gh-127350: Add Py_fopen() function

Rename _Py_fopen_obj() to Py_fopen(). The function now also accepts
bytes path on Windows.

Remove the private, undocumented, and untested function
_Py_fopen_obj().

Author: vstinner

Doc/c-api/sys.rst

@@ -216,6 +216,22 @@ Operating System Utilities
       The function now uses the UTF-8 encoding on Windows if
       :c:member:`PyPreConfig.legacy_windows_fs_encoding` is zero.
 
+.. c:function:: FILE* Py_fopen(PyObject *path, const char *mode)
+
+   Similar to the :c:func:`!fopen` function, but *path* is a Python object and
+   an exception is set on error.
+
+   *path* must be a :class:`str` object or a :class:`bytes` object.
+
+   On success, return the new file object.
+   On error, set an exception and return ``NULL``.
+
+   The file descriptor is created non-inheritable (:pep:`446`).
+
+   The caller must hold the GIL.
+
+   .. versionadded:: next
+
 
 .. _systemfunctions:
 

Doc/whatsnew/3.14.rst

@@ -1034,6 +1034,11 @@ New features
 * Add :c:func:`PyUnstable_Object_EnableDeferredRefcount` for enabling
   deferred reference counting, as outlined in :pep:`703`.
 
+* Add :c:func:`Py_fopen` function to open a file. Similar to the
+  :c:func:`!fopen` function, but the *path* parameter is a Python object and an
+  exception is set on error.
+  (Contributed by Victor Stinner in :gh:`127350`.)
+
 Porting to Python 3.14
 ----------------------
 

Include/cpython/fileutils.h

@@ -2,7 +2,6 @@
 #  error "this header file must not be included directly"
 #endif
 
-// Used by _testcapi which must not use the internal C API
-PyAPI_FUNC(FILE*) _Py_fopen_obj(
+PyAPI_FUNC(FILE*) Py_fopen(
     PyObject *path,
     const char *mode);

Lib/test/test_capi/test_file.py

@@ -0,0 +1,23 @@
+import os
+import unittest
+from test.support import import_helper
+
+_testcapi = import_helper.import_module('_testcapi')
+
+
+class CAPIFileTest(unittest.TestCase):
+    def test_py_fopen(self):
+        for filename in (__file__, os.fsencode(__file__)):
+            with self.subTest(filename=filename):
+                content = _testcapi.py_fopen(filename, "rb")
+                with open(filename, "rb") as fp:
+                    self.assertEqual(fp.read(256), content)
+
+        for invalid_type in (123, object()):
+            with self.subTest(filename=invalid_type):
+                with self.assertRaises(TypeError):
+                    _testcapi.py_fopen(invalid_type, "r")
+
+
+if __name__ == "__main__":
+    unittest.main()

Lib/test/test_ssl.py

@@ -1325,8 +1325,7 @@ def test_load_verify_cadata(self):
     def test_load_dh_params(self):
         ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
         ctx.load_dh_params(DHFILE)
-        if os.name != 'nt':
-            ctx.load_dh_params(BYTES_DHFILE)
+        ctx.load_dh_params(BYTES_DHFILE)
         self.assertRaises(TypeError, ctx.load_dh_params)
         self.assertRaises(TypeError, ctx.load_dh_params, None)
         with self.assertRaises(FileNotFoundError) as cm:

Misc/NEWS.d/next/C_API/2024-12-11-13-01-26.gh-issue-127350.uEBZZ4.rst

@@ -0,0 +1,3 @@
+Add :c:func:`Py_fopen` function to open a file. Similar to the :c:func:`!fopen`
+function, but the *path* parameter is a Python object and an exception is set
+on error. Patch by Victor Stinner.

Modules/_ssl.c

@@ -4377,7 +4377,7 @@ _ssl__SSLContext_load_dh_params_impl(PySSLContext *self, PyObject *filepath)
     FILE *f;
     DH *dh;
 
-    f = _Py_fopen_obj(filepath, "rb");
+    f = Py_fopen(filepath, "rb");
     if (f == NULL)
         return NULL;
 

Modules/_ssl/debughelpers.c

@@ -180,8 +180,8 @@ _PySSLContext_set_keylog_filename(PySSLContext *self, PyObject *arg, void *c) {
         return 0;
     }
 
-    /* _Py_fopen_obj() also checks that arg is of proper type. */
-    fp = _Py_fopen_obj(arg, "a" PY_STDIOTEXTMODE);
+    /* Py_fopen() also checks that arg is of proper type. */
+    fp = Py_fopen(arg, "a" PY_STDIOTEXTMODE);
     if (fp == NULL)
         return -1;
 

Modules/_testcapi/clinic/file.c.h

@@ -1,8 +1,43 @@
+// clinic/file.c.h uses internal pycore_modsupport.h API
+#define PYTESTCAPI_NEED_INTERNAL_API
+
 #include "parts.h"
 #include "util.h"
+#include "clinic/file.c.h"
+
+/*[clinic input]
+module _testcapi
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=6361033e795369fc]*/
+
+/*[clinic input]
+_testcapi.py_fopen
+
+    path: object
+    mode: str
+    /
+
+Call Py_fopen() and return fread(256).
+[clinic start generated code]*/
 
+static PyObject *
+_testcapi_py_fopen_impl(PyObject *module, PyObject *path, const char *mode)
+/*[clinic end generated code: output=5a900af000f759de input=0878c2f9333abd60]*/
+{
+    FILE *fp = Py_fopen(path, mode);
+    if (fp == NULL) {
+        return NULL;
+    }
+
+    char buffer[256];
+    size_t size = fread(buffer, 1, Py_ARRAY_LENGTH(buffer), fp);
+    fclose(fp);
+
+    return PyBytes_FromStringAndSize(buffer, size);
+}
 
 static PyMethodDef test_methods[] = {
+    _TESTCAPI_PY_FOPEN_METHODDEF
     {NULL},
 };