16.2. "io" --- 处理流的核心工具
*******************************

**源代码:** Lib/io.py

======================================================================


16.2.1. 概述
============

"io" 模块提供了 Python 用于处理各种 I/O 类型的主要工具。三种主要的 I/O
类型分别为: *文本 I/O*, *二进制 I/O* 和 *原始 I/O*。这些是泛型类型，有
很多种后端存储可以用在他们上面。一个隶属于任何这些类型的具体对象被称作
*file object*。 其他同类的术语还有 *流* 和 *类文件对象*。

独立于其类别，每个具体流对象也将具有各种功能：它可以是只读，只写或读写
。它还可以允许任意随机访问（向前或向后寻找任何位置），或仅允许顺序访问
（例如在套接字或管道的情况下）。

All streams are careful about the type of data you give to them.  For
example giving a "str" object to the "write()" method of a binary
stream will raise a "TypeError".  So will giving a "bytes" object to
the "write()" method of a text stream.

在 3.3 版更改: 由于 "IOError" 现在是 "OSError" 的别名，因此用于引发
"IOError" 的操作现在会引发 "OSError" 。


16.2.1.1. 文本 I/O
------------------

文本I/O预期并生成 "str" 对象。这意味着，无论何时后台存储是由字节组成的
（例如在文件的情况下），数据的编码和解码都是透明的，并且可以选择转换特
定于平台的换行符。

创建文本流的最简单方法是使用 "open()"，可以选择指定编码：

   f = open("myfile.txt", "r", encoding="utf-8")

内存中文本流也可以作为 "StringIO" 对象使用：

   f = io.StringIO("some initial text data")

"TextIOBase" 的文档中详细描述了文本流的API


16.2.1.2. 二进制 I/O
--------------------

二进制I/O（也称为缓冲I/O）预期 *bytes-like objects* 并产生  "bytes" 对
象。不执行编码、解码或换行转换。这种类型的流可以用于所有类型的非文本数
据，并且还可以在需要手动控制文本数据的处理时使用。

创建二进制流的最简单方法是使用 "open()"，并在模式字符串中指定 "'b'" ：

   f = open("myfile.jpg", "rb")

内存中二进制流也可以作为 "BytesIO" 对象使用：

   f = io.BytesIO(b"some initial binary data: \x00\x01")

"BufferedIOBase" 的文档中详细描述了二进制流 API。

其他库模块可以提供额外的方式来创建文本或二进制流。参见
"socket.socket.makefile()" 的示例。


16.2.1.3. 原始 I/O
------------------

原始 I/O（也称为 *非缓冲 I/O*）通常用作二进制和文本流的低级构建块。用
户代码直接操作原始流的用法非常罕见。不过，可以通过在禁用缓冲的情况下以
二进制模式打开文件来创建原始流：

   f = open("myfile.jpg", "rb", buffering=0)

The raw stream API is described in detail in the docs of "RawIOBase".


16.2.2. 高阶模块接口
====================

io.DEFAULT_BUFFER_SIZE

   包含模块缓冲 I/O 类使用的默认缓冲区大小的 int。 在可能的情况下
   "open()" 将使用文件的 blksize（由 "os.stat()" 获得）。

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

   这是内置的 "open()" 函数的别名。

exception io.BlockingIOError

   这是内置的 "BlockingIOError" 异常的兼容性别名。

exception io.UnsupportedOperation

   在流上调用不支持的操作时引发的继承 "OSError" 和 "ValueError" 的异常
   。


16.2.2.1. 内存中的流
--------------------

也可以使用 "str" 或 *bytes-like object* 作为文件进行读取和写入。对于字
符串， "StringIO" 可以像在文本模式下打开的文件一样使用。 "BytesIO" 可
以像以二进制模式打开的文件一样使用。两者都提供完整的随机读写功能。

参见:

  "sys"
     包含标准IO流: "sys.stdin", "sys.stdout" 和 "sys.stderr" 。


16.2.3. 类的层次结构
====================

I/O 流的实现被组织为类的层次结构。 首先是 *抽象基类* (ABC)，用于指定流
的各种类别，然后是提供标准流实现的具体类。

   注解: 抽象基类还提供某些方法的默认实现，以帮助实现具体的流类。例
     如 "BufferedIOBase" 提供了 "readinto()" 和 "readline()" 的未优化
     实现 。

I/O层次结构的顶部是抽象基类 "IOBase" 。它定义了流的基本接口。但是请注
意，对流的读取和写入之间没有分隔。如果实现不支持指定的操作，则会引发
"UnsupportedOperation" 。

"RawIOBase" ABC 是 "IOBase" 的子类。它负责将字节读取和写入流中。
"RawIOBase" 的子类 "FileIO" 提供计算机文件系统中文件的接口。

"BufferedIOBase" ABC处理原始字节流 ( "RawIOBase") 上的缓冲。其子类
"BufferedWriter"、"BufferedReader" 和 "BufferedRWPair" 缓冲流是可读、
可写以及可读写的。 "BufferedRandom" 为随机访问流提供缓冲接口。
"BufferedIOBase" 的另一个子类 "BytesIO" 是内存中字节流。

"TextIOBase" ABC是 "IOBase" 的另一个子类，它处理字节表示文本的流，并处
理字符串之间的编码和解码。 "TextIOWrapper" 是其一个扩展，它是原始缓冲
流（ "BufferedIOBase" ）的缓冲文本接口。最后， "StringIO" 是一个用于文
本的内存流。

参数名不是规范的一部分，只有 "open()" 的参数才用作关键字参数。

下表总结了抽象基类提供的 "io" 模块：

+---------------------------+--------------------+--------------------------+----------------------------------------------------+
| 抽象基类                  | 继承               | Stub 方法                | Mixin方法和属性                                    |
|===========================|====================|==========================|====================================================|
| "IOBase"                  |                    | "fileno", "seek", 和     | "close", "closed", "__enter__", "__exit__",        |
|                           |                    | "truncate"               | "flush", "isatty", "__iter__", "__next__",         |
|                           |                    |                          | "readable", "readline", "readlines", "seekable",   |
|                           |                    |                          | "tell", "writable", 和 "writelines"                |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+
| "RawIOBase"               | "IOBase"           | "readinto" 和 "write"    | 继承 "IOBase" 方法, "read", 和 "readall"           |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+
| "BufferedIOBase"          | "IOBase"           | "detach", "read",        | 继承 "IOBase" 方法, "readinto", 和 "readinto1"     |
|                           |                    | "read1", 和 "write"      |                                                    |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+
| "TextIOBase"              | "IOBase"           | "detach", "read",        | 继承 "IOBase" 方法, "encoding", "errors", 和       |
|                           |                    | "readline", 和 "write"   | "newlines"                                         |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+


16.2.3.1. I/O 基类
------------------

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 a "ValueError" (or
   "UnsupportedOperation") when operations they do not support are
   called.

   The basic type used for binary data read from or written to a file
   is "bytes".  Other *bytes-like objects* are accepted as method
   arguments too.  In some cases, such as "readinto()", a writable
   object such as "bytearray" is required.  Text I/O classes work with
   "str" data.

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

   "IOBase" (and its subclasses) supports 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 character 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 open('spam.txt', 'w') as file:
          file.write('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

      如果流关闭，则为True。

   fileno()

      Return the underlying file descriptor (an integer) of the stream
      if it exists.  An "OSError" 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 "OSError".

   readline(size=-1)

      Read and return one line from the stream.  If *size* is
      specified, at most *size* 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])

      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

      返回新的绝对位置。

      3.1 新版功能: "SEEK_*" 常量.

      3.3 新版功能: Some operating systems could support additional
      values, like "os.SEEK_HOLE" or "os.SEEK_DATA". The valid values
      for a file could depend on it being open in text or binary mode.

   seekable()

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

   tell()

      返回当前流的位置。

   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).  The new file size is
      returned.

   在 3.5 版更改: 现在Windows在扩展时将文件填充为零。

   writable()

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

   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(size=-1)

      Read up to *size* bytes from the object and return them.  As a
      convenience, if *size* is unspecified or -1, all bytes until EOF
      are returned. Otherwise, only one system call is ever made.
      Fewer than *size* bytes may be returned if the operating system
      call returns fewer than *size* bytes.

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

      The default implementation defers to "readall()" and
      "readinto()".

   readall()

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

   readinto(b)

      Read bytes into a pre-allocated, writable *bytes-like object*
      *b*, and return the number of bytes read.  If the object is in
      non-blocking mode and no bytes are available, "None" is
      returned.

   write(b)

      Write the given *bytes-like object*, *b*, to the underlying raw
      stream, and return the number of bytes written.  This can be
      less than the length of *b* in bytes, 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".

      3.1 新版功能.

   read(size=-1)

      Read and return up to *size* 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(size=-1)

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

   readinto(b)

      Read bytes into a pre-allocated, writable *bytes-like object*
      *b* and return the number of bytes read.

      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.

   readinto1(b)

      Read bytes into a pre-allocated, writable *bytes-like object*
      *b*, using at most one call to the underlying raw stream's
      "read()" (or "readinto()") method. Return the number of bytes
      read.

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

      3.5 新版功能.

   write(b)

      Write the given *bytes-like object*, *b*, and return the number
      of bytes written (always equal to the length of *b* in bytes,
      since if the write fails an "OSError" will be raised).
      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.


16.2.3.2. 原始文件 I/O
----------------------

class io.FileIO(name, mode='r', closefd=True, opener=None)

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

   *name* 可以是以下两项之一：

   * a character string or "bytes" object representing the path to
     the file which will be opened. In this case closefd must be
     "True" (the default) otherwise an error will be raised.

   * an integer representing the number of an existing OS-level file
     descriptor to which the resulting "FileIO" object will give
     access. When the FileIO object is closed this fd will be closed
     as well, unless *closefd* is set to "False".

   The *mode* can be "'r'", "'w'", "'x'" or "'a'" for reading
   (default), writing, exclusive creation 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.
   "FileExistsError" will be raised if it already exists when opened
   for creating. Opening a file for creating implies writing, so this
   mode behaves in a similar way to "'w'". 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.

   A custom opener can be used by passing a callable as *opener*. The
   underlying file descriptor for the file object is then obtained by
   calling *opener* with (*name*, *flags*). *opener* must return an
   open file descriptor (passing "os.open" as *opener* results in
   functionality similar to passing "None").

   新创建的文件是 不可继承的。

   有关 opener 参数的示例，请参见内置函数 "open()" 。

   在 3.3 版更改: 增加了 *opener* 参数。增加了 "'x'" 模式。

   在 3.4 版更改: 文件现在禁止继承。

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

   mode

      构造函数中给定的模式。

   name

      文件名。当构造函数中没有给定名称时，这是文件的文件描述符。


16.2.3.3. 缓冲流
----------------

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 buffer is discarded when the
   "close()" method is called.

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

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

   getbuffer()

      Return a readable and writable view over the contents of the
      buffer without copying them.  Also, mutating the view will
      transparently update the contents of the buffer:

         >>> b = io.BytesIO(b"abcdef")
         >>> view = b.getbuffer()
         >>> view[2:4] = b"56"
         >>> b.getvalue()
         b'ab56ef'

      注解: As long as the view exists, the "BytesIO" object cannot
        be resized or closed.

      3.2 新版功能.

   getvalue()

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

   read1()

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

   readinto1()

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

      3.5 新版功能.

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([size])

      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([size])

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

   read1(size)

      Read and return up to *size* 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 placed into an internal buffer.  The
   buffer will be written out to the underlying "RawIOBase" object
   under various conditions, including:

   * 当缓冲区对于所有挂起数据而言太小时；

   * 当 "flush()" 被调用时

   * 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".

   "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 the *bytes-like object*, *b*, and return the number of
      bytes written.  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".

   "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".

   "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.


16.2.3.4. 文本 I/O
------------------

class io.TextIOBase

   Base class for text streams.  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.  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

      解码器或编码器的错误设置。

   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 in 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".

      3.1 新版功能.

   read(size=-1)

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

   readline(size=-1)

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

      如果指定了 *size* ，最多将读取 *size* 个字符。

   seek(offset[, whence])

      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.

      3.1 新版功能: "SEEK_*" 常量.

   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 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, write_through=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(False)".

   *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. "'backslashreplace'"
   causes malformed data to be replaced by a backslashed escape
   sequence.  When writing, "'xmlcharrefreplace'" (replace with the
   appropriate XML character reference)  or "'namereplace'" (replace
   with "\N{...}" 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:

   * When reading input from the stream, 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.

   * 将输出写入流时，如果 *newline* 为 "None"，则写入的任何 "'\n'"
     字 符都将转换为系统默认行分隔符 "os.linesep"。如果 *newline* 是
     "''" 或 "'\n'"，则不进行翻译。如果 *newline* 是任何其他合法值，则
     写入 的任何 "'\n'" 字符将被转换为给定的字符串。

   If *line_buffering* is "True", "flush()" is implied when a call to
   write contains a newline character or a carriage return.

   If *write_through* is "True", calls to "write()" are guaranteed not
   to be buffered: any data written on the "TextIOWrapper" object is
   immediately handled to its underlying binary *buffer*.

   在 3.3 版更改: 已添加 *write_through* 参数

   在 3.3 版更改: The default *encoding* is now
   "locale.getpreferredencoding(False)" instead of
   "locale.getpreferredencoding()". Don't change temporary the locale
   encoding using "locale.setlocale()", use the current locale
   encoding instead of the user preferred encoding.

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

   line_buffering

      是否启用行缓冲。

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

   An in-memory stream for text I/O.  The text buffer is discarded
   when the "close()" method is called.

   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
   "TextIOBase" and its parents:

   getvalue()

      Return a "str" containing the entire contents of the buffer.
      Newlines are decoded as if by "read()", although the stream
      position is not changed.

   用法示例：

      import io

      output = io.StringIO()
      output.write('First line.\n')
      print('Second line.', file=output)

      # Retrieve file contents -- this will be
      # '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".


16.2.4. 性能
============

本节讨论所提供的具体 I/O 实现的性能。


16.2.4.1. 二进制 I/O
--------------------

By reading and writing only large chunks of data even when the user
asks for a single byte, buffered I/O hides any inefficiency in calling
and executing the operating system's unbuffered I/O routines.  The
gain depends on the OS and the kind of I/O which is performed.  For
example, on some modern 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 offers predictable performance regardless of the platform and the
backing device.  Therefore, it is almost always preferable to use
buffered I/O rather than unbuffered I/O for binary data.


16.2.4.2. 文本 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 requires
conversions between unicode and binary data using a character codec.
This can become noticeable handling huge amounts of text data like
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".


16.2.4.3. 多线程
----------------

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

二进制缓冲对象（例如 "BufferedReader", "BufferedWriter",
"BufferedRandom" 和 "BufferedRWPair"）使用锁来保护其内部结构；因此，可
以安全地一次从多个线程中调用它们。

"TextIOWrapper" 对象不再是线程安全的。


16.2.4.4. 可重入性
------------------

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 from doing I/O in a "signal" handler.  If a
thread tries to re-enter a buffered object which it is already
accessing, a "RuntimeError" is raised.  Note this doesn't prohibit a
different thread from entering the buffered object.

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.
