15.2. "io" — Core tools for working with streams
************************************************

2.6 版新加入.

The "io" module provides the Python interfaces to stream handling.
Under Python 2.x, this is proposed as an alternative to the built-in
"file" object, but in Python 3.x it is the default interface to access
files and streams.

備註: Since this module has been designed primarily for Python 3.x,
  you have to be aware that all uses of 「bytes」 in this document
  refer to the "str" type (of which "bytes" is an alias), and all uses
  of 「 text」 refer to the "unicode" type.  Furthermore, those two
  types are not interchangeable in the "io" APIs.

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 separation between reading and writing to streams;
implementations are allowed to raise 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 files in the machine’s file system.

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

Another "IOBase" subclass, "TextIOBase", deals with streams whose
bytes represent text, and handles encoding and decoding from and to
"unicode" strings.  "TextIOWrapper", which extends it, is a buffered
text interface to a buffered raw stream ("BufferedIOBase"). Finally,
"StringIO" is an in-memory stream for unicode text.

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


15.2.1. Module Interface
========================

io.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.

io.open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True)

   Open *file* and return a corresponding stream.  If the file cannot
   be opened, an "IOError" is raised.

   *file* is either a string giving the pathname (absolute or relative
   to 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 newlines mode (for backwards compatibility; should    |
   |           | not be used in 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 (including "'b'" in the *mode* argument) return
   contents as "bytes" objects without any decoding.  In text mode
   (the default, or when "'t'" is included in the *mode* argument),
   the contents of the file are returned as "unicode" 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. Pass 0 to switch buffering off (only allowed in binary
   mode), 1 to select line buffering (only usable in text mode), and
   an integer > 1 to indicate the size of a fixed-size chunk buffer.
   When no *buffering* argument is given, the default buffering policy
   works as follows:

   * Binary files are buffered in fixed-size chunks; the size of the
     buffer is chosen using a heuristic trying to determine the
     underlying device’s 「block size」 and falling back on
     "DEFAULT_BUFFER_SIZE". On many systems, the buffer will typically
     be 4096 or 8192 bytes long.

   * 「Interactive」 text files (files for which "isatty()" returns
     True) use line buffering.  Other text files use the policy
     described above for binary files.

   *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 (whatever "locale.getpreferredencoding()"
   returns), but any encoding supported by Python can be used.  See
   the "codecs" module for the list of supported encodings.

   *errors* is an optional string that specifies how encoding and
   decoding errors are to be handled—this cannot 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.) "'replace'" causes a replacement
   marker (such as "'?'") to be inserted where there is malformed
   data.  When writing, "'xmlcharrefreplace'" (replace with the
   appropriate XML character reference) or "'backslashreplace'"
   (replace with backslashed escape sequences) can be used.  Any other
   error handling name that has been registered with
   "codecs.register_error()" is also valid.

   *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 newlines 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" and a file descriptor rather than a
   filename was given, the underlying file descriptor will be kept
   open when the file is closed.  If a filename is given *closefd* has
   no effect and must be "True" (the default).

   The type of file object returned by the "open()" function depends
   on the mode.  When "open()" is used to open a file in a text mode
   ("'w'", "'r'", "'wt'", "'rt'", etc.), it returns a subclass of
   "TextIOBase" (specifically "TextIOWrapper").  When used to open a
   file in a binary mode with buffering, the returned class is a
   subclass of "BufferedIOBase".  The exact 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".  When buffering is
   disabled, the raw stream, a subclass of "RawIOBase", "FileIO", is
   returned.

   It is also possible to use an "unicode" or "bytes" string as a file
   for both reading and writing.  For "unicode" strings "StringIO" can
   be used like a file opened in text mode, and for "bytes" a
   "BytesIO" can be used like a file opened in a binary mode.

exception io.BlockingIOError

   Error raised when blocking would occur on a non-blocking stream.
   It inherits "IOError".

   In addition to those of "IOError", "BlockingIOError" has one
   attribute:

   characters_written

      An integer containing the number of characters written to the
      stream before it blocked.

exception io.UnsupportedOperation

   An exception inheriting "IOError" and "ValueError" that is raised
   when an unsupported operation is called on a stream.


15.2.2. I/O Base Classes
========================

class io.IOBase

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

   This class provides empty abstract 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 an "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" (also known as "str").  Method arguments may also be
   "bytearray" or "memoryview" of arrays of bytes. In some cases, such
   as "readinto()", a writable object such as "bytearray" is required.
   Text I/O classes work with "unicode" data.

   2.7 版更變: Implementations should support "memoryview" arguments.

   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. Lines are defined slightly differently depending on
   whether the stream is a binary stream (yielding "bytes"), or a text
   stream (yielding "unicode" strings).  See "readline()" below.

   IOBase is also a context manager and therefore supports the "with"
   statement.  In this example, *file* is closed after the "with"
   statement’s suite is finished—even if an exception occurs:

      with io.open('spam.txt', 'w') as file:
          file.write(u'Spam and eggs!')

   "IOBase" provides these data attributes and methods:

   close()

      Flush and close this stream. This method has no effect if the
      file is already closed. Once the file is closed, any operation
      on the file (e.g. reading or writing) will raise a "ValueError".

      As a convenience, it is allowed to call this method more than
      once; only the first call, however, will have an effect.

   closed

      True if the stream is closed.

   fileno()

      Return the underlying file descriptor (an integer) of the stream
      if it exists.  An "IOError" is raised if the IO object does not
      use a file descriptor.

   flush()

      Flush the write buffers of the stream if applicable.  This does
      nothing for read-only and non-blocking streams.

   isatty()

      Return "True" if the stream is interactive (i.e., connected to a
      terminal/tty device).

   readable()

      Return "True" if the stream can be read from.  If "False",
      "read()" will raise "IOError".

   readline(limit=-1)

      Read and return one 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 *newline* argument to "open()" can be used to select
      the line terminator(s) recognized.

   readlines(hint=-1)

      Read and 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*.

      Note that it’s already possible to iterate on file objects using
      "for line in file: ..." without calling "file.readlines()".

   seek(offset, whence=SEEK_SET)

      Change the stream position to the given byte *offset*.  *offset*
      is interpreted relative to the position indicated by *whence*.
      The default value for *whence* is "SEEK_SET".  Values for
      *whence* are:

      * "SEEK_SET" or "0" – start of the stream (the default);
        *offset* should be zero or positive

      * "SEEK_CUR" or "1" – current stream position; *offset* may be
        negative

      * "SEEK_END" or "2" – end of the stream; *offset* is usually
        negative

      Return the new absolute position.

      2.7 版新加入: The "SEEK_*" constants

   seekable()

      Return "True" if the stream supports random access.  If "False",
      "seek()", "tell()" and "truncate()" will raise "IOError".

   tell()

      Return the current stream position.

   truncate(size=None)

      Resize the stream to the given *size* in bytes (or the current
      position if *size* is not specified).  The current stream
      position isn’t changed. This resizing can extend or reduce the
      current file size.  In case of extension, the contents of the
      new file area depend on the platform (on most systems,
      additional bytes are zero-filled, on Windows they’re
      undetermined).  The new file size is returned.

   writable()

      Return "True" if the stream supports writing.  If "False",
      "write()" and "truncate()" will raise "IOError".

   writelines(lines)

      Write a list of lines to the stream.  Line separators are not
      added, so it is usual for each of the lines provided to have a
      line separator at the end.

   __del__()

      Prepare for object destruction. "IOBase" provides a default
      implementation of this method that calls the instance’s
      "close()" method.

class io.RawIOBase

   Base class for raw binary I/O.  It inherits "IOBase".  There is no
   public constructor.

   Raw binary I/O typically provides low-level access to an underlying
   OS device or API, and does not try to encapsulate it in high-level
   primitives (this is left to Buffered I/O and Text I/O, described
   later in this page).

   In addition to the attributes and methods from "IOBase", RawIOBase
   provides the following methods:

   read(n=-1)

      Read up to *n* bytes from the object and return them.  As a
      convenience, if *n* is unspecified or -1, "readall()" is called.
      Otherwise, only one system call is ever made.  Fewer than *n*
      bytes may be returned if the operating system call returns fewer
      than *n* bytes.

      If 0 bytes are returned, and *n* was not 0, this indicates end
      of file. If the object is in non-blocking mode and no bytes are
      available, "None" is returned.

   readall()

      Read and return all the bytes from the stream until EOF, using
      multiple calls to the stream if necessary.

   readinto(b)

      Read up to len(b) bytes into *b*, and return the number of bytes
      read.  The object *b* should be a pre-allocated, writable array
      of bytes, either "bytearray" or "memoryview". If the object is
      in non-blocking mode and no bytes are available, "None" is
      returned.

   write(b)

      Write *b* to the underlying raw stream, and return the number of
      bytes written.  The object *b* should be an array of bytes,
      either "bytes", "bytearray", or "memoryview".  The return value
      can be less than "len(b)", depending on specifics of the
      underlying raw stream, and especially if it is in non-blocking
      mode.  "None" is returned if the raw stream is set not to block
      and no single byte could be readily written to it.  The caller
      may release or mutate *b* after this method returns, so the
      implementation should only access *b* during the method call.

class io.BufferedIOBase

   Base class for binary streams that support some kind of buffering.
   It inherits "IOBase". There is no public constructor.

   The main difference with "RawIOBase" is that methods "read()",
   "readinto()" and "write()" will try (respectively) to read as much
   input as requested or to consume all given output, at the expense
   of making perhaps more than one system call.

   In addition, those methods can raise "BlockingIOError" if the
   underlying raw stream is in non-blocking mode and cannot take or
   give enough data; unlike their "RawIOBase" counterparts, they will
   never return "None".

   Besides, the "read()" method does not have a default implementation
   that defers to "readinto()".

   A typical "BufferedIOBase" implementation should not inherit from a
   "RawIOBase" implementation, but wrap one, like "BufferedWriter" and
   "BufferedReader" do.

   "BufferedIOBase" provides or overrides these methods and attribute
   in addition to those from "IOBase":

   raw

      The underlying raw stream (a "RawIOBase" instance) that
      "BufferedIOBase" deals with.  This is not part of the
      "BufferedIOBase" API and may not exist on some implementations.

   detach()

      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.

      Some buffers, like "BytesIO", do not have the concept of a
      single raw stream to return from this method.  They raise
      "UnsupportedOperation".

      2.7 版新加入.

   read(n=-1)

      Read and return up to *n* bytes.  If the argument is omitted,
      "None", or negative, data is read and returned until EOF is
      reached.  An empty bytes object is returned if the stream is
      already at 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, at most one raw read will be issued, and a short
      result does not imply that EOF is imminent.

      A "BlockingIOError" is raised if the underlying raw stream is in
      non blocking-mode, and has no data available at the moment.

   read1(n=-1)

      Read and return up to *n* bytes, with at most one call to the
      underlying raw stream’s "read()" method.  This can be useful if
      you are implementing your own buffering on top of a
      "BufferedIOBase" object.

   readinto(b)

      Read up to len(b) bytes into *b*, and return the number of bytes
      read. The object *b* should be a pre-allocated, writable array
      of bytes, either "bytearray" or "memoryview".

      Like "read()", multiple reads may be issued to the underlying
      raw stream, unless the latter is 『interactive』.

      A "BlockingIOError" is raised if the underlying raw stream is in
      non blocking-mode, and has no data available at the moment.

   write(b)

      Write *b*, and return the number of bytes written (always equal
      to "len(b)", since if the write fails an "IOError" will be
      raised).  The object *b* should be an array of bytes, either
      "bytes", "bytearray", or "memoryview".  Depending on the actual
      implementation, these bytes may be readily written to the
      underlying stream, or held in a buffer for performance and
      latency reasons.

      When in non-blocking mode, a "BlockingIOError" is raised if the
      data needed to be written to the raw stream but it couldn’t
      accept all the data without blocking.

      The caller may release or mutate *b* after this method returns,
      so the implementation should only access *b* during the method
      call.


15.2.3. Raw File I/O
====================

class io.FileIO(name, mode='r', closefd=True)

   "FileIO" represents an OS-level file containing bytes data. It
   implements the "RawIOBase" interface (and therefore the "IOBase"
   interface, too).

   The *name* can be one of two things:

   * a string representing the path to the file which will be
     opened;

   * an integer representing the number of an existing OS-level file
     descriptor to which the resulting "FileIO" object will give
     access.

   The *mode* can be "'r'", "'w'" or "'a'" for reading (default),
   writing, or appending.  The file will be created if it doesn’t
   exist when opened for writing or appending; it will be truncated
   when opened for writing.  Add a "'+'" to the mode to allow
   simultaneous reading and writing.

   The "read()" (when called with a positive argument), "readinto()"
   and "write()" methods on this class will only make one system call.

   In addition to the attributes and methods from "IOBase" and
   "RawIOBase", "FileIO" provides the following data attributes and
   methods:

   mode

      The mode as given in the constructor.

   name

      The file name.  This is the file descriptor of the file when no
      name is given in the constructor.


15.2.4. Buffered Streams
========================

Buffered I/O streams provide a higher-level interface to an I/O device
than raw I/O does.

class io.BytesIO([initial_bytes])

   A stream implementation using an in-memory bytes buffer.  It
   inherits "BufferedIOBase".

   The optional argument *initial_bytes* is a "bytes" object that
   contains initial data.

   "BytesIO" provides or overrides these methods in addition to those
   from "BufferedIOBase" and "IOBase":

   getvalue()

      Return "bytes" containing the entire contents of the buffer.

   read1()

      In "BytesIO", this is the same as "read()".

class io.BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffer providing higher-level access to a readable, sequential
   "RawIOBase" object.  It inherits "BufferedIOBase". When reading
   data from this object, a larger amount of data may be requested
   from the underlying raw stream, and kept in an internal buffer. The
   buffered data can then be returned directly on subsequent reads.

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

   "BufferedReader" provides or overrides these methods in addition to
   those from "BufferedIOBase" and "IOBase":

   peek([n])

      Return bytes from the stream without advancing the position.  At
      most one single read on the raw stream is done to satisfy the
      call. The number of bytes returned may be less or more than
      requested.

   read([n])

      Read and return *n* bytes, or if *n* is not given or negative,
      until EOF or if the read call would block in non-blocking mode.

   read1(n)

      Read and return up to *n* bytes with only one call on the raw
      stream.  If at least one byte is buffered, only buffered bytes
      are returned. Otherwise, one raw stream read call is made.

class io.BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffer providing higher-level access to a writeable, sequential
   "RawIOBase" object.  It inherits "BufferedIOBase". When writing to
   this object, data is normally held into an internal buffer.  The
   buffer will be written out to the underlying "RawIOBase" object
   under various conditions, including:

   * when the buffer gets too small for all pending data;

   * when "flush()" is called;

   * when a "seek()" is requested (for "BufferedRandom" objects);

   * when the "BufferedWriter" object is closed or destroyed.

   The constructor creates a "BufferedWriter" for the given writeable
   *raw* stream.  If the *buffer_size* is not given, it defaults to
   "DEFAULT_BUFFER_SIZE".

   A third argument, *max_buffer_size*, is supported, but unused and
   deprecated.

   "BufferedWriter" provides or overrides these methods in addition to
   those from "BufferedIOBase" and "IOBase":

   flush()

      Force bytes held in the buffer into the raw stream.  A
      "BlockingIOError" should be raised if the raw stream blocks.

   write(b)

      Write *b*, and return the number of bytes written. The object
      *b* should be an array of bytes, either "bytes", "bytearray", or
      "memoryview". When in non-blocking mode, a "BlockingIOError" is
      raised if the buffer needs to be written out but the raw stream
      blocks.

class io.BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffered interface to random access streams.  It inherits
   "BufferedReader" and "BufferedWriter", and further supports
   "seek()" and "tell()" functionality.

   The constructor creates a reader and writer for a seekable raw
   stream, given in the first argument.  If the *buffer_size* is
   omitted it defaults to "DEFAULT_BUFFER_SIZE".

   A third argument, *max_buffer_size*, is supported, but unused and
   deprecated.

   "BufferedRandom" is capable of anything "BufferedReader" or
   "BufferedWriter" can do.

class io.BufferedRWPair(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffered I/O object combining two unidirectional "RawIOBase"
   objects – one readable, the other writeable – into a single
   bidirectional endpoint.  It inherits "BufferedIOBase".

   *reader* and *writer* are "RawIOBase" objects that are readable and
   writeable respectively.  If the *buffer_size* is omitted it
   defaults to "DEFAULT_BUFFER_SIZE".

   A fourth argument, *max_buffer_size*, is supported, but unused and
   deprecated.

   "BufferedRWPair" implements all of "BufferedIOBase"’s methods
   except for "detach()", which raises "UnsupportedOperation".

   警告: "BufferedRWPair" does not attempt to synchronize accesses
     to its underlying raw streams.  You should not pass it the same
     object as reader and writer; use "BufferedRandom" instead.


15.2.5. Text I/O
================

class io.TextIOBase

   Base class for text streams.  This class provides a unicode
   character and line based interface to stream I/O.  There is no
   "readinto()" method because Python’s "unicode" strings are
   immutable. It inherits "IOBase".  There is no public constructor.

   "TextIOBase" provides or overrides these data attributes and
   methods in addition to those from "IOBase":

   encoding

      The name of the encoding used to decode the stream’s bytes into
      strings, and to encode strings into bytes.

   errors

      The error setting of the decoder or encoder.

   newlines

      A string, a tuple of strings, or "None", indicating the newlines
      translated so far.  Depending on the implementation and the
      initial constructor flags, this may not be available.

   buffer

      The underlying binary buffer (a "BufferedIOBase" instance) that
      "TextIOBase" deals with.  This is not part of the "TextIOBase"
      API and may not exist on some implementations.

   detach()

      Separate the underlying binary buffer from the "TextIOBase" and
      return it.

      After the underlying buffer has been detached, the "TextIOBase"
      is in an unusable state.

      Some "TextIOBase" implementations, like "StringIO", may not have
      the concept of an underlying buffer and calling this method will
      raise "UnsupportedOperation".

      2.7 版新加入.

   read(n=-1)

      Read and return at most *n* characters from the stream as a
      single "unicode".  If *n* is negative or "None", reads until
      EOF.

   readline(limit=-1)

      Read until newline or EOF and return a single "unicode".  If the
      stream is already at EOF, an empty string is returned.

      If *limit* is specified, at most *limit* characters will be
      read.

   seek(offset, whence=SEEK_SET)

      Change the stream position to the given *offset*.  Behaviour
      depends on the *whence* parameter.  The default value for
      *whence* is "SEEK_SET".

      * "SEEK_SET" or "0": seek from the start of the stream (the
        default); *offset* must either be a number returned by
        "TextIOBase.tell()", or zero.  Any other *offset* value
        produces undefined behaviour.

      * "SEEK_CUR" or "1": 「seek」 to the current position;
        *offset* must be zero, which is a no-operation (all other
        values are unsupported).

      * "SEEK_END" or "2": seek to the end of the stream; *offset*
        must be zero (all other values are unsupported).

      Return the new absolute position as an opaque number.

      2.7 版新加入: The "SEEK_*" constants.

   tell()

      Return the current stream position as an opaque number.  The
      number does not usually represent a number of bytes in the
      underlying binary storage.

   write(s)

      Write the "unicode" string *s* to the stream and return the
      number of characters written.

class io.TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False)

   A buffered text stream over a "BufferedIOBase" binary stream. It
   inherits "TextIOBase".

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

   *errors* is an optional string that specifies how encoding and
   decoding errors are to be handled.  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.)  "'replace'" causes a replacement marker (such as "'?'") to
   be inserted where there is malformed data.  When writing,
   "'xmlcharrefreplace'" (replace with the appropriate XML character
   reference) or "'backslashreplace'" (replace with backslashed escape
   sequences) can be used.  Any other error handling name that has
   been registered with "codecs.register_error()" is also valid.

   *newline* controls how line endings are handled.  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 newlines 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 *line_buffering* is "True", "flush()" is implied when a call to
   write contains a newline character or a carriage return.

   "TextIOWrapper" provides one attribute in addition to those of
   "TextIOBase" and its parents:

   line_buffering

      Whether line buffering is enabled.

class io.StringIO(initial_value=u'', newline=u'\n')

   An in-memory stream for unicode text.  It inherits "TextIOWrapper".

   The initial value of the buffer can be set by providing
   *initial_value*. If newline translation is enabled, newlines will
   be encoded as if by "write()".  The stream is positioned at the
   start of the buffer.

   The *newline* argument works like that of "TextIOWrapper". The
   default is to consider only "\n" characters as ends of lines and to
   do no newline translation.  If *newline* is set to "None", newlines
   are written as "\n" on all platforms, but universal newline
   decoding is still performed when reading.

   "StringIO" provides this method in addition to those from
   "TextIOWrapper" and its parents:

   getvalue()

      Return a "unicode" containing the entire contents of the buffer
      at any time before the "StringIO" object’s "close()" method is
      called.  Newlines are decoded as if by "read()", although the
      stream position is not changed.

   Example usage:

      import io

      output = io.StringIO()
      output.write(u'First line.\n')
      output.write(u'Second line.\n')

      # Retrieve file contents -- this will be
      # u'First line.\nSecond line.\n'
      contents = output.getvalue()

      # Close object and discard memory buffer --
      # .getvalue() will now raise an exception.
      output.close()

class io.IncrementalNewlineDecoder

   A helper codec that decodes newlines for *universal newlines* mode.
   It inherits "codecs.IncrementalDecoder".


15.2.6. Advanced topics
=======================

Here we will discuss several advanced topics pertaining to the
concrete I/O implementations described above.


15.2.6.1. Performance
---------------------


15.2.6.1.1. Binary I/O
~~~~~~~~~~~~~~~~~~~~~~

By reading and writing only large chunks of data even when the user
asks for a single byte, buffered I/O is designed to hide any
inefficiency in calling and executing the operating system’s
unbuffered I/O routines.  The gain will vary very much depending on
the OS and the kind of I/O which is performed (for example, on some
contemporary OSes such as Linux, unbuffered disk I/O can be as fast as
buffered I/O).  The bottom line, however, is that buffered I/O will
offer you predictable performance regardless of the platform and the
backing device.  Therefore, it is most always preferable to use
buffered I/O rather than unbuffered I/O.


15.2.6.1.2. Text I/O
~~~~~~~~~~~~~~~~~~~~

Text I/O over a binary storage (such as a file) is significantly
slower than binary I/O over the same storage, because it implies
conversions from unicode to binary data using a character codec.  This
can become noticeable if you handle huge amounts of text data (for
example very large log files). Also, "TextIOWrapper.tell()" and
"TextIOWrapper.seek()" are both quite slow due to the reconstruction
algorithm used.

"StringIO", however, is a native in-memory unicode container and will
exhibit similar speed to "BytesIO".


15.2.6.2. Multi-threading
-------------------------

"FileIO" objects are thread-safe to the extent that the operating
system calls (such as "read(2)" under Unix) they are wrapping are
thread-safe too.

Binary buffered objects (instances of "BufferedReader",
"BufferedWriter", "BufferedRandom" and "BufferedRWPair") protect their
internal structures using a lock; it is therefore safe to call them
from multiple threads at once.

"TextIOWrapper" objects are not thread-safe.


15.2.6.3. Reentrancy
--------------------

Binary buffered objects (instances of "BufferedReader",
"BufferedWriter", "BufferedRandom" and "BufferedRWPair") are not
reentrant.  While reentrant calls will not happen in normal
situations, they can arise if you are doing I/O in a "signal" handler.
If it is attempted to enter a buffered object again while already
being accessed *from the same thread*, then a "RuntimeError" is
raised.

The above implicitly extends to text files, since the "open()"
function will wrap a buffered object inside a "TextIOWrapper".  This
includes standard streams and therefore affects the built-in function
"print()" as well.
