[testing] auto-replay captured streams (#13803)

docs/source/testing.rst

@@ -1080,6 +1080,8 @@ If you need to capture both streams at once, use the parent :obj:`CaptureStd` cl
         function_that_writes_to_stdout_and_stderr()
     print(cs.err, cs.out)
 
+Also, to aid debugging test issues, by default these context managers automatically replay the captured streams on exit
+from the context.
 
 
 Capturing logger stream

src/transformers/testing_utils.py

@@ -610,34 +610,54 @@ class CaptureStd:
     """
     Context manager to capture:
 
-        - stdout, clean it up and make it available via obj.out
-        - stderr, and make it available via obj.err
+        - stdout: replay it, clean it up and make it available via ``obj.out``
+        - stderr: replay it and make it available via ``obj.err``
 
         init arguments:
 
-        - out - capture stdout: True/False, default True
-        - err - capture stdout: True/False, default True
+        - out - capture stdout:`` True``/``False``, default ``True``
+        - err - capture stdout: ``True``/``False``, default ``True``
+        - replay - whether to replay or not: ``True``/``False``, default ``True``. By default each
+        captured stream gets replayed back on context's exit, so that one can see what the test was doing. If this is a
+        not wanted behavior and the captured data shouldn't be replayed, pass ``replay=False`` to disable this feature.
 
         Examples::
 
+            # to capture stdout only with auto-replay
             with CaptureStdout() as cs:
                 print("Secret message")
-            print(f"captured: {cs.out}")
+            assert "message" in cs.out
 
+            # to capture stderr only with auto-replay
             import sys
             with CaptureStderr() as cs:
                 print("Warning: ", file=sys.stderr)
-            print(f"captured: {cs.err}")
+            assert "Warning" in cs.err
 
-            # to capture just one of the streams, but not the other
+            # to capture both streams with auto-replay
+            with CaptureStd() as cs:
+                print("Secret message")
+                print("Warning: ", file=sys.stderr)
+            assert "message" in cs.out
+            assert "Warning" in cs.err
+
+            # to capture just one of the streams, and not the other, with auto-replay
             with CaptureStd(err=False) as cs:
                 print("Secret message")
-            print(f"captured: {cs.out}")
+            assert "message" in cs.out
             # but best use the stream-specific subclasses
 
+            # to capture without auto-replay
+            with CaptureStd(replay=False) as cs:
+                print("Secret message")
+            assert "message" in cs.out
+
     """
 
-    def __init__(self, out=True, err=True):
+    def __init__(self, out=True, err=True, replay=True):
+
+        self.replay = replay
+
         if out:
             self.out_buf = StringIO()
             self.out = "error: CaptureStd context is unfinished yet, called too early"
@@ -666,11 +686,17 @@ def __enter__(self):
     def __exit__(self, *exc):
         if self.out_buf:
             sys.stdout = self.out_old
-            self.out = apply_print_resets(self.out_buf.getvalue())
+            captured = self.out_buf.getvalue()
+            if self.replay:
+                sys.stdout.write(captured)
+            self.out = apply_print_resets(captured)
 
         if self.err_buf:
             sys.stderr = self.err_old
-            self.err = self.err_buf.getvalue()
+            captured = self.err_buf.getvalue()
+            if self.replay:
+                sys.stderr.write(captured)
+            self.err = captured
 
     def __repr__(self):
         msg = ""
@@ -690,15 +716,15 @@ def __repr__(self):
 class CaptureStdout(CaptureStd):
     """Same as CaptureStd but captures only stdout"""
 
-    def __init__(self):
-        super().__init__(err=False)
+    def __init__(self, replay=True):
+        super().__init__(err=False, replay=replay)
 
 
 class CaptureStderr(CaptureStd):
     """Same as CaptureStd but captures only stderr"""
 
-    def __init__(self):
-        super().__init__(out=False)
+    def __init__(self, replay=True):
+        super().__init__(out=False, replay=replay)
 
 
 class CaptureLogger: