From 6eadac438bc75792581d4de57b478f46cd45446d Mon Sep 17 00:00:00 2001
From: Thijs Kramer <thijskramer@gmail.com>
Date: Tue, 19 Oct 2021 19:33:52 +0200
Subject: [PATCH] only write to stdout buffer when buffer object is available

---
 src/flake8/formatting/base.py     | 10 +++++++++-
 tests/unit/test_base_formatter.py | 26 ++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/flake8/formatting/base.py b/src/flake8/formatting/base.py
index 7919f92e9..c8f69ddac 100644
--- a/src/flake8/formatting/base.py
+++ b/src/flake8/formatting/base.py
@@ -184,7 +184,15 @@ def _write(self, output: str) -> None:
         if self.output_fd is not None:
             self.output_fd.write(output + self.newline)
         if self.output_fd is None or self.options.tee:
-            sys.stdout.buffer.write(output.encode() + self.newline.encode())
+            # sys.stdout might be replaced, and thus could also be
+            # a file like object like io.StringIO, which do not
+            # support the 'buffer' attribute.
+            try:
+                sys.stdout.buffer.write(
+                    output.encode() + self.newline.encode()
+                )
+            except AttributeError:
+                sys.stdout.write(output + self.newline)
 
     def write(self, line: Optional[str], source: Optional[str]) -> None:
         """Write the line either to the output file or stdout.
diff --git a/tests/unit/test_base_formatter.py b/tests/unit/test_base_formatter.py
index 895890317..463dff7ab 100644
--- a/tests/unit/test_base_formatter.py
+++ b/tests/unit/test_base_formatter.py
@@ -136,6 +136,32 @@ def test_write_produces_stdout(capsys):
     assert capsys.readouterr().out == f"{line}\n{source}\n"
 
 
+def test_write_without_stdout_buffer():
+    """Verify that stdout writes when it has no buffer."""
+    line = "Something to write"
+    source = "source"
+
+    buffer_side_effect = AttributeError("'_io.StringIO' object has no \
+        attribute 'buffer'")
+    buffer_write_mock = mock.Mock(side_effect=buffer_side_effect)
+
+    write_mock = mock.Mock()
+    mock.patch("flake8.formatting.base.sys.stdout.write", write_mock)
+
+    with mock.patch(
+        "flake8.formatting.base.sys.stdout.buffer.write",
+        buffer_write_mock
+    ):
+        formatter = base.BaseFormatter(options())
+        formatter.write(line, source)
+        assert write_mock.called is True
+        assert write_mock.call_count == 2
+        assert write_mock.mock_calls == [
+            mock.call(line + formatter.newline),
+            mock.call(source + formatter.newline),
+        ]
+
+
 class AfterInitFormatter(base.BaseFormatter):
     """Subclass for testing after_init."""