methane · Dec 21, 2017 · Feb 20, 2017 · Sep 15, 2017 · Sep 15, 2017 · Sep 16, 2017
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
@@ -901,7 +901,7 @@ Text I/O
       locale encoding using :func:`locale.setlocale`, use the current locale
       encoding instead of the user preferred encoding.
 
-   :class:`TextIOWrapper` provides one attribute in addition to those of
+   :class:`TextIOWrapper` provides these members in addition to those of
    :class:`TextIOBase` and its parents:
 
    .. attribute:: line_buffering
@@ -915,11 +915,19 @@ Text I/O
 
       .. versionadded:: 3.7
 
-   .. method:: reconfigure(*, line_buffering=None, write_through=None)
+   .. method:: reconfigure(*, encoding, errors, newline, \
+                           line_buffering, write_through)
 
-      Reconfigure this text stream using new settings for *line_buffering*
-      and *write_through*.  Passing ``None`` as an argument will retain
-      the current setting for that parameter.
+      Reconfigure this text stream using new settings for *encoding*,
+      *errors*, *newline*, *line_buffering* and *write_through*.
+
+      Parameters not specified keep current settings, except
+      ``errors='strict`` is used when *encoding* is specified but
+      *errors* is not specified.
+
+      It is not possible to change the encoding if some data has already
+      been read from the stream. On the other hand, changing encoding
+      after write is possible.
 
       This method does an implicit stream flush before setting the
       new parameters.

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
@@ -1923,10 +1923,7 @@ class TextIOWrapper(TextIOBase):
     # so that the signature can match the signature of the C version.
     def __init__(self, buffer, encoding=None, errors=None, newline=None,
                  line_buffering=False, write_through=False):
-        if newline is not None and not isinstance(newline, str):
-            raise TypeError("illegal newline type: %r" % (type(newline),))
-        if newline not in (None, "", "\n", "\r", "\r\n"):
-            raise ValueError("illegal newline value: %r" % (newline,))
+        self._check_newline(newline)
         if encoding is None:
             try:
                 encoding = os.device_encoding(buffer.fileno())
@@ -1956,22 +1953,38 @@ def __init__(self, buffer, encoding=None, errors=None, newline=None,
                 raise ValueError("invalid errors: %r" % errors)
 
         self._buffer = buffer
+        self._decoded_chars = ''  # buffer for text returned from decoder
+        self._decoded_chars_used = 0  # offset into _decoded_chars for read()
+        self._snapshot = None  # info for reconstructing decoder state
+        self._seekable = self._telling = self.buffer.seekable()
+        self._has_read1 = hasattr(self.buffer, 'read1')
+        self._configure(encoding, errors, newline,
+                        line_buffering, write_through)
+
+    def _check_newline(self, newline):
+        if newline is not None and not isinstance(newline, str):
+            raise TypeError("illegal newline type: %r" % (type(newline),))
+        if newline not in (None, "", "\n", "\r", "\r\n"):
+            raise ValueError("illegal newline value: %r" % (newline,))
+
+    def _configure(self, encoding=None, errors=None, newline=None,
+                   line_buffering=False, write_through=False):
         self._encoding = encoding
         self._errors = errors
+        self._encoder = None
+        self._decoder = None
+        self._b2cratio = 0.0
+
         self._readuniversal = not newline
         self._readtranslate = newline is None
         self._readnl = newline
         self._writetranslate = newline != ''
         self._writenl = newline or os.linesep
-        self._encoder = None
-        self._decoder = None
-        self._decoded_chars = ''  # buffer for text returned from decoder
-        self._decoded_chars_used = 0  # offset into _decoded_chars for read()
-        self._snapshot = None  # info for reconstructing decoder state
-        self._seekable = self._telling = self.buffer.seekable()
-        self._has_read1 = hasattr(self.buffer, 'read1')
-        self._b2cratio = 0.0
 
+        self._line_buffering = line_buffering
+        self._write_through = write_through
+
+        # don't write a BOM in the middle of a file
         if self._seekable and self.writable():
             position = self.buffer.tell()
             if position != 0:
@@ -1981,12 +1994,6 @@ def __init__(self, buffer, encoding=None, errors=None, newline=None,
                     # Sometimes the encoder doesn't exist
                     pass
 
-        self._configure(line_buffering, write_through)
-
-    def _configure(self, line_buffering=False, write_through=False):
-        self._line_buffering = line_buffering
-        self._write_through = write_through
-
     # self._snapshot is either None, or a tuple (dec_flags, next_input)
     # where dec_flags is the second (integer) item of the decoder state
     # and next_input is the chunk of input bytes that comes next after the
@@ -2033,17 +2040,45 @@ def write_through(self):
     def buffer(self):
         return self._buffer
 
-    def reconfigure(self, *, line_buffering=None, write_through=None):
+    def reconfigure(self, *,
+                    encoding=None, errors=None, newline=Ellipsis,
+                    line_buffering=None, write_through=None):
         """Reconfigure the text stream with new parameters.
 
         This also flushes the stream.
         """
+        if errors is None:
+            if encoding is None:
+                errors = self._errors
+            else:
+                errors = 'strict'
+        elif not isinstance(errors, str):
+            raise TypeError("invalid errors: %r" % errors)
+
+        if encoding is None:
+            encoding = self._encoding
+        else:
+            if not isinstance(encoding, str):
+                raise TypeError("invalid encoding: %r" % encoding)
+
+        if (self._decoder is not None
+                and (encoding != self._encoding or errors != self._errors)):
+            raise UnsupportedOperation(
+                "It is not possible to set the encoding of stream after "
+                "the first read")
+
+        if newline is Ellipsis:
+            newline = self._readnl
+        self._check_newline(newline)
+
         if line_buffering is None:
             line_buffering = self.line_buffering
         if write_through is None:
             write_through = self.write_through
+
         self.flush()
-        self._configure(line_buffering, write_through)
+        self._configure(encoding, errors, newline,
+                        line_buffering, write_through)
 
     def seekable(self):
         if self.closed:

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -3329,6 +3329,120 @@ def seekable(self): return True
         F.tell = lambda x: 0
         t = self.TextIOWrapper(F(), encoding='utf-8')
 
+    def test_reconfigure_encoding_read(self):
+        # latin1 -> utf8
+        # (latin1 can decode utf-8 encoded string)
+        data = 'abc\xe9\n'.encode('latin1') + 'd\xe9f\n'.encode('utf8')
+        raw = self.BytesIO(data)
+        txt = self.TextIOWrapper(raw, encoding='latin1', newline='\n')
+        self.assertEqual(txt.readline(), 'abc\xe9\n')
+        with self.assertRaises(self.UnsupportedOperation):
+            txt.reconfigure(encoding='utf-8')
+
+    def test_reconfigure_write_fromascii(self):
+        # ascii has a specific encodefunc in the C implementation,
+        # but utf-8-sig has not. Make sure that we get rid of the
+        # cached encodefunc when we switch encoders.
+        raw = self.BytesIO()
+        txt = self.TextIOWrapper(raw, encoding='ascii', newline='\n')
+        txt.write('foo\n')
+        txt.reconfigure(encoding='utf-8-sig')
+        txt.write('\xe9\n')
+        txt.flush()
+        self.assertEqual(raw.getvalue(), b'foo\n\xc3\xa9\n')
+
+    def test_reconfigure_write(self):
+        # latin -> utf8
+        raw = self.BytesIO()
+        txt = self.TextIOWrapper(raw, encoding='latin1', newline='\n')
+        txt.write('abc\xe9\n')
+        txt.reconfigure(encoding='utf-8')
+        self.assertEqual(raw.getvalue(), b'abc\xe9\n')
+        txt.write('d\xe9f\n')
+        txt.flush()
+        self.assertEqual(raw.getvalue(), b'abc\xe9\nd\xc3\xa9f\n')
+
+        # ascii -> utf-8-sig: ensure that no BOM is written in the middle of
+        # the file
+        raw = self.BytesIO()
+        txt = self.TextIOWrapper(raw, encoding='ascii', newline='\n')
+        txt.write('abc\n')
+        txt.reconfigure(encoding='utf-8-sig')
+        txt.write('d\xe9f\n')
+        txt.flush()
+        self.assertEqual(raw.getvalue(), b'abc\nd\xc3\xa9f\n')
+
+    def test_reconfigure_write_non_seekable(self):
+        raw = self.BytesIO()
+        raw.seekable = lambda: False
+        raw.seek = None
+        txt = self.TextIOWrapper(raw, encoding='ascii', newline='\n')
+        txt.write('abc\n')
+        txt.reconfigure(encoding='utf-8-sig')
+        txt.write('d\xe9f\n')
+        txt.flush()
+
+        # If the raw stream is not seekable, there'll be a BOM
+        self.assertEqual(raw.getvalue(),  b'abc\n\xef\xbb\xbfd\xc3\xa9f\n')
+
+    def test_reconfigure_defaults(self):
+        txt = self.TextIOWrapper(self.BytesIO(), 'ascii', 'replace', '\n')
+        txt.reconfigure(encoding=None)
+        self.assertEqual(txt.encoding, 'ascii')
+        self.assertEqual(txt.errors, 'replace')
+        txt.write('LF\n')
+
+        txt.reconfigure(newline='\r\n')
+        self.assertEqual(txt.encoding, 'ascii')
+        self.assertEqual(txt.errors, 'replace')
+
+        txt.reconfigure(errors='ignore')
+        self.assertEqual(txt.encoding, 'ascii')
+        txt.write('CRLF\n')
+
+        txt.reconfigure(encoding='utf-8', newline=None)
+        self.assertEqual(txt.errors, 'strict')
+        txt.seek(0)
+        self.assertEqual(txt.read(), 'LF\nCRLF\n')
+
+        self.assertEqual(txt.detach().getvalue(), b'LF\nCRLF\r\n')
+
+    def test_reconfigure_newline(self):
+        raw = self.BytesIO(b'CR\rEOF')
+        txt = self.TextIOWrapper(raw, 'ascii', newline='\n')
+        txt.reconfigure(newline=None)
+        self.assertEqual(txt.readline(), 'CR\n')
+        raw = self.BytesIO(b'CR\rEOF')
+        txt = self.TextIOWrapper(raw, 'ascii', newline='\n')
+        txt.reconfigure(newline='')
+        self.assertEqual(txt.readline(), 'CR\r')
+        raw = self.BytesIO(b'CR\rLF\nEOF')
+        txt = self.TextIOWrapper(raw, 'ascii', newline='\r')
+        txt.reconfigure(newline='\n')
+        self.assertEqual(txt.readline(), 'CR\rLF\n')
+        raw = self.BytesIO(b'LF\nCR\rEOF')
+        txt = self.TextIOWrapper(raw, 'ascii', newline='\n')
+        txt.reconfigure(newline='\r')
+        self.assertEqual(txt.readline(), 'LF\nCR\r')
+        raw = self.BytesIO(b'CR\rCRLF\r\nEOF')
+        txt = self.TextIOWrapper(raw, 'ascii', newline='\r')
+        txt.reconfigure(newline='\r\n')
+        self.assertEqual(txt.readline(), 'CR\rCRLF\r\n')
+
+        txt = self.TextIOWrapper(self.BytesIO(), 'ascii', newline='\r')
+        txt.reconfigure(newline=None)
+        txt.write('linesep\n')
+        txt.reconfigure(newline='')
+        txt.write('LF\n')
+        txt.reconfigure(newline='\n')
+        txt.write('LF\n')
+        txt.reconfigure(newline='\r')
+        txt.write('CR\n')
+        txt.reconfigure(newline='\r\n')
+        txt.write('CRLF\n')
+        expected = 'linesep' + os.linesep + 'LF\nLF\nCR\rCRLF\r\n'
+        self.assertEqual(txt.detach().getvalue().decode('ascii'), expected)
+
 
 class MemviewBytesIO(io.BytesIO):
     '''A BytesIO object whose read method returns memoryviews

diff --git a/Misc/NEWS.d/next/Library/2017-09-16-02-56-33.bpo-15216.lqXCTT.rst b/Misc/NEWS.d/next/Library/2017-09-16-02-56-33.bpo-15216.lqXCTT.rst
@@ -0,0 +1,2 @@
+``TextIOWrapper.reconfigure()`` supports changing *encoding*, *errors*, and
+*newline*.
diff --git a/Modules/_io/clinic/textio.c.h b/Modules/_io/clinic/textio.c.h
@@ -149,7 +149,7 @@ PyDoc_STRVAR(_io_TextIOWrapper___init____doc__,
 
 static int
 _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
-                                const char *encoding, const char *errors,
+                                const char *encoding, PyObject *errors,
                                 const char *newline, int line_buffering,
                                 int write_through);
 
@@ -158,10 +158,10 @@ _io_TextIOWrapper___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 {
     int return_value = -1;
     static const char * const _keywords[] = {"buffer", "encoding", "errors", "newline", "line_buffering", "write_through", NULL};
-    static _PyArg_Parser _parser = {"O|zzzii:TextIOWrapper", _keywords, 0};
+    static _PyArg_Parser _parser = {"O|zOzii:TextIOWrapper", _keywords, 0};
     PyObject *buffer;
     const char *encoding = NULL;
-    const char *errors = NULL;
+    PyObject *errors = Py_None;
     const char *newline = NULL;
     int line_buffering = 0;
     int write_through = 0;
@@ -177,7 +177,8 @@ _io_TextIOWrapper___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 }
 
 PyDoc_STRVAR(_io_TextIOWrapper_reconfigure__doc__,
-"reconfigure($self, /, *, line_buffering=None, write_through=None)\n"
+"reconfigure($self, /, *, encoding=None, errors=None, newline=None,\n"
+"            line_buffering=None, write_through=None)\n"
 "--\n"
 "\n"
 "Reconfigure the text stream with new parameters.\n"
@@ -188,24 +189,28 @@ PyDoc_STRVAR(_io_TextIOWrapper_reconfigure__doc__,
     {"reconfigure", (PyCFunction)_io_TextIOWrapper_reconfigure, METH_FASTCALL|METH_KEYWORDS, _io_TextIOWrapper_reconfigure__doc__},
 
 static PyObject *
-_io_TextIOWrapper_reconfigure_impl(textio *self,
+_io_TextIOWrapper_reconfigure_impl(textio *self, PyObject *encoding,
+                                   PyObject *errors, PyObject *newline_obj,
                                    PyObject *line_buffering_obj,
                                    PyObject *write_through_obj);
 
 static PyObject *
 _io_TextIOWrapper_reconfigure(textio *self, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
 {
     PyObject *return_value = NULL;
-    static const char * const _keywords[] = {"line_buffering", "write_through", NULL};
-    static _PyArg_Parser _parser = {"|$OO:reconfigure", _keywords, 0};
+    static const char * const _keywords[] = {"encoding", "errors", "newline", "line_buffering", "write_through", NULL};
+    static _PyArg_Parser _parser = {"|$OOOOO:reconfigure", _keywords, 0};
+    PyObject *encoding = Py_None;
+    PyObject *errors = Py_None;
+    PyObject *newline_obj = NULL;
     PyObject *line_buffering_obj = Py_None;
     PyObject *write_through_obj = Py_None;
 
     if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
-        &line_buffering_obj, &write_through_obj)) {
+        &encoding, &errors, &newline_obj, &line_buffering_obj, &write_through_obj)) {
         goto exit;
     }
-    return_value = _io_TextIOWrapper_reconfigure_impl(self, line_buffering_obj, write_through_obj);
+    return_value = _io_TextIOWrapper_reconfigure_impl(self, encoding, errors, newline_obj, line_buffering_obj, write_through_obj);
 
 exit:
     return return_value;
@@ -499,4 +504,4 @@ _io_TextIOWrapper_close(textio *self, PyObject *Py_UNUSED(ignored))
 {
     return _io_TextIOWrapper_close_impl(self);
 }
-/*[clinic end generated code: output=937989df0a8abfc3 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=f01ba6b6d36cb708 input=a9049054013a1b77]*/
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		``TextIOWrapper.reconfigure()`` supports changing encoding, errors, and
		newline.