implement a detach() method for BufferedIOBase and TextIOBase #5883

3 files changed, 86 insertions, 6 deletions

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index e580366..e3e7c3d 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -642,6 +642,15 @@ class BufferedIOBase(IOBase):
         """
         self._unsupported("write")
 
+    def detach(self) -> None:
+        """
+        Separate the underlying raw stream from the buffer and return it.
+
+        After the raw stream has been detached, the buffer is in an unusable
+        state.
+        """
+        self._unsupported("detach")
+
 io.BufferedIOBase.register(BufferedIOBase)
 
 
@@ -689,13 +698,21 @@ class _BufferedIOMixin(BufferedIOBase):
         self.raw.flush()
 
     def close(self):
-        if not self.closed:
+        if not self.closed and self.raw is not None:
             try:
                 self.flush()
             except IOError:
                 pass  # If flush() fails, just give up
             self.raw.close()
 
+    def detach(self):
+        if self.raw is None:
+            raise ValueError("raw stream already detached")
+        self.flush()
+        raw = self.raw
+        self.raw = None
+        return raw
+
     ### Inquiries ###
 
     def seekable(self):
@@ -1236,6 +1253,15 @@ class TextIOBase(IOBase):
         """
         self._unsupported("readline")
 
+    def detach(self) -> None:
+        """
+        Separate the underlying buffer from the TextIOBase and return it.
+
+        After the underlying buffer has been detached, the TextIO is in an
+        unusable state.
+        """
+        self._unsupported("detach")
+
     @property
     def encoding(self):
         """Subclasses should override."""
@@ -1448,11 +1474,12 @@ class TextIOWrapper(TextIOBase):
         self._telling = self._seekable
 
     def close(self):
-        try:
-            self.flush()
-        except IOError:
-            pass  # If flush() fails, just give up
-        self.buffer.close()
+        if self.buffer is not None:
+            try:
+                self.flush()
+            except IOError:
+                pass  # If flush() fails, just give up
+            self.buffer.close()
 
     @property
     def closed(self):
@@ -1647,6 +1674,14 @@ class TextIOWrapper(TextIOBase):
         self.seek(pos)
         return self.buffer.truncate()
 
+    def detach(self):
+        if self.buffer is None:
+            raise ValueError("buffer is already detached")
+        self.flush()
+        buffer = self.buffer
+        self.buffer = None
+        return buffer
+
     def seek(self, cookie, whence=0):
         if self.closed:
             raise ValueError("tell on closed file")
@@ -1865,3 +1900,7 @@ class StringIO(TextIOWrapper):
     @property
     def encoding(self):
         return None
+
+    def detach(self):
+        # This doesn't make sense on StringIO.
+        self._unsupported("detach")
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index a8c878e..1a525dc 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -526,6 +526,12 @@ class PyIOTest(IOTest):
 class CommonBufferedTests:
     # Tests common to BufferedReader, BufferedWriter and BufferedRandom
 
+    def test_detach(self):
+        raw = self.MockRawIO()
+        buf = self.tp(raw)
+        self.assertIs(buf.detach(), raw)
+        self.assertRaises(ValueError, buf.detach)
+
     def test_fileno(self):
         rawio = self.MockRawIO()
         bufio = self.tp(rawio)
@@ -811,6 +817,14 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
         bufio.flush()
         self.assertEquals(b"".join(rawio._write_stack), b"abcghi")
 
+    def test_detach_flush(self):
+        raw = self.MockRawIO()
+        buf = self.tp(raw)
+        buf.write(b"howdy!")
+        self.assertFalse(raw._write_stack)
+        buf.detach()
+        self.assertEqual(raw._write_stack, [b"howdy!"])
+
     def test_write(self):
         # Write to the buffered IO but don't overflow the buffer.
         writer = self.MockRawIO()
@@ -1052,6 +1066,10 @@ class BufferedRWPairTest(unittest.TestCase):
         pair = self.tp(self.MockRawIO(), self.MockRawIO())
         self.assertFalse(pair.closed)
 
+    def test_detach(self):
+        pair = self.tp(self.MockRawIO(), self.MockRawIO())
+        self.assertRaises(self.UnsupportedOperation, pair.detach)
+
     def test_constructor_max_buffer_size_deprecation(self):
         with support.check_warnings() as w:
             warnings.simplefilter("always", DeprecationWarning)
@@ -1480,6 +1498,19 @@ class TextIOWrapperTest(unittest.TestCase):
         self.assertRaises(TypeError, t.__init__, b, newline=42)
         self.assertRaises(ValueError, t.__init__, b, newline='xyzzy')
 
+    def test_detach(self):
+        r = self.BytesIO()
+        b = self.BufferedWriter(r)
+        t = self.TextIOWrapper(b)
+        self.assertIs(t.detach(), b)
+
+        t = self.TextIOWrapper(b, encoding="ascii")
+        t.write("howdy")
+        self.assertFalse(r.getvalue())
+        t.detach()
+        self.assertEqual(r.getvalue(), b"howdy")
+        self.assertRaises(ValueError, t.detach)
+
     def test_repr(self):
         raw = self.BytesIO("hello".encode("utf-8"))
         b = self.BufferedReader(raw)
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index ad04613..d94d9dd 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -57,6 +57,10 @@ class MemorySeekTestMixin:
 
 class MemoryTestMixin:
 
+    def test_detach(self):
+        buf = self.ioclass()
+        self.assertRaises(self.UnsupportedOperation, buf.detach)
+
     def write_ops(self, f, t):
         self.assertEqual(f.write(t("blah.")), 5)
         self.assertEqual(f.seek(0), 0)
@@ -336,6 +340,9 @@ class MemoryTestMixin:
 
 
 class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
+
+    UnsupportedOperation = pyio.UnsupportedOperation
+
     @staticmethod
     def buftype(s):
         return s.encode("ascii")
@@ -413,6 +420,7 @@ class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
 class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
     buftype = str
     ioclass = pyio.StringIO
+    UnsupportedOperation = pyio.UnsupportedOperation
     EOF = ""
 
     # TextIO-specific behaviour.
@@ -518,9 +526,11 @@ class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
 
 class CBytesIOTest(PyBytesIOTest):
     ioclass = io.BytesIO
+    UnsupportedOperation = io.UnsupportedOperation
 
 class CStringIOTest(PyStringIOTest):
     ioclass = io.StringIO
+    UnsupportedOperation = io.UnsupportedOperation
 
     # XXX: For the Python version of io.StringIO, this is highly
     # dependent on the encoding used for the underlying buffer.