fix: preserve LF line endings when reading from stdin on Windows

isort/main.py

@@ -1060,7 +1060,18 @@ def main(argv: Sequence[str] | None = None, stdin: TextIOWrapper | None = None)
         if show_files:
             sys.exit("Error: can't show files for streaming input.")
 
-        input_stream = sys.stdin if stdin is None else stdin
+        if stdin is None:
+            raw = sys.stdin.buffer.read()
+            if b"\r\n" in raw:
+                line_ending = "\r\n"
+            elif b"\r" in raw:
+                line_ending = "\r"
+            else:
+                line_ending = "\n"
+            import io
+            input_stream = io.TextIOWrapper(io.BytesIO(raw), encoding=sys.stdin.encoding or "utf-8")
+        else:
+            input_stream = stdin
         if check:
             incorrectly_sorted = not api.check_stream(
                 input_stream=input_stream,

tests/unit/test_regressions.py

@@ -1985,3 +1985,19 @@ def test_comment_on_opening_line_of_aliased_import_does_not_move():
         isort.code(short_line, profile="black")
         == "from mod import attr as alias  # type: ignore[attr-defined]  # My comment\n"
     )
+
+
+def test_lf_line_endings_preserved_from_stdin_issue_2453():
+    """LF line endings should be preserved when reading from stdin on Windows.
+    See: https://github.com/PyCQA/isort/issues/2453
+    """
+    import io
+    lf_content = "import os\nimport re\n"
+    input_stream = io.TextIOWrapper(
+        io.BytesIO(lf_content.encode("utf-8")), encoding="utf-8"
+    )
+    output_stream = io.StringIO()
+    from isort import core
+    core.process(input_stream, output_stream)
+    result = output_stream.getvalue()
+    assert "\r\n" not in result, "LF input should not be converted to CRLF"