Merge #17987: properly document support.captured_xxx.

Lib/test/support.py

@@ -1186,16 +1186,31 @@ def captured_output(stream_name):
 def captured_stdout():
     """Capture the output of sys.stdout:
 
-       with captured_stdout() as s:
+       with captured_stdout() as stdout:
            print("hello")
-       self.assertEqual(s.getvalue(), "hello")
+       self.assertEqual(stdout.getvalue(), "hello\n")
     """
     return captured_output("stdout")
 
 def captured_stderr():
+    """Capture the output of sys.stderr:
+
+       with captured_stderr() as stderr:
+           print("hello", file=sys.stderr)
+       self.assertEqual(stderr.getvalue(), "hello\n")
+    """
     return captured_output("stderr")
 
 def captured_stdin():
+    """Capture the input to sys.stdin:
+
+       with captured_stdin() as stdin:
+           stdin.write('hello\n')
+           stdin.seek(0)
+           # call test code that consumes from sys.stdin
+           captured = input()
+       self.assertEqual(captured, "hello")
+    """
     return captured_output("stdin")