io --- 处理流的核心工具

源代码: Lib/io.py

---

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）。

所有流对提供给它们的数据类型都很敏感。例如将 str 对象给二进制流的 write() 方法会引发 TypeError。将 bytes 对象提供给文本流的 write() 方法也是如此。

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 。

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

二进制 I/O

二进制I/O（也称为缓冲I/O）预期 bytes-like objects 并生成 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的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。

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)

RawIOBase 的文档中详细描述了原始流的API

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整数。（如果可能） 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。

open 带有 path, mode, flags 参数的，将引发 审计事件

io.open_code(path)

以 'rb' 模式打开提供的文件。如果目的是将其做为可执行代码，则应使用此函数。

path 应当是绝对路径。

此函数的行为可能会被先前对 PyFile_SetOpenCodeHook() 的调用所覆盖，但是，应该始终认为它与 open(path, 'rb') 可相互替换。重写行为是为了对文件进行额外的验证或预处理。

3.8 新版功能.

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。

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 。

类的层次结构

I/O 流被安排为按类的层次结构实现。 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离。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。

The BufferedIOBase ABC extends IOBase. It deals with buffering on a raw binary stream (RawIOBase). Its subclasses, BufferedWriter, BufferedReader, and BufferedRWPair buffer raw binary streams that are readable, writable, and both readable and writable, respectively. BufferedRandom provides a buffered interface to seekable streams. Another BufferedIOBase subclass, BytesIO, is a stream of in-memory bytes.

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

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	抽象方法	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

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() 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. 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=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

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 streams. It inherits IOBase. There is no public constructor.

Raw binary streams typically provide low-level access to an underlying OS device or API, and do not try to encapsulate it in high-level primitives (this functionality is done at a higher-level in buffered binary streams and text streams, described later in this page).

RawIOBase provides these methods in addition to those from IOBase:

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. For example, b might be a bytearray. 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 data attributes and methods 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])

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.

If size is -1 (the default), an arbitrary number of bytes are returned (more than zero unless EOF is reached).

readinto(b)

Read bytes into a pre-allocated, writable bytes-like object b and return the number of bytes read. For example, b might be a bytearray.

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.

原始文件 I/O

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

A raw binary stream representing an OS-level file containing bytes data. It inherits RawIOBase.

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。

FileIO provides these data attributes in addition to those from RawIOBase and IOBase:

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。

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 binary stream 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([size])

在 BytesIO 中，这与 read() 相同。

在 3.7 版更改: size 参数现在是可选的。

readinto1(b)

在 BytesIO 中，这与 readinto() 相同。

3.5 新版功能.

class io.BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)

A buffered binary stream providing higher-level access to a readable, non seekable RawIOBase raw binary stream. 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.

在 3.7 版更改: size 参数现在是可选的。

class io.BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)

A buffered binary stream providing higher-level access to a writeable, non seekable RawIOBase raw binary stream. 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 binary stream providing higher-level access to a seekable RawIOBase raw binary stream. It inherits BufferedReader and BufferedWriter.

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. In addition, seek() and tell() are guaranteed to be implemented.

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

A buffered binary stream providing higher-level access to two non seekable RawIOBase raw binary streams---one readable, the other writeable. 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.

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. 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=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.

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 providing higher-level access to a BufferedIOBase buffered 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 newline is '', universal newlines mode is enabled, but line endings are returned to the caller untranslated. If newline 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 these data attributes and methods in addition to those from TextIOBase and IOBase:

line_buffering

是否启用行缓冲。

write_through

Whether writes are passed immediately to the underlying binary buffer.

3.7 新版功能.

reconfigure(*[, encoding][, errors][, newline][, line_buffering][, write_through])

Reconfigure this text stream using new settings for encoding, errors, newline, line_buffering and write_through.

Parameters not specified keep current settings, except errors='strict' is used when encoding is specified but errors is not specified.

It is not possible to change the encoding or newline if some data has already been read from the stream. On the other hand, changing encoding after write is possible.

This method does an implicit stream flush before setting the new parameters.

3.7 新版功能.

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

A text stream using an in-memory text buffer. It inherits TextIOBase.

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, except that when writing output to the stream, if newline is None, newlines are written as \n on all platforms.

StringIO provides this method in addition to those from TextIOBase and IOBase:

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.

性能

本节讨论所提供的具体 I/O 实现的性能。

二进制 I/O

即使在用户请求单个字节时，也只读取和写入大块数据。通过该方法，缓冲 I/O 隐藏了操作系统调用和执行无缓冲 I/O 例程时的任何低效性。增益取决于操作系统和执行的 I/O 类型。例如，在某些现代操作系统上（例如Linux），无缓冲磁盘 I/O 可以与缓冲 I/O 一样快。但最重要的是，无论平台和支持设备如何，缓冲 I/O 都能提供可预测的性能。因此，对于二进制数据，几乎总是首选使用缓冲的 I/O 而不是未缓冲的 I/O 。

文本 I/O

二进制存储（如文件）上的文本 I/O 比同一存储上的二进制 I/O 慢得多，因为它需要使用字符编解码器在Unicode和二进制数据之间进行转换。这在处理大量文本数据（如大型日志文件）时会变得非常明显。此外，由于使用的重构算法 TextIOWrapper.tell() 和 TextIOWrapper.seek() 都相当慢。

StringIO 是原生的内存 Unicode 容器，速度与 BytesIO 相似。

多线程

FileIO 对象是线程安全的，只要它们封装的操作系统调用（比如Unix下的 read(2) ）也是线程安全的。

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。

可重入性

二进制缓冲对象（ BufferedReader ， BufferedWriter ， BufferedRandom 和 BufferedRWPair 的实例）不是可重入的。虽然在正常情况下不会发生可重入调用，但是可能会在 signal 处理程序中执行 I/O 而产生。如果线程尝试重新输入已经访问的缓冲对象，则会引发 RuntimeError 。注意，这并不禁止其他线程进入缓冲对象。

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 print() function as well.