16.2. io — 用于处理流的核心工具 ¶

16.2.1.1. 文本 I/O ¶

Text I/O expects and produces str objects. This means that whenever the backing store is natively made of bytes (such as in the case of a file), encoding and decoding of data is made transparently as well as optional translation of platform-specific newline characters.

The easiest way to create a text stream is with open() , optionally specifying an encoding:

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

In-memory text streams are also available as StringIO 对象：

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

The text stream API is described in detail in the documentation of TextIOBase .

16.2.1.2. 二进制 I/O ¶

Binary I/O (also called buffered I/O ) expects 像字节对象 and produces bytes objects. No encoding, decoding, or newline translation is performed. This category of streams can be used for all kinds of non-text data, and also when manual control over the handling of text data is desired.

The easiest way to create a binary stream is with open() with 'b' in the mode string:

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

In-memory binary streams are also available as BytesIO 对象：

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

The binary stream API is described in detail in the docs of BufferedIOBase .

Other library modules may provide additional ways to create text or binary streams. See socket.socket.makefile() 例如。

16.2.1.3. 原生 I/O ¶

Raw I/O (also called unbuffered I/O ) is generally used as a low-level building-block for binary and text streams; it is rarely useful to directly manipulate a raw stream from user code. Nevertheless, you can create a raw stream by opening a file in binary mode with buffering disabled:

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

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

16.2.2.1. 内存中的流 ¶

It is also possible to use a str or 像字节对象 as a file for both reading and writing. For strings StringIO can be used like a file opened in text mode. BytesIO can be used like a file opened in binary mode. Both provide full read-write capabilities with random access.

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() ，或 write() because their signatures will vary, implementations and clients should consider those methods part of the interface. Also, implementations may raise a ValueError (或 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 像字节对象 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 在这种情况下。

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 语句。在此范例中， 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 提供这些数据属性和方法：

close ( ) ¶

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

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

closed ¶

True 若流被关闭。

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 ( ) ¶

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

readable ( ) ¶

返回 True 若流可以被读取。若 False , read() 会引发 OSError .

readline ( size=-1 ) ¶

从流读取并返回一行。若 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 自变量为 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 文件： ... 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 是：

• 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 版新增： SEEK_* constants.

3.3 版新增： 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 ( ) ¶

返回 True if the stream supports random access. If False , seek() , tell() and truncate() 会引发 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 版改变： Windows will now zero-fill files when extending.

writable ( ) ¶

返回 True if the stream supports writing. If False , write() and truncate() 会引发 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() 方法。

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 字节。

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 被返回。

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 像字节对象 b , and return the number of bytes read. If the object is in non-blocking mode and no bytes are available, None 被返回。

write ( b ) ¶

写入给定 像字节对象 , 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 ¶

底层原生流 ( 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 版新增。

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() (或 readinto() ) method. This can be useful if you are implementing your own buffering on top of a BufferedIOBase 对象。

readinto ( b ) ¶

Read bytes into a pre-allocated, writable 像字节对象 b and return the number of bytes read.

像 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 像字节对象 b , using at most one call to the underlying raw stream’s read() (或 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 版新增。

write ( b ) ¶

写入给定 像字节对象 , 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. 原生文件 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 can be one of two things:

• 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 被设为 False .

mode 可以是 '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.

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 ).

新近创建的文件 不可继承 .

见 open() built-in function for examples on using the opener 参数。

3.3 版改变： opener 参数被添加。 'x' 模式被添加。

3.4 版改变： 文件现在不可继承。

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

mode ¶

The mode as given in the constructor.

name ¶

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

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.

可选自变量 initial_bytes 是 像字节对象 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'
													

注意

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

3.2 版新增。

getvalue ( ) ¶

返回 bytes containing the entire contents of the buffer.

read1 ( ) ¶

在 BytesIO ，这如同 read() .

readinto1 ( ) ¶

在 BytesIO ，这如同 readinto() .

3.5 版新增。

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.

构造函数创建 BufferedReader for the given readable raw stream and buffer_size 。若 buffer_size is omitted, DEFAULT_BUFFER_SIZE 被使用。

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:

• when the buffer gets too small for all pending data;
• when flush() is called;
• when a seek() is requested (for BufferedRandom objects);
• when the BufferedWriter object is closed or destroyed.

构造函数创建 BufferedWriter for the given writeable raw stream. If the buffer_size 不给定，它默认为 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 像字节对象 , 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() 功能。

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 代替。

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 ¶

The error setting of the decoder or encoder.

newlines ¶

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

buffer ¶

The underlying binary buffer (a BufferedIOBase instance) that TextIOBase deals with. This is not part of the TextIOBase API and may not exist 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 版新增。

read ( size=-1 ) ¶

Read and return at most size characters from the stream as a single str 。若 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.

若 size is specified, at most size characters will be read.

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 版新增： SEEK_* constants.

tell ( ) ¶

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

write ( s ) ¶

Write the 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' ，和 '\r\n' . It works as follows:

• When reading input from the stream, if newline is None , 通用换行符 mode is enabled. Lines in the input can end in '\n' , '\r' ，或 '\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.
• When writing output to the stream, if newline is None , any '\n' characters written are translated to the system default line separator, os.linesep 。若 newline is '' or '\n' , no translation takes place. If newline is any of the other legal values, any '\n' characters written are translated to the given string.

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

若 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 版改变： write_through argument has been added.

3.3 版改变： 默认 encoding 现为 locale.getpreferredencoding(False) 而不是 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 ¶

行缓冲是否被启用。

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.

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 被设为 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 ( ) ¶

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

用法范例：

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 通用换行符 mode. It inherits codecs.IncrementalDecoder .

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.

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

TextIOWrapper 对象不是线程安全的。

16.2.4.4. 重入 ¶

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.