🐛 Keep CliRunner output buffers open when its stream wrappers close

tests/test_termui.py

@@ -5,8 +5,9 @@
 
 import io
 import os
+import sys
 from contextlib import contextmanager
-from typing import Literal
+from typing import Literal, TextIO
 
 import pytest
 import typer
@@ -103,6 +104,38 @@ def main() -> None:
     assert os.environ.get(env_key) == "present"
 
 
+def test_clirunner_stream_replaced_during_invoke() -> None:
+    # pytest's live logging suspends capturing mid-command: it swaps sys.stdout
+    # and sys.stderr, which drops the last reference to the runner's wrappers
+    runner = CliRunner()
+    app = typer.Typer()
+
+    @app.command()
+    def main() -> None:
+        sys.stdout = io.StringIO()
+        sys.stderr = io.StringIO()
+
+    result = runner.invoke(app, [])
+    assert result.exit_code == 0, result.output
+    assert result.output == ""
+
+
+def test_clirunner_stream_usable_after_invoke() -> None:
+    # A logging.StreamHandler created inside the command keeps writing here
+    runner = CliRunner()
+    app = typer.Typer()
+    streams: list[TextIO] = []
+
+    @app.command()
+    def main() -> None:
+        streams.append(sys.stderr)
+
+    result = runner.invoke(app, [])
+    assert result.exit_code == 0, result.output
+    streams[0].write("late")
+    streams[0].flush()
+
+
 @pytest.mark.parametrize(
     ("exit_value", "expected_exit_code", "expected_stdout"),
     [

typer/testing.py

@@ -53,19 +53,18 @@ def __init__(self) -> None:
         self.stdout: io.BytesIO = BytesIOCopy(copy_to=self.output)
         self.stderr: io.BytesIO = BytesIOCopy(copy_to=self.output)
 
-    def __del__(self) -> None:
-        """Guarantee that file-like objects are closed in a predictable order"""
-        self.stderr.close()
-        self.stdout.close()
-        self.output.close()
-
 
 class _NamedTextIOWrapper(io.TextIOWrapper):
     def __init__(self, buffer: BinaryIO, name: str, mode: str, **kwargs: Any) -> None:
         super().__init__(buffer, **kwargs)
         self._name = name
         self._mode = mode
 
+    def close(self) -> None:
+        """The buffer belongs to `StreamMixer`, so keep it open when the
+        wrapper is closed or garbage collected.
+        """
+
     @property
     def name(self) -> str:
         return self._name  # pragma: no cover