diff --git a/Lib/test/test_fileio.py b/Lib/test/test_fileio.py
index b74cec2..e7955cc 100644
--- a/Lib/test/test_fileio.py
+++ b/Lib/test/test_fileio.py
@@ -305,7 +305,7 @@
         finally:
             os.unlink(TESTFN)
 
-    def testModeStrings(self):
+    def testInvalidModeStrings(self):
         # check invalid mode strings
         for mode in ("", "aU", "wU+", "rw", "rt"):
             try:
@@ -316,6 +316,21 @@
                 f.close()
                 self.fail('%r is an invalid file mode' % mode)
 
+    def testModeStrings(self):
+        # test that the mode attribute is correct for various mode strings
+        # given as init args
+        try:
+            for modes in [('w', 'wb'), ('wb', 'wb'), ('wb+', 'rb+'),
+                          ('w+b', 'rb+'), ('a', 'ab'), ('ab', 'ab'),
+                          ('ab+', 'ab+'), ('a+b', 'ab+'), ('r', 'rb'),
+                          ('rb', 'rb'), ('rb+', 'rb+'), ('r+b', 'rb+')]:
+                # read modes are last so that TESTFN will exist first
+                with _FileIO(TESTFN, modes[0]) as f:
+                    self.assertEqual(f.mode, modes[1])
+        finally:
+            if os.path.exists(TESTFN):
+                os.unlink(TESTFN)
+
     def testUnicodeOpen(self):
         # verify repr works for unicode too
         f = _FileIO(str(TESTFN), "w")
diff --git a/Misc/ACKS b/Misc/ACKS
index 8977c02..8d29290 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -124,6 +124,7 @@
 Georg Brandl
 Christopher Brannon
 Terrence Brannon
+Erik Bray
 Brian Brazil
 Dave Brennan
 Tom Bridgman
diff --git a/Misc/NEWS b/Misc/NEWS
index afb7464..7ef400e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,9 @@
 Library
 -------
 
+- Issue #18876: The FileIO.mode attribute now better reflects the actual mode
+  under which the file was opened.  Patch by Erik Bray.
+
 - Issue #18851: Avoid a double close of subprocess pipes when the child
   process fails starting.
 
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 6cd7d81..5946e6a 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -47,6 +47,7 @@
     int fd;
     unsigned int readable : 1;
     unsigned int writable : 1;
+    unsigned int appending : 1;
     signed int seekable : 2; /* -1 means unknown */
     unsigned int closefd : 1;
     PyObject *weakreflist;
@@ -124,6 +125,7 @@
         self->fd = -1;
         self->readable = 0;
         self->writable = 0;
+        self->appending = 0;
         self->seekable = -1;
         self->closefd = 1;
         self->weakreflist = NULL;
@@ -184,7 +186,7 @@
     Py_UNICODE *widename = NULL;
 #endif
     int ret = 0;
-    int rwa = 0, plus = 0, append = 0;
+    int rwa = 0, plus = 0;
     int flags = 0;
     int fd = -1;
     int closefd = 1;
@@ -279,8 +281,8 @@
                 goto bad_mode;
             rwa = 1;
             self->writable = 1;
-            flags |= O_CREAT;
-            append = 1;
+            self->appending = 1;
+            flags |= O_APPEND | O_CREAT;
             break;
         case 'b':
             break;
@@ -311,11 +313,6 @@
     flags |= O_BINARY;
 #endif
 
-#ifdef O_APPEND
-    if (append)
-        flags |= O_APPEND;
-#endif
-
     if (fd >= 0) {
         if (check_fd(fd))
             goto error;
@@ -356,7 +353,7 @@
     if (PyObject_SetAttrString((PyObject *)self, "name", nameobj) < 0)
         goto error;
 
-    if (append) {
+    if (self->appending) {
         /* For consistent behaviour, we explicitly seek to the
            end of file (otherwise, it might be done only on the
            first write()). */
@@ -898,7 +895,13 @@
 static char *
 mode_string(fileio *self)
 {
-    if (self->readable) {
+    if (self->appending) {
+        if (self->readable)
+            return "ab+";
+        else
+            return "ab";
+    }
+    else if (self->readable) {
         if (self->writable)
             return "rb+";
         else