15.2. io — Core tools for working with streams

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The io module provides the Python interfaces to stream handling. The built-in open() function is defined in this module.

At the top of the I/O hierarchy is the abstract base class IOBase. It defines the basic interface to a stream. Note, however, that there is no separation between reading and writing to streams; implementations are allowed to throw an IOError if they do not support a given operation.

Extending IOBase is RawIOBase which deals simply with the reading and writing of raw bytes to a stream. FileIO subclasses RawIOBase to provide an interface to files in the machine’s file system.

BufferedIOBase deals with buffering on a raw byte stream (RawIOBase). Its subclasses, BufferedWriter, BufferedReader, and BufferedRWPair buffer streams that are readable, writable, and both readable and writable. BufferedRandom provides a buffered interface to random access streams. BytesIO is a simple stream of in-memory bytes.

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

Argument names are not part of the specification, and only the arguments of open() are intended to be used as keyword arguments.

15.2.1. Module Interface

io.DEFAULT_BUFFER_SIZE

An int containing the default buffer size used by the module’s buffered I/O classes. open() uses the file’s blksize (as obtained by os.stat()) if possible.

io.open(file[, mode[, buffering[, encoding[, errors[, newline[, closefd=True]]]]]])

Open file and return a stream. If the file cannot be opened, an IOError is raised.

file is either a string giving the name (and the path if the file isn’t in the current working directory) of the file to be opened or a file descriptor of the file to be opened. (If a file descriptor is given, for example, from os.fdopen(), it is closed when the returned I/O object is closed, unless closefd is set to False.)

mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode. Other common values are 'w' for writing (truncating the file if it already exists), and 'a' for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are:

Character	Meaning
'r'	open for reading (default)
'w'	open for writing, truncating the file first
'a'	open for writing, appending to the end of the file if it exists
'b'	binary mode
't'	text mode (default)
'+'	open a disk file for updating (reading and writing)
'U'	universal newline mode (for backwards compatibility; should not be used in new code)

The default mode is 'rt' (open for reading text). For binary random access, the mode 'w+b' opens and truncates the file to 0 bytes, while 'r+b' opens the file without truncation.

Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn’t. Files opened in binary mode (including 'b' in the mode argument) return contents as bytes objects without any decoding. In text mode (the default, or when 't' is included in the mode argument), the contents of the file are returned as strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given.

buffering is an optional integer used to set the buffering policy. Pass 0 to switch buffering off (only allowed in binary mode), 1 to select line buffering (only usable in text mode), and an integer > 1 to indicate the size of a fixed-size chunk buffer. When no buffering argument is given, the default buffering policy works as follows:

• Binary files are buffered in fixed-size chunks; the size of the buffer is chosen using a heuristic trying to determine the underlying device’s 「block size」 and falling back on DEFAULT_BUFFER_SIZE. On many systems, the buffer will typically be 4096 or 8192 bytes long.
• 「Interactive」 text files (files for which isatty() returns True) use line buffering. Other text files use the policy described above for binary files.

encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent, but any encoding supported by Python can be used. See the codecs module for the list of supported encodings.

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. When writing, 'xmlcharrefreplace' (replace with the appropriate XML character reference) or 'backslashreplace' (replace with backslashed escape sequences) can be used. Any other error handling name that has been registered with codecs.register_error() is also valid.

newline controls how universal newlines works (it only applies to text mode). It can be None, '', '\n', '\r', and '\r\n'. It works as follows:

• On input, 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 newline 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.
• On output, if newline is None, any '\n' characters written are translated to the system default line separator, os.linesep. If newline is '', no translation takes place. If newline is any of the other legal values, any '\n' characters written are translated to the given string.

If closefd is False and a file descriptor rather than a filename was given, the underlying file descriptor will be kept open when the file is closed. If a filename is given closefd has no effect but must be True (the default).

The type of file object returned by the open() function depends on the mode. When open() is used to open a file in a text mode ('w', 'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open a file in a binary mode, the returned class varies: in read binary mode, it returns a BufferedReader; in write binary and append binary modes, it returns a BufferedWriter, and in read/write mode, it returns a BufferedRandom.

It is also possible to use a string or bytearray as a file for both reading and writing. For strings StringIO can be used like a file opened in a text mode, and for bytearrays a BytesIO can be used like a file opened in a binary mode.

exception io.BlockingIOError

Error raised when blocking would occur on a non-blocking stream. It inherits IOError.

In addition to those of IOError, BlockingIOError has one attribute:

characters_written

An integer containing the number of characters written to the stream before it blocked.

exception io.UnsupportedOperation

An exception inheriting IOError and ValueError that is raised when an unsupported operation is called on a stream.

15.2.2. I/O Base Classes

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 IOError when operations they do not support are called.

The basic type used for binary data read from or written to a file is bytes. bytearrays are accepted too, and in some cases (such as readinto) 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 IOError in this case.

IOBase (and its subclasses) support the iterator protocol, meaning that an IOBase object can be iterated over yielding the lines in a stream.

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 an 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 if the stream is closed.

fileno()

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

readline([limit])

Read and return one line from the stream. If limit is specified, at most limit bytes will be read.

The line terminator is always b'\n' for binary files; for text files, the newlines argument to open() can be used to select the line terminator(s) recognized.

readlines([hint])

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.

seek(offset[, whence])

Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. Values for whence are:

• 0 – start of the stream (the default); offset should be zero or positive
• 1 – current stream position; offset may be negative
• 2 – end of the stream; offset is usually negative

Return the new absolute position.

seekable()

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

tell()

Return the current stream position.

truncate([size])

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, on Windows they’re undetermined). The new file size is returned.

writable()

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

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.

class io.RawIOBase

Base class for raw binary I/O. It inherits IOBase. There is no public constructor.

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

read([n])

Read and return all the bytes from the stream until EOF, or if n is specified, up to n bytes. Only one system call is ever made. An empty bytes object is returned on EOF; None is returned if the object is set not to block and has no data to read.

readall()

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

readinto(b)

Read up to len(b) bytes into bytearray b and return the number of bytes read.

write(b)

Write the given bytes or bytearray object, b, to the underlying raw stream and return the number of bytes written (This is never less than len(b), since if the write fails, an IOError will be raised).

class io.BufferedIOBase

Base class for streams that support buffering. It inherits IOBase. There is no public constructor.

The main difference with RawIOBase is that the read() method supports omitting the size argument, and does not have a default implementation that defers to readinto().

In addition, read(), readinto(), and write() may raise BlockingIOError if the underlying raw stream is in non-blocking mode and not ready; unlike their raw counterparts, they will never return None.

A typical implementation should not inherit from a RawIOBase implementation, but wrap one like BufferedWriter and BufferedReader.

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

read([n])

Read and return up to n 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 has no data at the moment.

readinto(b)

Read up to len(b) bytes into bytearray 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 has no data at the moment.

write(b)

Write the given bytes or bytearray object, b, to the underlying raw stream and return the number of bytes written (never less than len(b), since if the write fails an IOError will be raised).

A BlockingIOError is raised if the buffer is full, and the underlying raw stream cannot accept more data at the moment.

15.2.3. Raw File I/O

class io.FileIO(name[, mode])

FileIO represents a file containing bytes data. It implements the RawIOBase interface (and therefore the IOBase interface, too).

The mode can be 'r', 'w' or 'a' for reading (default), writing, 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. Add a '+' to the mode to allow simultaneous reading and writing.

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

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.

read([n])

Read and return at most n bytes. Only one system call is made, so it is possible that less data than was requested is returned. Use len() on the returned bytes object to see how many bytes were actually returned. (In non-blocking mode, None is returned when no data is available.)

readall()

Read and return the entire file’s contents in a single bytes object. As much as immediately available is returned in non-blocking mode. If the EOF has been reached, b'' is returned.

write(b)

Write the bytes or bytearray object, b, to the file, and return the number actually written. Only one system call is made, so it is possible that only some of the data is written.

Note that the inherited readinto() method should not be used on FileIO objects.

15.2.4. Buffered Streams

class io.BytesIO([initial_bytes])

A stream implementation using an in-memory bytes buffer. It inherits BufferedIOBase.

The argument initial_bytes is an optional initial bytearray.

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

getvalue()

Return bytes containing the entire contents of the buffer.

read1()

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

class io.BufferedReader(raw[, buffer_size])

A buffer for a readable, sequential RawIOBase object. It inherits BufferedIOBase.

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([n])

Return 1 (or n if specified) bytes from a buffer without advancing the position. Only a single read on the raw stream is done to satisfy the call. The number of bytes returned may be less than requested since at most all the buffer’s bytes from the current position to the end are returned.

read([n])

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

read1(n)

Read and return up to n 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[, max_buffer_size]])

A buffer for a writeable sequential RawIO object. It inherits BufferedIOBase.

The constructor creates a BufferedWriter for the given writeable raw stream. If the buffer_size is not given, it defaults to DEAFULT_BUFFER_SIZE. If max_buffer_size is omitted, it defaults to twice the 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 or bytearray object, b, onto the raw stream and return the number of bytes written. A BlockingIOError is raised when the raw stream blocks.

class io.BufferedRWPair(reader, writer[, buffer_size[, max_buffer_size]])

A combined buffered writer and reader object for a raw stream that can be written to and read from. It has and supports both read(), write(), and their variants. This is useful for sockets and two-way pipes. 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. The max_buffer_size (for the buffered writer) defaults to twice the buffer size.

BufferedRWPair implements all of BufferedIOBase’s methods.

class io.BufferedRandom(raw[, buffer_size[, max_buffer_size]])

A buffered interface to random access streams. 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. The max_buffer_size (for the buffered writer) defaults to twice the buffer size.

BufferedRandom is capable of anything BufferedReader or BufferedWriter can do.

15.2.5. Text 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.

newlines

A string, a tuple of strings, or None, indicating the newlines translated so far.

read(n)

Read and return at most n characters from the stream as a single str. If n is negative or None, reads to EOF.

readline()

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

write(s)

Write the string s to the stream and return the number of characters written.

class io.TextIOWrapper(buffer[, encoding[, errors[, newline[, line_buffering]]]])

A buffered text stream over a BufferedIOBase raw stream, buffer. It inherits TextIOBase.

encoding gives the name of the encoding that the stream will be decoded or encoded with. It defaults to locale.getpreferredencoding().

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. When writing, 'xmlcharrefreplace' (replace with the appropriate XML character reference) or 'backslashreplace' (replace with backslashed escape sequences) can be used. Any other error handling name that has been registered with codecs.register_error() is also valid.

newline can be None, '', '\n', '\r', or '\r\n'. It controls the handling of line endings. If it is None, universal newlines is enabled. With this enabled, on input, the lines endings '\n', '\r', or '\r\n' are translated to '\n' before being returned to the caller. Conversely, on output, '\n' is translated to the system default line separator, os.linesep. If newline is any other of its legal values, that newline becomes the newline when the file is read and it is returned untranslated. On output, '\n' is converted to the newline.

If line_buffering is True, flush() is implied when a call to write contains a newline character.

TextIOWrapper provides these data attributes in addition to those of TextIOBase and its parents:

errors

The encoding and decoding error setting.

line_buffering

Whether line buffering is enabled.

class io.StringIO([initial_value[, encoding[, errors[, newline]]]])

An in-memory stream for text. It inherits TextIOWrapper.

Create a new StringIO stream with an inital value, encoding, error handling, and newline setting. See TextIOWrapper’s constructor for more information.

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

getvalue()

Return a str containing the entire contents of the buffer.

class io.IncrementalNewlineDecoder

A helper codec that decodes newlines for universal newlines mode. It inherits codecs.IncrementalDecoder.