appear in dir()
+        names = dir(self.module.datonly)
+        assert "data_array" in names
+        assert "max_value" in names
+
 
 @pytest.mark.slow
 class TestUsedModule(util.F2PyTest):