Lib/io.py

"""
The io module provides the Python interfaces to stream handling. The
builtin open function is defined in this module.

At the top of the I/O hierarchy is the abstract base class IOBase. It
defines the basic interface to a stream. Note, however, that there is no
seperation between reading and writing to streams; implementations are
allowed to throw an IOError if they do not support a given operation.

Extending IOBase is RawIOBase which deals simply with the reading and
writing of raw bytes to a stream. FileIO subclasses RawIOBase to provide
an interface to OS files.

BufferedIOBase deals with buffering on a raw byte stream (RawIOBase). Its
subclasses, BufferedWriter, BufferedReader, and BufferedRWPair buffer
streams that are readable, writable, and both respectively.
BufferedRandom provides a buffered interface to random access
streams. BytesIO is a simple stream of in-memory bytes.

Another IOBase subclass, TextIOBase, deals with the encoding and decoding
of streams into text. TextIOWrapper, which extends it, is a buffered text
interface to a buffered raw stream (`BufferedIOBase`). Finally, StringIO
is a in-memory stream for text.

Argument names are not part of the specification, and only the arguments
of open() are intended to be used as keyword arguments.

data:

DEFAULT_BUFFER_SIZE

   An int containing the default buffer size used by the module's buffered
   I/O classes. open() uses the file's blksize (as obtained by os.stat) if
   possible.
"""
# New I/O library conforming to PEP 3116.

# This is a prototype; hopefully eventually some of this will be
# reimplemented in C.

# XXX edge cases when switching between reading/writing
# XXX need to support 1 meaning line-buffered
# XXX whenever an argument is None, use the default value
# XXX read/write ops should check readable/writable
# XXX buffered readinto should work with arbitrary buffer objects
# XXX use incremental encoder for text output, at least for UTF-16 and UTF-8-SIG
# XXX check writable, readable and seekable in appropriate places


__author__ = ("Guido van Rossum <guido@python.org>, "
              "Mike Verdone <mike.verdone@gmail.com>, "
              "Mark Russell <mark.russell@zen.co.uk>")

__all__ = ["BlockingIOError", "open", "IOBase", "RawIOBase", "FileIO",
           "BytesIO", "StringIO", "BufferedIOBase",
           "BufferedReader", "BufferedWriter", "BufferedRWPair",
           "BufferedRandom", "TextIOBase", "TextIOWrapper"]

import os
import abc
import sys
import codecs
import _fileio
import warnings

# open() uses st_blksize whenever we can
DEFAULT_BUFFER_SIZE = 8 * 1024  # bytes


class BlockingIOError(IOError):

    """Exception raised when I/O would block on a non-blocking I/O stream."""

    def __init__(self, errno, strerror, characters_written=0):
        IOError.__init__(self, errno, strerror)
        self.characters_written = characters_written


def open(file, mode="r", buffering=None, encoding=None, errors=None,
         newline=None, closefd=True):
    r"""
    Open file and return a stream. If the file cannot be opened, an
    IOError is raised.

    file is either a string giving the name (and the path if the file
    isn't in the current working directory) of the file to be opened or an
    integer file descriptor of the file to be wrapped. (If a file
    descriptor is given, it is closed when the returned I/O object is
    closed, unless closefd is set to False.)

    mode is an optional string that specifies the mode in which the file
    is opened. It defaults to 'r' which means open for reading in text
    mode.  Other common values are 'w' for writing (truncating the file if
    it already exists), and 'a' for appending (which on some Unix systems,
    means that all writes append to the end of the file regardless of the
    current seek position). In text mode, if encoding is not specified the
    encoding used is platform dependent. (For reading and writing raw
    bytes use binary mode and leave encoding unspecified.) The available
    modes are:

    ========= ===============================================================
    Character Meaning
    --------- ---------------------------------------------------------------
    'r'       open for reading (default)
    'w'       open for writing, truncating the file first
    'a'       open for writing, appending to the end of the file if it exists
    'b'       binary mode
    't'       text mode (default)
    '+'       open a disk file for updating (reading and writing)
    'U'       universal newline mode (for backwards compatibility; unneeded
              for new code)
    ========= ===============================================================

    The default mode is 'rt' (open for reading text). For binary random
    access, the mode 'w+b' opens and truncates the file to 0 bytes, while
    'r+b' opens the file without truncation.

    Python distinguishes between files opened in binary and text modes,
    even when the underlying operating system doesn't. Files opened in
    binary mode (appending 'b' to the mode argument) return contents as
    bytes objects without any decoding. In text mode (the default, or when
    't' is appended to the mode argument), the contents of the file are
    returned as strings, the bytes having been first decoded using a
    platform-dependent encoding or using the specified encoding if given.

    buffering is an optional integer used to set the buffering policy. By
    default full buffering is on. Pass 0 to switch buffering off (only
    allowed in binary mode), 1 to set line buffering, and an integer > 1
    for full buffering.

    encoding is the name of the encoding used to decode or encode the
    file. This should only be used in text mode. The default encoding is
    platform dependent, but any encoding supported by Python can be
    passed.  See the codecs module for the list of supported encodings.

    errors is an optional string that specifies how encoding errors are to
    be handled---this argument should not be used in binary mode. Pass
    'strict' to raise a ValueError exception if there is an encoding error
    (the default of None has the same effect), or pass 'ignore' to ignore
    errors. (Note that ignoring encoding errors can lead to data loss.)
    See the documentation for codecs.register for a list of the permitted
    encoding error strings.

    newline controls how universal newlines works (it only applies to text
    mode). It can be None, '', '\n', '\r', and '\r\n'.  It works as
    follows:

    * On input, if newline is None, universal newlines mode is
      enabled. Lines in the input can end in '\n', '\r', or '\r\n', and
      these are translated into '\n' before being returned to the
      caller. If it is '', universal newline mode is enabled, but line
      endings are returned to the caller untranslated. If it has any of
      the other legal values, input lines are only terminated by the given
      string, and the line ending is returned to the caller untranslated.

    * On output, if newline is None, any '\n' characters written are
      translated to the system default line separator, os.linesep. If
      newline is '', no translation takes place. If newline is any of the
      other legal values, any '\n' characters written are translated to
      the given string.

    If closefd is False, the underlying file descriptor will be kept open
    when the file is closed. This does not work when a file name is given
    and must be True in that case.

    open() returns a file object whose type depends on the mode, and
    through which the standard file operations such as reading and writing
    are performed. When open() is used to open a file in a text mode ('w',
    'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open
    a file in a binary mode, the returned class varies: in read binary
    mode, it returns a BufferedReader; in write binary and append binary
    modes, it returns a BufferedWriter, and in read/write mode, it returns
    a BufferedRandom.

    It is also possible to use a string or bytearray as a file for both
    reading and writing. For strings StringIO can be used like a file
    opened in a text mode, and for bytes a BytesIO can be used like a file
    opened in a binary mode.
    """
    if not isinstance(file, (str, int)):
        raise TypeError("invalid file: %r" % file)
    if not isinstance(mode, str):
        raise TypeError("invalid mode: %r" % mode)
    if buffering is not None and not isinstance(buffering, int):
        raise TypeError("invalid buffering: %r" % buffering)
    if encoding is not None and not isinstance(encoding, str):
        raise TypeError("invalid encoding: %r" % encoding)
    if errors is not None and not isinstance(errors, str):
        raise TypeError("invalid errors: %r" % errors)
    modes = set(mode)
    if modes - set("arwb+tU") or len(mode) > len(modes):
        raise ValueError("invalid mode: %r" % mode)
    reading = "r" in modes
    writing = "w" in modes
    appending = "a" in modes
    updating = "+" in modes
    text = "t" in modes
    binary = "b" in modes
    if "U" in modes:
        if writing or appending:
            raise ValueError("can't use U and writing mode at once")
        reading = True
    if text and binary:
        raise ValueError("can't have text and binary mode at once")
    if reading + writing + appending > 1:
        raise ValueError("can't have read/write/append mode at once")
    if not (reading or writing or appending):
        raise ValueError("must have exactly one of read/write/append mode")
    if binary and encoding is not None:
        raise ValueError("binary mode doesn't take an encoding argument")
    if binary and errors is not None:
        raise ValueError("binary mode doesn't take an errors argument")
    if binary and newline is not None:
        raise ValueError("binary mode doesn't take a newline argument")
    raw = FileIO(file,
                 (reading and "r" or "") +
                 (writing and "w" or "") +
                 (appending and "a" or "") +
                 (updating and "+" or ""),
                 closefd)
    if buffering is None:
        buffering = -1
    line_buffering = False
    if buffering == 1 or buffering < 0 and raw.isatty():
        buffering = -1
        line_buffering = True
    if buffering < 0:
        buffering = DEFAULT_BUFFER_SIZE
        try:
            bs = os.fstat(raw.fileno()).st_blksize
        except (os.error, AttributeError):
            pass
        else:
            if bs > 1:
                buffering = bs
    if buffering < 0:
        raise ValueError("invalid buffering size")
    if buffering == 0:
        if binary:
            raw._name = file
            raw._mode = mode
            return raw
        raise ValueError("can't have unbuffered text I/O")
    if updating:
        buffer = BufferedRandom(raw, buffering)
    elif writing or appending:
        buffer = BufferedWriter(raw, buffering)
    elif reading:
        buffer = BufferedReader(raw, buffering)
    else:
        raise ValueError("unknown mode: %r" % mode)
    if binary:
        buffer.name = file
        buffer.mode = mode
        return buffer
    text = TextIOWrapper(buffer, encoding, errors, newline, line_buffering)
    text.name = file
    text.mode = mode
    return text

class _DocDescriptor:
    """Helper for builtins.open.__doc__
    """
    def __get__(self, obj, typ):
        return (
            "open(file, mode='r', buffering=None, encoding=None, "
                 "errors=None, newline=None, closefd=True)\n\n" +
            open.__doc__)

class OpenWrapper:
    """Wrapper for builtins.open

    Trick so that open won't become a bound method when stored
    as a class variable (as dumbdbm does).

    See initstdio() in Python/pythonrun.c.
    """
    __doc__ = _DocDescriptor()

    def __new__(cls, *args, **kwargs):
        return open(*args, **kwargs)


class UnsupportedOperation(ValueError, IOError):
    pass


class IOBase(metaclass=abc.ABCMeta):

    """
    The abstract base class for all I/O classes, acting on streams of
    bytes. There is no public constructor.

    This class provides dummy implementations for many methods that
    derived classes can override selectively; the default implementations
    represent a file that cannot be read, written or seeked.

    Even though IOBase does not declare read, readinto, or write because
    their signatures will vary, implementations and clients should
    consider those methods part of the interface. Also, implementations
    may raise a IOError when operations they do not support are called.

    The basic type used for binary data read from or written to a file is
    bytes. bytearrays are accepted too, and in some cases (such as
    readinto) needed. Text I/O classes work with str data.

    Note that calling any method (even inquiries) on a closed stream is
    undefined. Implementations may raise IOError in this case.

    IOBase (and its subclasses) support the iterator protocol, meaning
    that an IOBase object can be iterated over yielding the lines in a
    stream.

    IOBase also supports the :keyword:`with` statement. In this example,
    fp is closed after the suite of the with statment is complete:

    with open('spam.txt', 'r') as fp:
        fp.write('Spam and eggs!')
    """

    ### Internal ###

    def _unsupported(self, name: str) -> IOError:
        """Internal: raise an exception for unsupported operations."""
        raise UnsupportedOperation("%s.%s() not supported" %
                                   (self.__class__.__name__, name))

    ### Positioning ###

    def seek(self, pos: int, whence: int = 0) -> int:
        """seek(pos: int, whence: int = 0) -> int.  Change stream position.

        Change the stream position to byte offset offset. offset is
        interpreted relative to the position indicated by whence.  Values
        for whence are:

        * 0 -- start of stream (the default); offset should be zero or positive
        * 1 -- current stream position; offset may be negative
        * 2 -- end of stream; offset is usually negative

        Return the new absolute position.
        """
        self._unsupported("seek")

    def tell(self) -> int:
        """tell() -> int.  Return current stream position."""
        return self.seek(0, 1)

    def truncate(self, pos: int = None) -> int:
        """truncate(pos: int = None) -> int. Truncate file to pos bytes.

        Pos defaults to the current IO position as reported by tell().
        Returns the new size.
        """
        self._unsupported("truncate")

    ### Flush and close ###

    def flush(self) -> None:
        """flush() -> None.  Flushes write buffers, if applicable.

        This is not implemented for read-only and non-blocking streams.
        """
        # XXX Should this return the number of bytes written???

    __closed = False

    def close(self) -> None:
        """close() -> None.  Flushes and closes the IO object.

        This method has no effect if the file is already closed.
        """
        if not self.__closed:
            try:
                self.flush()
            except IOError:
                pass  # If flush() fails, just give up
            self.__closed = True

    def __del__(self) -> None:
        """Destructor.  Calls close()."""
        # The try/except block is in case this is called at program
        # exit time, when it's possible that globals have already been
        # deleted, and then the close() call might fail.  Since
        # there's nothing we can do about such failures and they annoy
        # the end users, we suppress the traceback.
        try:
            self.close()
        except:
            pass

    ### Inquiries ###

    def seekable(self) -> bool:
        """seekable() -> bool.  Return whether object supports random access.

        If False, seek(), tell() and truncate() will raise IOError.
        This method may need to do a test seek().
        """
        return False

    def _checkSeekable(self, msg=None):
        """Internal: raise an IOError if file is not seekable
        """
        if not self.seekable():
            raise IOError("File or stream is not seekable."
                          if msg is None else msg)


    def readable(self) -> bool:
        """readable() -> bool.  Return whether object was opened for reading.

        If False, read() will raise IOError.
        """
        return False

    def _checkReadable(self, msg=None):
        """Internal: raise an IOError if file is not readable
        """
        if not self.readable():
            raise IOError("File or stream is not readable."
                          if msg is None else msg)

    def writable(self) -> bool:
        """writable() -> bool.  Return whether object was opened for writing.

        If False, write() and truncate() will raise IOError.
        """
        return False

    def _checkWritable(self, msg=None):
        """Internal: raise an IOError if file is not writable
        """
        if not self.writable():
            raise IOError("File or stream is not writable."
                          if msg is None else msg)

    @property
    def closed(self):
        """closed: bool.  True iff the file has been closed.

        For backwards compatibility, this is a property, not a predicate.
        """
        return self.__closed

    def _checkClosed(self, msg=None):
        """Internal: raise an ValueError if file is closed
        """
        if self.closed:
            raise ValueError("I/O operation on closed file."
                             if msg is None else msg)

    ### Context manager ###

    def __enter__(self) -> "IOBase":  # That's a forward reference
        """Context management protocol.  Returns self."""
        self._checkClosed()
        return self

    def __exit__(self, *args) -> None:
        """Context management protocol.  Calls close()"""
        self.close()

    ### Lower-level APIs ###

    # XXX Should these be present even if unimplemented?

    def fileno(self) -> int:
        """fileno() -> int.  Returns underlying file descriptor if one exists.

        Raises IOError if the IO object does not use a file descriptor.
        """
        self._unsupported("fileno")

    def isatty(self) -> bool:
        """isatty() -> int.  Returns whether this is an 'interactive' stream.
        """
        self._checkClosed()
        return False

    ### Readline[s] and writelines ###

    def readline(self, limit: int = -1) -> bytes:
        r"""readline(limit: int = -1) -> bytes Read and return a line from the
        stream.

        If limit is specified, at most limit bytes will be read.

        The line terminator is always b'\n' for binary files; for text
        files, the newlines argument to open can be used to select the line
        terminator(s) recognized.
        """
        # For backwards compatibility, a (slowish) readline().
        if hasattr(self, "peek"):
            def nreadahead():
                readahead = self.peek(1)
                if not readahead:
                    return 1
                n = (readahead.find(b"\n") + 1) or len(readahead)
                if limit >= 0:
                    n = min(n, limit)
                return n
        else:
            def nreadahead():
                return 1
        if limit is None:
            limit = -1
        res = bytearray()
        while limit < 0 or len(res) < limit:
            b = self.read(nreadahead())
            if not b:
                break
            res += b
            if res.endswith(b"\n"):
                break
        return bytes(res)

    def __iter__(self):
        self._checkClosed()
        return self

    def __next__(self):
        line = self.readline()
        if not line:
            raise StopIteration
        return line

    def readlines(self, hint=None):
        """readlines(hint=None) -> list Return a list of lines from the stream.

        hint can be specified to control the number of lines read: no more
        lines will be read if the total size (in bytes/characters) of all
        lines so far exceeds hint.
        """
        if hint is None:
            return list(self)
        n = 0
        lines = []
        for line in self:
            lines.append(line)
            n += len(line)
            if n >= hint:
                break
        return lines

    def writelines(self, lines):
        self._checkClosed()
        for line in lines:
            self.write(line)


class RawIOBase(IOBase):

    """Base class for raw binary I/O."""

    # The read() method is implemented by calling readinto(); derived
    # classes that want to support read() only need to implement
    # readinto() as a primitive operation.  In general, readinto() can be
    # more efficient than read().

    # (It would be tempting to also provide an implementation of
    # readinto() in terms of read(), in case the latter is a more suitable
    # primitive operation, but that would lead to nasty recursion in case
    # a subclass doesn't implement either.)

    def read(self, n: int = -1) -> bytes:
        """read(n: int) -> bytes.  Read and return up to n bytes.

        Returns an empty bytes object on EOF, or None if the object is
        set not to block and has no data to read.
        """
        if n is None:
            n = -1
        if n < 0:
            return self.readall()
        b = bytearray(n.__index__())
        n = self.readinto(b)
        del b[n:]
        return bytes(b)

    def readall(self):
        """readall() -> bytes.  Read until EOF, using multiple read() calls."""
        res = bytearray()
        while True:
            data = self.read(DEFAULT_BUFFER_SIZE)
            if not data:
                break
            res += data
        return bytes(res)

    def readinto(self, b: bytearray) -> int:
        """readinto(b: bytearray) -> int.  Read up to len(b) bytes into b.

        Returns number of bytes read (0 for EOF), or None if the object
        is set not to block as has no data to read.
        """
        self._unsupported("readinto")

    def write(self, b: bytes) -> int:
        """write(b: bytes) -> int.  Write the given buffer to the IO stream.

        Returns the number of bytes written, which may be less than len(b).
        """
        self._unsupported("write")


class FileIO(_fileio._FileIO, RawIOBase):

    """Raw I/O implementation for OS files."""

    # This multiply inherits from _FileIO and RawIOBase to make
    # isinstance(io.FileIO(), io.RawIOBase) return True without requiring
    # that _fileio._FileIO inherits from io.RawIOBase (which would be hard
    # to do since _fileio.c is written in C).

    def close(self):
        _fileio._FileIO.close(self)
        RawIOBase.close(self)

    @property
    def name(self):
        return self._name

    # XXX(gb): _FileIO already has a mode property
    @property
    def mode(self):
        return self._mode


class BufferedIOBase(IOBase):

    """Base class for buffered IO objects.

    The main difference with RawIOBase is that the read() method
    supports omitting the size argument, and does not have a default
    implementation that defers to readinto().

    In addition, read(), readinto() and write() may raise
    BlockingIOError if the underlying raw stream is in non-blocking
    mode and not ready; unlike their raw counterparts, they will never
    return None.

    A typical implementation should not inherit from a RawIOBase
    implementation, but wrap one.
    """

    def read(self, n: int = None) -> bytes:
        """read(n: int = None) -> bytes.  Read and return up to n bytes.

        If the argument is omitted, None, or negative, reads and
        returns all data until EOF.

        If the argument is positive, and the underlying raw stream is
        not 'interactive', multiple raw reads may be issued to satisfy
        the byte count (unless EOF is reached first).  But for
        interactive raw streams (XXX and for pipes?), at most one raw
        read will be issued, and a short result does not imply that
        EOF is imminent.

        Returns an empty bytes array on EOF.

        Raises BlockingIOError if the underlying raw stream has no
        data at the moment.
        """
        self._unsupported("read")

    def readinto(self, b: bytearray) -> int:
        """readinto(b: bytearray) -> int.  Read up to len(b) bytes into b.

        Like read(), this may issue multiple reads to the underlying raw
        stream, unless the latter is 'interactive'.

        Returns the number of bytes read (0 for EOF).

        Raises BlockingIOError if the underlying raw stream has no
        data at the moment.
        """
        # XXX This ought to work with anything that supports the buffer API
        data = self.read(len(b))
        n = len(data)
        try:
            b[:n] = data
        except TypeError as err:
            import array
            if not isinstance(b, array.array):
                raise err
            b[:n] = array.array('b', data)
        return n

    def write(self, b: bytes) -> int:
        """write(b: bytes) -> int.  Write the given buffer to the IO stream.

        Returns the number of bytes written, which is never less than
        len(b).

        Raises BlockingIOError if the buffer is full and the
        underlying raw stream cannot accept more data at the moment.
        """
        self._unsupported("write")


class _BufferedIOMixin(BufferedIOBase):

    """A mixin implementation of BufferedIOBase with an underlying raw stream.

    This passes most requests on to the underlying raw stream.  It
    does *not* provide implementations of read(), readinto() or
    write().
    """

    def __init__(self, raw):
        self.raw = raw

    ### Positioning ###

    def seek(self, pos, whence=0):
        return self.raw.seek(pos, whence)

    def tell(self):
        return self.raw.tell()

    def truncate(self, pos=None):
        # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
        # and a flush may be necessary to synch both views of the current
        # file state.
        self.flush()

        if pos is None:
            pos = self.tell()
        return self.raw.truncate(pos)

    ### Flush and close ###

    def flush(self):
        self.raw.flush()

    def close(self):
        if not self.closed:
            try:
                self.flush()
            except IOError:
                pass  # If flush() fails, just give up
            self.raw.close()

    ### Inquiries ###

    def seekable(self):
        return self.raw.seekable()

    def readable(self):
        return self.raw.readable()

    def writable(self):
        return self.raw.writable()

    @property
    def closed(self):
        return self.raw.closed

    ### Lower-level APIs ###

    def fileno(self):
        return self.raw.fileno()

    def isatty(self):
        return self.raw.isatty()


class BytesIO(BufferedIOBase):

    """Buffered I/O implementation using an in-memory bytes buffer."""

    def __init__(self, initial_bytes=None):
        buf = bytearray()
        if initial_bytes is not None:
            buf += initial_bytes
        self._buffer = buf
        self._pos = 0

    def getvalue(self):
        """getvalue() -> bytes Return the bytes value (contents) of the buffer
        """
        return bytes(self._buffer)

    def read(self, n=None):
        if n is None:
            n = -1
        if n < 0:
            n = len(self._buffer)
        newpos = min(len(self._buffer), self._pos + n)
        b = self._buffer[self._pos : newpos]
        self._pos = newpos
        return bytes(b)

    def read1(self, n):
        """In BytesIO, this is the same as read.
        """
        return self.read(n)

    def write(self, b):
        if self.closed:
            raise ValueError("write to closed file")
        if isinstance(b, str):
            raise TypeError("can't write str to binary stream")
        n = len(b)
        newpos = self._pos + n
        if newpos > len(self._buffer):
            # Inserts null bytes between the current end of the file
            # and the new write position.
            padding = b'\x00' * (newpos - len(self._buffer) - n)
            self._buffer[self._pos:newpos - n] = padding
        self._buffer[self._pos:newpos] = b
        self._pos = newpos
        return n

    def seek(self, pos, whence=0):
        try:
            pos = pos.__index__()
        except AttributeError as err:
            raise TypeError("an integer is required") from err
        if whence == 0:
            self._pos = max(0, pos)
        elif whence == 1:
            self._pos = max(0, self._pos + pos)
        elif whence == 2:
            self._pos = max(0, len(self._buffer) + pos)
        else:
            raise IOError("invalid whence value")
        return self._pos

    def tell(self):
        return self._pos

    def truncate(self, pos=None):
        if pos is None:
            pos = self._pos
        del self._buffer[pos:]
        return pos

    def readable(self):
        return True

    def writable(self):
        return True

    def seekable(self):
        return True


class BufferedReader(_BufferedIOMixin):

    """BufferedReader(raw[, buffer_size])

    A buffer for a readable, sequential BaseRawIO object.

    The constructor creates a BufferedReader for the given readable raw
    stream and buffer_size. If buffer_size is omitted, DEFAULT_BUFFER_SIZE
    is used.
    """

    def __init__(self, raw, buffer_size=DEFAULT_BUFFER_SIZE):
        """Create a new buffered reader using the given readable raw IO object.
        """
        raw._checkReadable()
        _BufferedIOMixin.__init__(self, raw)
        self._read_buf = b""
        self.buffer_size = buffer_size

    def read(self, n=None):
        """Read n bytes.

        Returns exactly n bytes of data unless the underlying raw IO
        stream reaches EOF or if the call would block in non-blocking
        mode. If n is negative, read until EOF or until read() would
        block.
        """
        if n is None:
            n = -1
        nodata_val = b""
        while n < 0 or len(self._read_buf) < n:
            to_read = max(self.buffer_size,
                          n if n is not None else 2*len(self._read_buf))
            current = self.raw.read(to_read)
            if current in (b"", None):
                nodata_val = current
                break
            self._read_buf += current
        if self._read_buf:
            if n < 0:
                n = len(self._read_buf)
            out = self._read_buf[:n]
            self._read_buf = self._read_buf[n:]
        else:
            out = nodata_val
        return out

    def peek(self, n=0):
        """Returns buffered bytes without advancing the position.

        The argument indicates a desired minimal number of bytes; we
        do at most one raw read to satisfy it.  We never return more
        than self.buffer_size.
        """
        want = min(n, self.buffer_size)
        have = len(self._read_buf)
        if have < want:
            to_read = self.buffer_size - have
            current = self.raw.read(to_read)
            if current:
                self._read_buf += current
        return self._read_buf

    def read1(self, n):
        """Reads up to n bytes, with at most one read() system call."""
        # Returns up to n bytes.  If at least one byte is buffered, we
        # only return buffered bytes.  Otherwise, we do one raw read.
        if n <= 0:
            return b""
        self.peek(1)
        return self.read(min(n, len(self._read_buf)))

    def tell(self):
        return self.raw.tell() - len(self._read_buf)

    def seek(self, pos, whence=0):
        if whence == 1:
            pos -= len(self._read_buf)
        pos = self.raw.seek(pos, whence)
        self._read_buf = b""
        return pos


class BufferedWriter(_BufferedIOMixin):

    """BufferedWriter(raw[, buffer_size[, max_buffer_size]])

    A buffer for a writeable sequential RawIO object.

    The constructor creates a BufferedWriter for the given writeable raw
    stream. If the buffer_size is not given, it defaults to
    DEAFULT_BUFFER_SIZE. If max_buffer_size is omitted, it defaults to
    twice the buffer size.
    """

    def __init__(self, raw,
                 buffer_size=DEFAULT_BUFFER_SIZE, max_buffer_size=None):
        raw._checkWritable()
        _BufferedIOMixin.__init__(self, raw)
        self.buffer_size = buffer_size
        self.max_buffer_size = (2*buffer_size
                                if max_buffer_size is None
                                else max_buffer_size)
        self._write_buf = bytearray()

    def write(self, b):
        if self.closed:
            raise ValueError("write to closed file")
        if isinstance(b, str):
            raise TypeError("can't write str to binary stream")
        # XXX we can implement some more tricks to try and avoid partial writes
        if len(self._write_buf) > self.buffer_size:
            # We're full, so let's pre-flush the buffer
            try:
                self.flush()
            except BlockingIOError as e:
                # We can't accept anything else.
                # XXX Why not just let the exception pass through?
                raise BlockingIOError(e.errno, e.strerror, 0)
        before = len(self._write_buf)
        self._write_buf.extend(b)
        written = len(self._write_buf) - before
        if len(self._write_buf) > self.buffer_size:
            try:
                self.flush()
            except BlockingIOError as e:
                if (len(self._write_buf) > self.max_buffer_size):
                    # We've hit max_buffer_size. We have to accept a partial
                    # write and cut back our buffer.
                    overage = len(self._write_buf) - self.max_buffer_size
                    self._write_buf = self._write_buf[:self.max_buffer_size]
                    raise BlockingIOError(e.errno, e.strerror, overage)
        return written

    def flush(self):
        if self.closed:
            raise ValueError("flush of closed file")
        written = 0
        try:
            while self._write_buf:
                n = self.raw.write(self._write_buf)
                del self._write_buf[:n]
                written += n
        except BlockingIOError as e:
            n = e.characters_written
            del self._write_buf[:n]
            written += n
            raise BlockingIOError(e.errno, e.strerror, written)

    def tell(self):
        return self.raw.tell() + len(self._write_buf)

    def seek(self, pos, whence=0):
        self.flush()
        return self.raw.seek(pos, whence)


class BufferedRWPair(BufferedIOBase):

    """A buffered reader and writer object together.

    A buffered reader object and buffered writer object put together to
    form a sequential IO object that can read and write. This is typically
    used with a socket or two-way pipe.

    reader and writer are RawIOBase objects that are readable and
    writeable respectively. If the buffer_size is omitted it defaults to
    DEFAULT_BUFFER_SIZE. The max_buffer_size (for the buffered writer)
    defaults to twice the buffer size.
    """

    # XXX The usefulness of this (compared to having two separate IO
    # objects) is questionable.

    def __init__(self, reader, writer,
                 buffer_size=DEFAULT_BUFFER_SIZE, max_buffer_size=None):
        """Constructor.

        The arguments are two RawIO instances.
        """
        reader._checkReadable()
        writer._checkWritable()
        self.reader = BufferedReader(reader, buffer_size)
        self.writer = BufferedWriter(writer, buffer_size, max_buffer_size)

    def read(self, n=None):
        if n is None:
            n = -1
        return self.reader.read(n)

    def readinto(self, b):
        return self.reader.readinto(b)

    def write(self, b):
        return self.writer.write(b)

    def peek(self, n=0):
        return self.reader.peek(n)

    def read1(self, n):
        return self.reader.read1(n)

    def readable(self):
        return self.reader.readable()

    def writable(self):
        return self.writer.writable()

    def flush(self):
        return self.writer.flush()

    def close(self):
        self.writer.close()
        self.reader.close()

    def isatty(self):
        return self.reader.isatty() or self.writer.isatty()

    @property
    def closed(self):
        return self.writer.closed()


class BufferedRandom(BufferedWriter, BufferedReader):

    """BufferedRandom(raw[, buffer_size[, max_buffer_size]])

    A buffered interface to random access streams.

    The constructor creates a reader and writer for a seekable stream,
    raw, given in the first argument. If the buffer_size is omitted it
    defaults to DEFAULT_BUFFER_SIZE. The max_buffer_size (for the buffered
    writer) defaults to twice the buffer size.
    """

    def __init__(self, raw,
                 buffer_size=DEFAULT_BUFFER_SIZE, max_buffer_size=None):
        raw._checkSeekable()
        BufferedReader.__init__(self, raw, buffer_size)
        BufferedWriter.__init__(self, raw, buffer_size, max_buffer_size)

    def seek(self, pos, whence=0):
        self.flush()
        # First do the raw seek, then empty the read buffer, so that
        # if the raw seek fails, we don't lose buffered data forever.
        pos = self.raw.seek(pos, whence)
        self._read_buf = b""
        return pos

    def tell(self):
        if (self._write_buf):
            return self.raw.tell() + len(self._write_buf)
        else:
            return self.raw.tell() - len(self._read_buf)

    def read(self, n=None):
        if n is None:
            n = -1
        self.flush()
        return BufferedReader.read(self, n)

    def readinto(self, b):
        self.flush()
        return BufferedReader.readinto(self, b)

    def peek(self, n=0):
        self.flush()
        return BufferedReader.peek(self, n)

    def read1(self, n):
        self.flush()
        return BufferedReader.read1(self, n)

    def write(self, b):
        if self._read_buf:
            self.raw.seek(-len(self._read_buf), 1) # Undo readahead
            self._read_buf = b""
        return BufferedWriter.write(self, b)


class TextIOBase(IOBase):

    """Base class for text I/O.

    This class provides a character and line based interface to stream
    I/O. There is no readinto method because Python's character strings
    are immutable. There is no public constructor.
    """

    def read(self, n: int = -1) -> str:
        """read(n: int = -1) -> str.  Read at most n characters from stream.

        Read from underlying buffer until we have n characters or we hit EOF.
        If n is negative or omitted, read until EOF.
        """
        self._unsupported("read")

    def write(self, s: str) -> int:
        """write(s: str) -> int.  Write string s to stream."""
        self._unsupported("write")

    def truncate(self, pos: int = None) -> int:
        """truncate(pos: int = None) -> int.  Truncate size to pos."""
        self.flush()
        if pos is None:
            pos = self.tell()
        self.seek(pos)
        return self.buffer.truncate()

    def readline(self) -> str:
        """readline() -> str.  Read until newline or EOF.

        Returns an empty string if EOF is hit immediately.
        """
        self._unsupported("readline")

    @property
    def encoding(self):
        """Subclasses should override."""
        return None

    @property
    def newlines(self):
        """newlines -> None | str | tuple of str. Line endings translated
        so far.

        Only line endings translated during reading are considered.

        Subclasses should override.
        """
        return None


class IncrementalNewlineDecoder(codecs.IncrementalDecoder):
    r"""Codec used when reading a file in universal newlines mode.  It wraps
    another incremental decoder, translating \r\n and \r into \n.  It also
    records the types of newlines encountered.  When used with
    translate=False, it ensures that the newline sequence is returned in
    one piece.
    """
    def __init__(self, decoder, translate, errors='strict'):
        codecs.IncrementalDecoder.__init__(self, errors=errors)
        self.buffer = b''
        self.translate = translate
        self.decoder = decoder
        self.seennl = 0

    def decode(self, input, final=False):
        # decode input (with the eventual \r from a previous pass)
        if self.buffer:
            input = self.buffer + input

        output = self.decoder.decode(input, final=final)

        # retain last \r even when not translating data:
        # then readline() is sure to get \r\n in one pass
        if output.endswith("\r") and not final:
            output = output[:-1]
            self.buffer = b'\r'
        else:
            self.buffer = b''

        # Record which newlines are read
        crlf = output.count('\r\n')
        cr = output.count('\r') - crlf
        lf = output.count('\n') - crlf
        self.seennl |= (lf and self._LF) | (cr and self._CR) \
                    | (crlf and self._CRLF)

        if self.translate:
            if crlf:
                output = output.replace("\r\n", "\n")
            if cr:
                output = output.replace("\r", "\n")

        return output

    def getstate(self):
        buf, flag = self.decoder.getstate()
        return buf + self.buffer, flag

    def setstate(self, state):
        buf, flag = state
        if buf.endswith(b'\r'):
            self.buffer = b'\r'
            buf = buf[:-1]
        else:
            self.buffer = b''
        self.decoder.setstate((buf, flag))

    def reset(self):
        self.seennl = 0
        self.buffer = b''
        self.decoder.reset()

    _LF = 1
    _CR = 2
    _CRLF = 4

    @property
    def newlines(self):
        return (None,
                "\n",
                "\r",
                ("\r", "\n"),
                "\r\n",
                ("\n", "\r\n"),
                ("\r", "\r\n"),
                ("\r", "\n", "\r\n")
               )[self.seennl]


class TextIOWrapper(TextIOBase):

    r"""TextIOWrapper(buffer[, encoding[, errors[, newline[, line_buffering]]]])

    Character and line based layer over a BufferedIOBase object, buffer.

    encoding gives the name of the encoding that the stream will be
    decoded or encoded with. It defaults to locale.getpreferredencoding.

    errors determines the strictness of encoding and decoding (see the
    codecs.register) and defaults to "strict".

    newline can be None, '', '\n', '\r', or '\r\n'.  It controls the
    handling of line endings. If it is None, universal newlines is
    enabled.  With this enabled, on input, the lines endings '\n', '\r',
    or '\r\n' are translated to '\n' before being returned to the
    caller. Conversely, on output, '\n' is translated to the system
    default line seperator, os.linesep. If newline is any other of its
    legal values, that newline becomes the newline when the file is read
    and it is returned untranslated. On output, '\n' is converted to the
    newline.

    If line_buffering is True, a call to flush is implied when a call to
    write contains a newline character.
    """

    _CHUNK_SIZE = 128

    def __init__(self, buffer, encoding=None, errors=None, newline=None,
                 line_buffering=False):
        if newline not in (None, "", "\n", "\r", "\r\n"):
            raise ValueError("illegal newline value: %r" % (newline,))
        if encoding is None:
            try:
                encoding = os.device_encoding(buffer.fileno())
            except (AttributeError, UnsupportedOperation):
                pass
            if encoding is None:
                try:
                    import locale
                except ImportError:
                    # Importing locale may fail if Python is being built
                    encoding = "ascii"
                else:
                    encoding = locale.getpreferredencoding()

        if not isinstance(encoding, str):
            raise ValueError("invalid encoding: %r" % encoding)

        if errors is None:
            errors = "strict"
        else:
            if not isinstance(errors, str):
                raise ValueError("invalid errors: %r" % errors)

        self.buffer = buffer
        self._line_buffering = line_buffering
        self._encoding = encoding
        self._errors = errors
        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._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
    # snapshot point.  We use this to reconstruct decoder states in tell().

    # Naming convention:
    #   - "bytes_..." for integer variables that count input bytes
    #   - "chars_..." for integer variables that count decoded characters

    @property
    def encoding(self):
        return self._encoding

    @property
    def errors(self):
        return self._errors

    @property
    def line_buffering(self):
        return self._line_buffering

    def seekable(self):
        return self._seekable

    def flush(self):
        self.buffer.flush()
        self._telling = self._seekable

    def close(self):
        try:
            self.flush()
        except:
            pass  # If flush() fails, just give up
        self.buffer.close()

    @property
    def closed(self):
        return self.buffer.closed

    def fileno(self):
        return self.buffer.fileno()

    def isatty(self):
        return self.buffer.isatty()

    def write(self, s: str):
        if self.closed:
            raise ValueError("write to closed file")
        if not isinstance(s, str):
            raise TypeError("can't write %s to text stream" %
                            s.__class__.__name__)
        length = len(s)
        haslf = (self._writetranslate or self._line_buffering) and "\n" in s
        if haslf and self._writetranslate and self._writenl != "\n":
            s = s.replace("\n", self._writenl)
        encoder = self._encoder or self._get_encoder()
        # XXX What if we were just reading?
        b = encoder.encode(s)
        self.buffer.write(b)
        if self._line_buffering and (haslf or "\r" in s):
            self.flush()
        self._snapshot = None
        if self._decoder:
            self._decoder.reset()
        return length

    def _get_encoder(self):
        make_encoder = codecs.getincrementalencoder(self._encoding)
        self._encoder = make_encoder(self._errors)
        return self._encoder

    def _get_decoder(self):
        make_decoder = codecs.getincrementaldecoder(self._encoding)
        decoder = make_decoder(self._errors)
        if self._readuniversal:
            decoder = IncrementalNewlineDecoder(decoder, self._readtranslate)
        self._decoder = decoder
        return decoder

    # The following three methods implement an ADT for _decoded_chars.
    # Text returned from the decoder is buffered here until the client
    # requests it by calling our read() or readline() method.
    def _set_decoded_chars(self, chars):
        """Set the _decoded_chars buffer."""
        self._decoded_chars = chars
        self._decoded_chars_used = 0

    def _get_decoded_chars(self, n=None):
        """Advance into the _decoded_chars buffer."""
        offset = self._decoded_chars_used
        if n is None:
            chars = self._decoded_chars[offset:]
        else:
            chars = self._decoded_chars[offset:offset + n]
        self._decoded_chars_used += len(chars)
        return chars

    def _rewind_decoded_chars(self, n):
        """Rewind the _decoded_chars buffer."""
        if self._decoded_chars_used < n:
            raise AssertionError("rewind decoded_chars out of bounds")
        self._decoded_chars_used -= n

    def _read_chunk(self):
        """
        Read and decode the next chunk of data from the BufferedReader.
        """

        # The return value is True unless EOF was reached.  The decoded
        # string is placed in self._decoded_chars (replacing its previous
        # value).  The entire input chunk is sent to the decoder, though
        # some of it may remain buffered in the decoder, yet to be
        # converted.

        if self._decoder is None:
            raise ValueError("no decoder")

        if self._telling:
            # To prepare for tell(), we need to snapshot a point in the
            # file where the decoder's input buffer is empty.

            dec_buffer, dec_flags = self._decoder.getstate()
            # Given this, we know there was a valid snapshot point
            # len(dec_buffer) bytes ago with decoder state (b'', dec_flags).

        # Read a chunk, decode it, and put the result in self._decoded_chars.
        input_chunk = self.buffer.read1(self._CHUNK_SIZE)
        eof = not input_chunk
        self._set_decoded_chars(self._decoder.decode(input_chunk, eof))

        if self._telling:
            # At the snapshot point, len(dec_buffer) bytes before the read,
            # the next input to be decoded is dec_buffer + input_chunk.
            self._snapshot = (dec_flags, dec_buffer + input_chunk)

        return not eof

    def _pack_cookie(self, position, dec_flags=0,
                           bytes_to_feed=0, need_eof=0, chars_to_skip=0):
        # The meaning of a tell() cookie is: seek to position, set the
        # decoder flags to dec_flags, read bytes_to_feed bytes, feed them
        # into the decoder with need_eof as the EOF flag, then skip
        # chars_to_skip characters of the decoded result.  For most simple
        # decoders, tell() will often just give a byte offset in the file.
        return (position | (dec_flags<<64) | (bytes_to_feed<<128) |
               (chars_to_skip<<192) | bool(need_eof)<<256)

    def _unpack_cookie(self, bigint):
        rest, position = divmod(bigint, 1<<64)
        rest, dec_flags = divmod(rest, 1<<64)
        rest, bytes_to_feed = divmod(rest, 1<<64)
        need_eof, chars_to_skip = divmod(rest, 1<<64)
        return position, dec_flags, bytes_to_feed, need_eof, chars_to_skip

    def tell(self):
        if not self._seekable:
            raise IOError("underlying stream is not seekable")
        if not self._telling:
            raise IOError("telling position disabled by next() call")
        self.flush()
        position = self.buffer.tell()
        decoder = self._decoder
        if decoder is None or self._snapshot is None:
            if self._decoded_chars:
                # This should never happen.
                raise AssertionError("pending decoded text")
            return position

        # Skip backward to the snapshot point (see _read_chunk).
        dec_flags, next_input = self._snapshot
        position -= len(next_input)

        # How many decoded characters have been used up since the snapshot?
        chars_to_skip = self._decoded_chars_used
        if chars_to_skip == 0:
            # We haven't moved from the snapshot point.
            return self._pack_cookie(position, dec_flags)

        # Starting from the snapshot position, we will walk the decoder
        # forward until it gives us enough decoded characters.
        saved_state = decoder.getstate()
        try:
            # Note our initial start point.
            decoder.setstate((b'', dec_flags))
            start_pos = position
            start_flags, bytes_fed, chars_decoded = dec_flags, 0, 0
            need_eof = 0

            # Feed the decoder one byte at a time.  As we go, note the
            # nearest "safe start point" before the current location
            # (a point where the decoder has nothing buffered, so seek()
            # can safely start from there and advance to this location).
            next_byte = bytearray(1)
            for next_byte[0] in next_input:
                bytes_fed += 1
                chars_decoded += len(decoder.decode(next_byte))
                dec_buffer, dec_flags = decoder.getstate()
                if not dec_buffer and chars_decoded <= chars_to_skip:
                    # Decoder buffer is empty, so this is a safe start point.
                    start_pos += bytes_fed
                    chars_to_skip -= chars_decoded
                    start_flags, bytes_fed, chars_decoded = dec_flags, 0, 0
                if chars_decoded >= chars_to_skip:
                    break
            else:
                # We didn't get enough decoded data; signal EOF to get more.
                chars_decoded += len(decoder.decode(b'', final=True))
                need_eof = 1
                if chars_decoded < chars_to_skip:
                    raise IOError("can't reconstruct logical file position")

            # The returned cookie corresponds to the last safe start point.
            return self._pack_cookie(
                start_pos, start_flags, bytes_fed, need_eof, chars_to_skip)
        finally:
            decoder.setstate(saved_state)

    def seek(self, cookie, whence=0):
        if not self._seekable:
            raise IOError("underlying stream is not seekable")
        if whence == 1: # seek relative to current position
            if cookie != 0:
                raise IOError("can't do nonzero cur-relative seeks")
            # Seeking to the current position should attempt to
            # sync the underlying buffer with the current position.
            whence = 0
            cookie = self.tell()
        if whence == 2: # seek relative to end of file
            if cookie != 0:
                raise IOError("can't do nonzero end-relative seeks")
            self.flush()
            position = self.buffer.seek(0, 2)
            self._set_decoded_chars('')
            self._snapshot = None
            if self._decoder:
                self._decoder.reset()
            return position
        if whence != 0:
            raise ValueError("invalid whence (%r, should be 0, 1 or 2)" %
                             (whence,))
        if cookie < 0:
            raise ValueError("negative seek position %r" % (cookie,))
        self.flush()

        # The strategy of seek() is to go back to the safe start point
        # and replay the effect of read(chars_to_skip) from there.
        start_pos, dec_flags, bytes_to_feed, need_eof, chars_to_skip = \
            self._unpack_cookie(cookie)

        # Seek back to the safe start point.
        self.buffer.seek(start_pos)
        self._set_decoded_chars('')
        self._snapshot = None

        # Restore the decoder to its state from the safe start point.
        if self._decoder or dec_flags or chars_to_skip:
            self._decoder = self._decoder or self._get_decoder()
            self._decoder.setstate((b'', dec_flags))
            self._snapshot = (dec_flags, b'')

        if chars_to_skip:
            # Just like _read_chunk, feed the decoder and save a snapshot.
            input_chunk = self.buffer.read(bytes_to_feed)
            self._set_decoded_chars(
                self._decoder.decode(input_chunk, need_eof))
            self._snapshot = (dec_flags, input_chunk)

            # Skip chars_to_skip of the decoded characters.
            if len(self._decoded_chars) < chars_to_skip:
                raise IOError("can't restore logical file position")
            self._decoded_chars_used = chars_to_skip

        return cookie

    def read(self, n=None):
        if n is None:
            n = -1
        decoder = self._decoder or self._get_decoder()
        if n < 0:
            # Read everything.
            result = (self._get_decoded_chars() +
                      decoder.decode(self.buffer.read(), final=True))
            self._set_decoded_chars('')
            self._snapshot = None
            return result
        else:
            # Keep reading chunks until we have n characters to return.
            eof = False
            result = self._get_decoded_chars(n)
            while len(result) < n and not eof:
                eof = not self._read_chunk()
                result += self._get_decoded_chars(n - len(result))
            return result

    def __next__(self):
        self._telling = False
        line = self.readline()
        if not line:
            self._snapshot = None
            self._telling = self._seekable
            raise StopIteration
        return line

    def readline(self, limit=None):
        if limit is None:
            limit = -1

        # Grab all the decoded text (we will rewind any extra bits later).
        line = self._get_decoded_chars()

        start = 0
        decoder = self._decoder or self._get_decoder()

        pos = endpos = None
        while True:
            if self._readtranslate:
                # Newlines are already translated, only search for \n
                pos = line.find('\n', start)
                if pos >= 0:
                    endpos = pos + 1
                    break
                else:
                    start = len(line)

            elif self._readuniversal:
                # Universal newline search. Find any of \r, \r\n, \n
                # The decoder ensures that \r\n are not split in two pieces

                # In C we'd look for these in parallel of course.
                nlpos = line.find("\n", start)
                crpos = line.find("\r", start)
                if crpos == -1:
                    if nlpos == -1:
                        # Nothing found
                        start = len(line)
                    else:
                        # Found \n
                        endpos = nlpos + 1
                        break
                elif nlpos == -1:
                    # Found lone \r
                    endpos = crpos + 1
                    break
                elif nlpos < crpos:
                    # Found \n
                    endpos = nlpos + 1
                    break
                elif nlpos == crpos + 1:
                    # Found \r\n
                    endpos = crpos + 2
                    break
                else:
                    # Found \r
                    endpos = crpos + 1
                    break
            else:
                # non-universal
                pos = line.find(self._readnl)
                if pos >= 0:
                    endpos = pos + len(self._readnl)
                    break

            if limit >= 0 and len(line) >= limit:
                endpos = limit  # reached length limit
                break

            # No line ending seen yet - get more data
            more_line = ''
            while self._read_chunk():
                if self._decoded_chars:
                    break
            if self._decoded_chars:
                line += self._get_decoded_chars()
            else:
                # end of file
                self._set_decoded_chars('')
                self._snapshot = None
                return line

        if limit >= 0 and endpos > limit:
            endpos = limit  # don't exceed limit

        # Rewind _decoded_chars to just after the line ending we found.
        self._rewind_decoded_chars(len(line) - endpos)
        return line[:endpos]

    @property
    def newlines(self):
        return self._decoder.newlines if self._decoder else None

class StringIO(TextIOWrapper):
    """StringIO([initial_value[, encoding, [errors, [newline]]]])

    An in-memory stream for text. The initial_value argument sets the
    value of object. The other arguments are like those of TextIOWrapper's
    constructor.
    """

    # XXX This is really slow, but fully functional

    def __init__(self, initial_value="", encoding="utf-8",
                 errors="strict", newline="\n"):
        super(StringIO, self).__init__(BytesIO(),
                                       encoding=encoding,
                                       errors=errors,
                                       newline=newline)
        if initial_value:
            if not isinstance(initial_value, str):
                initial_value = str(initial_value)
            self.write(initial_value)
            self.seek(0)

    def getvalue(self):
        self.flush()
        return self.buffer.getvalue().decode(self._encoding, self._errors)