Fix EncodedFile.write return value

Make EncodedFile, used for captured output streams, method .write return
the number of characters written. Add test for captured stderr write.
Fixes #6557.

Co-Authored-By: Bruno Oliveira <nicoddemus@gmail.com>

Author: gavento

AUTHORS

@@ -256,6 +256,7 @@ Tim Hoffmann
 Tim Strazny
 Tom Dalton
 Tom Viner
+Tomáš Gavenčiak
 Tomer Keren
 Trevor Bekolay
 Tyler Goodlet

changelog/6557.bugfix.rst

@@ -0,0 +1 @@
+Make capture output streams ``.write()`` method return the same return value from original streams.

src/_pytest/capture.py

@@ -424,7 +424,7 @@ def write(self, obj):
             raise TypeError(
                 "write() argument must be str, not {}".format(type(obj).__name__)
             )
-        self.buffer.write(obj)
+        return self.buffer.write(obj)
 
     def writelines(self, linelist):
         data = "".join(linelist)

testing/test_capture.py

@@ -1492,3 +1492,8 @@ def test_fails():
     result_with_capture.stdout.fnmatch_lines(
         ["E * TypeError: write() argument must be str, not bytes"]
     )
+
+
+def test_stderr_write_returns_len(capsys):
+    """Write on Encoded files, namely captured stderr, should return number of characters written."""
+    assert sys.stderr.write("Foo") == 3