#22709: Use stdin as-is if it does not have a buffer attribute.

This restores backward compatibility lost in the fix for #21075, and
is better duck typing.

Patch by Akira Li.

Lib/fileinput.py

@@ -328,7 +328,7 @@ class FileInput:
             if self._filename == '-':
                 self._filename = '<stdin>'
                 if 'b' in self._mode:
-                    self._file = sys.stdin.buffer
+                    self._file = getattr(sys.stdin, 'buffer', sys.stdin)
                 else:
                     self._file = sys.stdin
                 self._isstdin = True

Lib/test/test_fileinput.py

@@ -240,6 +240,17 @@ class FileInputTests(unittest.TestCase):
             lines = list(fi)
             self.assertEqual(lines, [b'spam, bacon, sausage, and spam'])
 
+    def test_detached_stdin_binary_mode(self):
+        orig_stdin = sys.stdin
+        try:
+            sys.stdin = BytesIO(b'spam, bacon, sausage, and spam')
+            self.assertFalse(hasattr(sys.stdin, 'buffer'))
+            fi = FileInput(files=['-'], mode='rb')
+            lines = list(fi)
+            self.assertEqual(lines, [b'spam, bacon, sausage, and spam'])
+        finally:
+            sys.stdin = orig_stdin
+
     def test_file_opening_hook(self):
         try:
             # cannot use openhook and inplace mode

Misc/NEWS

@@ -41,6 +41,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25447: fileinput now uses sys.stdin as-is if it does not have a
+  buffer attribute (restores backward compatibility).
+
 - Issue #25447: Copying the lru_cache() wrapper object now always works,
   independedly from the type of the wrapped object (by returning the original
   object unchanged).