Replace buffers inside of stdout and stderr

The current implementation replaces the streams in `sys`,
which is not effective if those streams have already been
stored in other modules, and can be problematic if the
temporary replacement streams are copied into other modules
as they are not able to be cleaned up by the context manager.

Related to #75

.travis.yml

@@ -23,6 +23,6 @@ jobs:
 script:
  - python --version
  - pip list
- - pytest --cov=src -p no:warnings
+ - pytest --cov=src -p no:warnings -s
  - if (( $PYTHON_MAJOR == 3 && $PYTHON_MINOR == 7 )); then tox -e flake8; else echo "No flake8."; fi
  - if (( $PYTHON_MAJOR == 3 && $PYTHON_MINOR == 7 )); then codecov; else echo "No codecov."; fi

src/stdio_mgr/stdio_mgr.py

@@ -37,7 +37,7 @@
     TextIOWrapper,
 )
 
-from .types import _MultiCloseContextManager, ReplaceSysIoContextManager
+from .types import _OutStreamsCloseContextManager, InjectSysIoContextManager
 
 
 class _PersistedBytesIO(BytesIO):
@@ -266,7 +266,7 @@ class SafeCloseTeeStdin(_SafeCloseIOBase, TeeStdin):
     """
 
 
-class StdioManager(ReplaceSysIoContextManager, _MultiCloseContextManager):
+class StdioManager(InjectSysIoContextManager, _OutStreamsCloseContextManager):
     r"""Substitute temporary text buffers for `stdio` in a managed context.
 
     Context manager.

src/stdio_mgr/types.py

@@ -177,3 +177,44 @@ def __exit__(self, exc_type, exc_value, traceback):
         (sys.stdin, sys.stdout, sys.stderr) = self._prior_streams
 
         return super().__exit__(exc_type, exc_value, traceback)
+
+
+class _OutStreamsCloseContextManager(_MultiCloseContextManager):
+    """Close stdout and stderr only."""
+
+    def __enter__(self):
+        """Enter context of all members."""
+        with ExitStack() as stack:
+            with suppress(AttributeError):
+                stack.enter_context(self.stdout)
+            with suppress(AttributeError):
+                stack.enter_context(self.stderr)
+
+            self._close_files = stack.pop_all().close
+
+        return super().__enter__()
+
+
+class InjectSysIoContextManager(StdioTuple):
+    """Replace sys stdio with members of the tuple."""
+
+    def __enter__(self):
+        """Enter context, replacing sys stdio objects with capturing streams."""
+        self._prior_stdin = sys.stdin
+        self._prior_raw_out = (sys.stdout.buffer.raw, sys.stderr.buffer.raw)
+
+        super().__enter__()
+
+        sys.stdin = self.stdin
+        sys.stdout.buffer.__init__(self.stdout.buffer.raw)
+        sys.stderr.buffer.__init__(self.stderr.buffer.raw)
+
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        """Exit context, restoring state of sys module."""
+        sys.stdin = self._prior_stdin
+        sys.stdout.buffer.__init__(self._prior_raw_out[0])
+        sys.stderr.buffer.__init__(self._prior_raw_out[1])
+
+        return super().__exit__(exc_type, exc_value, traceback)

tests/test_stdiomgr_base.py

@@ -481,11 +481,6 @@ def test_stdout_detached(convert_newlines):
 
         assert convert_newlines("test str\n") == o.getvalue()
 
-        with pytest.raises(ValueError) as err:
-            print("anything")
-
-        assert str(err.value) == "underlying buffer has been detached"
-
         f.write(convert_newlines("second test str\n").encode("utf8"))
         f.flush()
 
@@ -529,7 +524,7 @@ def test_stdout_access_buffer_after_close(convert_newlines):
         with pytest.raises(ValueError) as err:
             print("anything")
 
-        assert str(err.value) == "I/O operation on closed file."
+        assert str(err.value) == "write to closed file"
 
         assert convert_newlines("test str\nsecond test str\n") == o.getvalue()
 

tox.ini

@@ -7,7 +7,7 @@ envlist=
 
 [testenv]
 commands=
-    pytest -p no:warnings
+    pytest -p no:warnings -s
 deps=
     pytest
 
@@ -43,7 +43,8 @@ commands=
 [pytest]
 addopts = -v -rsxX
 xfail_strict = True
-
+faulthandler_timeout = 5
+timeout = 5
 
 [flake8]
 # W503: black formats binary operators to start of line