testing utils: capturing std streams context manager (#6231)

* testing utils: capturing std streams context manager

* style

* missing import

* add the origin of this code

src/transformers/testing_utils.py

 import os
+import re
+import sys
 import unittest
 from distutils.util import strtobool
+from io import StringIO
 
 from .file_utils import _tf_available, _torch_available, _torch_tpu_available
 
@@ -136,3 +139,119 @@ def require_torch_and_cuda(test_case):
         return unittest.skip("test requires CUDA")
     else:
         return test_case
+
+
+#
+# Helper functions for dealing with testing text outputs
+# The original code came from:
+# https://github.com/fastai/fastai/blob/master/tests/utils/text.py
+
+# When any function contains print() calls that get overwritten, like progress bars,
+# a special care needs to be applied, since under pytest -s captured output (capsys
+# or contextlib.redirect_stdout) contains any temporary printed strings, followed by
+# \r's. This helper function ensures that the buffer will contain the same output
+# with and without -s in pytest, by turning:
+# foo bar\r tar mar\r final message
+# into:
+# final message
+# it can handle a single string or a multiline buffer
+def apply_print_resets(buf):
+    return re.sub(r"^.*\r", "", buf, 0, re.M)
+
+
+def assert_screenout(out, what):
+    out_pr = apply_print_resets(out).lower()
+    match_str = out_pr.find(what.lower())
+    assert match_str != -1, f"expecting to find {what} in output: f{out_pr}"
+
+
+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
+
+    init arguments:
+    - out - capture stdout: True/False, default True
+    - err - capture stdout: True/False, default True
+
+    Examples:
+
+    with CaptureStdout() as cs:
+        print("Secret message")
+    print(f"captured: {cs.out}")
+
+    import sys
+    with CaptureStderr() as cs:
+        print("Warning: ", file=sys.stderr)
+    print(f"captured: {cs.err}")
+
+    # to capture just one of the streams, but not the other
+    with CaptureStd(err=False) as cs:
+        print("Secret message")
+    print(f"captured: {cs.out}")
+    # but best use the stream-specific subclasses
+
+    """
+
+    def __init__(self, out=True, err=True):
+        if out:
+            self.out_buf = StringIO()
+            self.out = "error: CaptureStd context is unfinished yet, called too early"
+        else:
+            self.out_buf = None
+            self.out = "not capturing stdout"
+
+        if err:
+            self.err_buf = StringIO()
+            self.err = "error: CaptureStd context is unfinished yet, called too early"
+        else:
+            self.err_buf = None
+            self.err = "not capturing stderr"
+
+    def __enter__(self):
+        if self.out_buf:
+            self.out_old = sys.stdout
+            sys.stdout = self.out_buf
+
+        if self.err_buf:
+            self.err_old = sys.stderr
+            sys.stderr = self.err_buf
+
+        return self
+
+    def __exit__(self, *exc):
+        if self.out_buf:
+            sys.stdout = self.out_old
+            self.out = apply_print_resets(self.out_buf.getvalue())
+
+        if self.err_buf:
+            sys.stderr = self.err_old
+            self.err = self.err_buf.getvalue()
+
+    def __repr__(self):
+        msg = ""
+        if self.out_buf:
+            msg += f"stdout: {self.out}\n"
+        if self.err_buf:
+            msg += f"stderr: {self.err}\n"
+        return msg
+
+
+# in tests it's the best to capture only the stream that's wanted, otherwise
+# it's easy to miss things, so unless you need to capture both streams, use the
+# subclasses below (less typing). Or alternatively, configure `CaptureStd` to
+# disable the stream you don't need to test.
+
+
+class CaptureStdout(CaptureStd):
+    """ Same as CaptureStd but captures only stdout """
+
+    def __init__(self):
+        super().__init__(err=False)
+
+
+class CaptureStderr(CaptureStd):
+    """ Same as CaptureStd but captures only stderr """
+
+    def __init__(self):
+        super().__init__(out=False)