# 16.2. `io` — 处理流的核心工具

源代码: Lib/io.py

## 16.2.1. 總覽

`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.

3.3 版更變: 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。

也參考

`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。

> 備註

> 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 `truncate` | `close`, `closed`, `__enter__`, `__exit__`, `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`, `read1`, 和 `write` | 继承 `IOBase` 方法, `readinto`, 和 `readinto1`
`TextIOBase` | `IOBase` | `detach`, `read`, `readline`, 和 `write` | 继承 `IOBase` 方法, `encoding`, `errors`, 和 `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 版新加入: `SEEK_*` 常量.

    3.3 版新加入: 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.

  3.5 版更變: 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 版新加入.

  `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 版新加入.

  `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()` 。

  3.3 版更變: 增加了 opener 参数。增加了 `'x'` 模式。

  3.4 版更變: 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'

    備註

    As long as the view exists, the `BytesIO` object cannot be resized or closed.

    3.2 版新加入.

  `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 版新加入.

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 版新加入.

  `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 版新加入: `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.

  3.3 版更變: 已添加 write_through 参数

  3.3 版更變: 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.