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.opencode(_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。

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。其一个子类 TextIOWrapper 是原始缓冲流（ BufferedIOBase ）的缓冲文本接口。另一个子类 StringIO 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：

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 methodsthat derived classes can override selectively; the defaultimplementations represent a file that cannot be read, written orseeked.

Even though IOBase does not declare read()or write() because their signatures will vary, implementations andclients 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 isbytes. Other bytes-like objects areaccepted as method arguments too. Text I/O classes work with str data.

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

IOBase (and its subclasses) supports the iterator protocol, meaningthat an IOBase object can be iterated over yielding the lines in astream. Lines are defined slightly differently depending on whether thestream is a binary stream (yielding bytes), or a text stream (yieldingcharacter strings). See readline() below.

IOBase is also a context manager and therefore supports thewith statement. In this example, file is closed after thewith 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 isalready 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 itexists. An OSError is raised if the IO object does not use a filedescriptor.

• flush()

• Flush the write buffers of the stream if applicable. This does nothingfor read-only and non-blocking streams.

• isatty()

• Return True if the stream is interactive (i.e., connected toa 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, atmost 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 lineterminator(s) recognized.

• readlines(hint=-1)
• Read and return a list of lines from the stream. hint can be specifiedto control the number of lines read: no more lines will be read if thetotal size (in bytes/characters) of all lines so far exceeds hint.

Note that it's already possible to iterate on file objects using forline in file: … without calling file.readlines().

• seek(offset, whence=SEEK_SET)

• Change the stream position to the given byte offset. offset isinterpreted relative to the position indicated by whence. The defaultvalue for whence is SEEKSET. Values for _whence are:

  • SEEKSET or 0 — start of the stream (the default);_offset should be zero or positive

  • SEEKCUR or 1 — current stream position; _offset maybe negative

  • SEEKEND or 2 — end of the stream; _offset is usuallynegative

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, likeos.SEEK_HOLE or os.SEEK_DATA. The valid valuesfor 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 positionif size is not specified). The current stream position isn't changed.This resizing can extend or reduce the current file size. In case ofextension, the contents of the new file area depend on the platform(on most systems, additional bytes are zero-filled). The new file sizeis 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 itis usual for each of the lines provided to have a line separator at theend.

• del()

• Prepare for object destruction. IOBase provides a defaultimplementation of this method that calls the instance'sclose() method.

• class io.RawIOBase
• Base class for raw binary I/O. It inherits IOBase. There is nopublic constructor.

Raw binary I/O typically provides low-level access to an underlying OSdevice 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 maybe 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() andreadinto().

• readall()

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

• readinto(b)

• Read bytes into a pre-allocated, writablebytes-like objectb, and return thenumber of bytes read. For example, b might be a bytearray.If the object is in non-blocking mode and no bytesare available, None is returned.

• write(b)

• Write the given bytes-like object, b, to theunderlying raw stream, and return the number ofbytes written. This can be less than the length of b inbytes, depending on specifics of the underlying rawstream, and especially if it is in non-blocking mode. None isreturned if the raw stream is set not to block and no single byte couldbe readily written to it. The caller may release or mutate b afterthis 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 muchinput as requested or to consume all given output, at the expense ofmaking perhaps more than one system call.

In addition, those methods can raise BlockingIOError if theunderlying raw stream is in non-blocking mode and cannot take or giveenough data; unlike their RawIOBase counterparts, they willnever return None.

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

A typical BufferedIOBase implementation should not inherit from aRawIOBase implementation, but wrap one, likeBufferedWriter and BufferedReader do.

BufferedIOBase provides or overrides these methods and attribute inaddition to those from IOBase:

• raw

• The underlying raw stream (a RawIOBase instance) thatBufferedIOBase deals with. This is not part of theBufferedIOBase 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 unusablestate.

Some buffers, like BytesIO, do not have the concept of a singleraw stream to return from this method. They raiseUnsupportedOperation.

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 emptybytes object is returned if the stream is already at EOF.

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

A BlockingIOError is raised if the underlying raw stream is innon 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 theunderlying raw stream's read() (orreadinto()) method. This can be useful if you areimplementing your own buffering on top of a BufferedIOBaseobject.

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

• readinto(b)
• Read bytes into a pre-allocated, writablebytes-like objectb and return the number of bytes read.For example, b might be a bytearray.

Like read(), multiple reads may be issued to the underlying rawstream, unless the latter is interactive.

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

• readinto1(b)
• Read bytes into a pre-allocated, writablebytes-like objectb, using at most one call tothe underlying raw stream's read() (orreadinto()) method. Return the number of bytes read.

A BlockingIOError is raised if the underlying raw stream is in nonblocking-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 numberof bytes written (always equal to the length of b in bytes, since ifthe write fails an OSError will be raised). Depending on theactual implementation, these bytes may be readily written to theunderlying stream, or held in a buffer for performance and latencyreasons.

When in non-blocking mode, a BlockingIOError is raised if thedata needed to be written to the raw stream but it couldn't acceptall 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)
• FileIO represents an OS-level file containing bytes data.It implements the RawIOBase interface (and therefore theIOBase interface, too).

name 可以是以下两项之一：

• a character string or bytes object representing the path to thefile 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 descriptorto which the resulting FileIO object will give access. When theFileIO 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 becreated if it doesn't exist when opened for writing or appending; it will betruncated when opened for writing. FileExistsError will be raised ifit already exists when opened for creating. Opening a file for creatingimplies 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 underlyingfile descriptor for the file object is then obtained by calling opener with(name, flags). opener must return an open file descriptor (passingos.open as opener results in functionality similar to passingNone).

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 andRawIOBase, FileIO provides the following dataattributes:

• 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 devicethan raw I/O does.

• class io.BytesIO([initial_bytes])
• A stream implementation using an in-memory bytes buffer. It inheritsBufferedIOBase. The buffer is discarded when theclose() method is called.

The optional argument initial_bytes is a bytes-like object thatcontains initial data.

BytesIO provides or overrides these methods in addition to thosefrom BufferedIOBase and IOBase:

• getbuffer()
• Return a readable and writable view over the contents of the bufferwithout copying them. Also, mutating the view will transparentlyupdate 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 beresized 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 buffer providing higher-level access to a readable, sequentialRawIOBase object. It inherits BufferedIOBase.When reading data from this object, a larger amount of data may berequested 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 readableraw stream and buffer_size. If buffer_size is omitted,DEFAULT_BUFFER_SIZE is used.

BufferedReader provides or overrides these methods in addition tothose from BufferedIOBase and IOBase:

• peek([size])

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

• read([size])

• Read and return size bytes, or if size is not given or negative, untilEOF 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 buffer providing higher-level access to a writeable, sequentialRawIOBase object. It inherits BufferedIOBase.When writing to this object, data is normally placed into an internalbuffer. The buffer will be written out to the underlying RawIOBaseobject 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 writeableraw stream. If the buffer_size is not given, it defaults toDEFAULT_BUFFER_SIZE.

BufferedWriter provides or overrides these methods in addition tothose from BufferedIOBase and IOBase:

• flush()

• Force bytes held in the buffer into the raw stream. ABlockingIOError should be raised if the raw stream blocks.

• write(b)

• Write the bytes-like object, b, and return thenumber of bytes written. When in non-blocking mode, aBlockingIOError is raised if the buffer needs to be written out butthe raw stream blocks.

• class io.BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)
• A buffered interface to random access streams. It inheritsBufferedReader and BufferedWriter.

The constructor creates a reader and writer for a seekable raw stream, givenin the first argument. If the buffer_size is omitted it defaults toDEFAULT_BUFFER_SIZE.

BufferedRandom is capable of anything BufferedReader orBufferedWriter can do. In addition, seek() and tell()are guaranteed to be implemented.

• class io.BufferedRWPair(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)
• A buffered I/O object combining two unidirectional RawIOBaseobjects — one readable, the other writeable — into a single bidirectionalendpoint. It inherits BufferedIOBase.

reader and writer are RawIOBase objects that are readable andwriteable respectively. If the buffer_size is omitted it defaults toDEFAULT_BUFFER_SIZE.

BufferedRWPair implements all of BufferedIOBase's methodsexcept for detach(), which raisesUnsupportedOperation.

警告

BufferedRWPair does not attempt to synchronize accesses toits underlying raw streams. You should not pass it the same objectas 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 basedinterface to stream I/O. It inherits IOBase.There is no public constructor.

TextIOBase provides or overrides these data attributes andmethods in addition to those from IOBase:

• encoding

• The name of the encoding used to decode the stream's bytes intostrings, 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 newlinestranslated so far. Depending on the implementation and the initialconstructor flags, this may not be available.

• buffer

• The underlying binary buffer (a BufferedIOBase instance) thatTextIOBase deals with. This is not part of theTextIOBase API and may not exist in some implementations.

• detach()

• Separate the underlying binary buffer from the TextIOBase andreturn it.

After the underlying buffer has been detached, the TextIOBase isin an unusable state.

Some TextIOBase implementations, like StringIO, may nothave the concept of an underlying buffer and calling this method willraise UnsupportedOperation.

3.1 新版功能.

• read(size=-1)

• Read and return at most size characters from the stream as a singlestr. 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 isalready 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 onthe whence parameter. The default value for whence isSEEK_SET.

  • SEEKSET or 0: seek from the start of the stream(the default); _offset must either be a number returned byTextIOBase.tell(), or zero. Any other offset valueproduces undefined behaviour.

  • SEEKCUR or 1: "seek" to the current position;_offset must be zero, which is a no-operation (all other valuesare unsupported).

  • SEEKEND 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 numberdoes not usually represent a number of bytes in the underlyingbinary storage.

• write(s)

• Write the string s to the stream and return the number of characterswritten.

• 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 orencoded with. It defaults tolocale.getpreferredencoding(False).

errors is an optional string that specifies how encoding and decodingerrors are to be handled. Pass 'strict' to raise a ValueErrorexception if there is an encoding error (the default of None has the sameeffect), or pass 'ignore' to ignore errors. (Note that ignoring encodingerrors 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 abackslashed escape sequence. When writing, 'xmlcharrefreplace'(replace with the appropriate XML character reference) or 'namereplace'(replace with \N{…} escape sequences) can be used. Any other errorhandling name that has been registered withcodecs.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 newlinesmode is enabled, but line endings are returned to the caller untranslated.If it has any of the other legal values, input lines are only terminatedby the given string, and the line ending is returned to the calleruntranslated.

• 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 towrite contains a newline character or a carriage return.

If write_through is True, calls to write() are guaranteednot to be buffered: any data written on the TextIOWrapperobject 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 thelocale encoding using locale.setlocale(), use the current localeencoding instead of the user preferred encoding.

TextIOWrapper provides these members in addition to those ofTextIOBase and its parents:

• line_buffering

• 是否启用行缓冲。

• write_through

• Whether writes are passed immediately to the underlying binarybuffer.

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, excepterrors='strict' is used when encoding is specified buterrors is not specified.

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

This method does an implicit stream flush before setting thenew parameters.

3.7 新版功能.

• class io.StringIO(initial_value='', newline='\n')
• An in-memory stream for text I/O. The text buffer is discarded when theclose() 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 bywrite(). The stream is positioned at the start ofthe buffer.

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

StringIO provides this method in addition to those fromTextIOBase and its parents:

• getvalue()
• Return a str containing the entire contents of the buffer.Newlines are decoded as if by read(), althoughthe 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

By reading and writing only large chunks of data even when the user asks for asingle byte, buffered I/O hides any inefficiency in calling and executing theoperating system's unbuffered I/O routines. The gain depends on the OS and thekind 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 platformand the backing device. Therefore, it is almost always preferable to usebuffered I/O rather than unbuffered I/O for binary data.

文本 I/O

Text I/O over a binary storage (such as a file) is significantly slower thanbinary I/O over the same storage, because it requires conversions betweenunicode and binary data using a character codec. This can become noticeablehandling huge amounts of text data like large log files. Also,TextIOWrapper.tell() and TextIOWrapper.seek() are both quite slowdue to the reconstruction algorithm used.

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

多线程

FileIO objects are thread-safe to the extent that the operating systemcalls (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。

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 tore-enter a buffered object which it is already accessing, a RuntimeErroris raised. Note this doesn't prohibit a different thread from entering thebuffered object.

The above implicitly extends to text files, since the open() functionwill wrap a buffered object inside a TextIOWrapper. This includesstandard streams and therefore affects the built-in function print() aswell.