python-clinic / Doc / library / io.rst

:mod:`io` --- Core tools for working with streams


The :mod:`io` module provides Python's main facilities for dealing with various types of I/O. There are three main types of I/O: text I/O, binary I/O and raw I/O. These are generic categories, and various backing stores can be used for each of them. A concrete object belonging to any of these categories is called a :term:`file object`. Other common terms are stream and file-like object.

Independently of its category, each concrete stream object will also have various capabilities: it can be read-only, write-only, or read-write. It can also allow arbitrary random access (seeking forwards or backwards to any location), or only sequential access (for example in the case of a socket or pipe).

All streams are careful about the type of data you give to them. For example giving a :class:`str` object to the write() method of a binary stream will raise a TypeError. So will giving a :class:`bytes` object to the write() method of a text stream.

Text I/O

Text I/O expects and produces :class:`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 :meth:`open()`, optionally specifying an encoding:

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

In-memory text streams are also available as :class:`StringIO` objects:

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

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

Binary I/O

Binary I/O (also called buffered I/O) expects and produces :class:`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 :meth:`open()` with 'b' in the mode string:

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

In-memory binary streams are also available as :class:`BytesIO` objects:

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

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

Other library modules may provide additional ways to create text or binary streams. See :meth:`socket.socket.makefile` for example.

Raw 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 :class:`RawIOBase`.

High-level Module Interface

In-memory streams

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

Class hierarchy

The implementation of I/O streams is organized as a hierarchy of classes. First :term:`abstract base classes <abstract base class>` (ABCs), which are used to specify the various categories of streams, then concrete classes providing the standard stream implementations.


The abstract base classes also provide default implementations of some methods in order to help implementation of concrete stream classes. For example, :class:`BufferedIOBase` provides unoptimized implementations of readinto() and readline().

At the top of the I/O hierarchy is the abstract base class :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 raise :exc:`UnsupportedOperation` if they do not support a given operation.

The :class:`RawIOBase` ABC extends :class:`IOBase`. It deals with the reading and writing of bytes to a stream. :class:`FileIO` subclasses :class:`RawIOBase` to provide an interface to files in the machine's file system.

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

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

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

The following table summarizes the ABCs provided by the :mod:`io` module:

ABC Inherits Stub Methods Mixin Methods and Properties
:class:`IOBase`   fileno, seek, and truncate close, closed, __enter__, __exit__, flush, isatty, __iter__, __next__, readable, readline, readlines, seekable, tell, writable, and writelines
:class:`RawIOBase` :class:`IOBase` readinto and write Inherited :class:`IOBase` methods, read, and readall
:class:`BufferedIOBase` :class:`IOBase` detach, read, read1, and write Inherited :class:`IOBase` methods, readinto
:class:`TextIOBase` :class:`IOBase` detach, read, readline, and write Inherited :class:`IOBase` methods, encoding, errors, and newlines

I/O Base Classes

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 :class:`IOBase` does not declare :meth:`read`, :meth:`readinto`, or :meth:`write` because their signatures will vary, implementations and clients should consider those methods part of the interface. Also, implementations may raise a :exc:`ValueError` (or :exc:`UnsupportedOperation`) when operations they do not support are called.

The basic type used for binary data read from or written to a file is :class:`bytes`. :class:`bytearray`s are accepted too, and in some cases (such as :class:`readinto`) required. Text I/O classes work with :class:`str` data.

Note that calling any method (even inquiries) on a closed stream is undefined. Implementations may raise :exc:`ValueError` in this case.

:class:`IOBase` (and its subclasses) supports the iterator protocol, meaning that an :class:`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 :meth:`~IOBase.readline` below.

:class:`IOBase` is also a context manager and therefore supports the :keyword:`with` statement. In this example, file is closed after the :keyword:`with` statement's suite is finished---even if an exception occurs:

with open('spam.txt', 'w') as file:
    file.write('Spam and eggs!')

:class:`IOBase` provides these data attributes and methods:

Base class for raw binary I/O. It inherits :class:`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 :class:`IOBase`, :class:`RawIOBase` provides the following methods:

Base class for binary streams that support some kind of buffering. It inherits :class:`IOBase`. There is no public constructor.

The main difference with :class:`RawIOBase` is that methods :meth:`read`, :meth:`readinto` and :meth:`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 :exc:`BlockingIOError` if the underlying raw stream is in non-blocking mode and cannot take or give enough data; unlike their :class:`RawIOBase` counterparts, they will never return None.

Besides, the :meth:`read` method does not have a default implementation that defers to :meth:`readinto`.

A typical :class:`BufferedIOBase` implementation should not inherit from a :class:`RawIOBase` implementation, but wrap one, like :class:`BufferedWriter` and :class:`BufferedReader` do.

:class:`BufferedIOBase` provides or overrides these methods and attribute in addition to those from :class:`IOBase`:

Raw File I/O

:class:`FileIO` represents an OS-level file containing bytes data. It implements the :class:`RawIOBase` interface (and therefore the :class:`IOBase` interface, too).

The name can be one of two things:

  • a character string or :class:`bytes` object representing the path to the file which will be opened;
  • an integer representing the number of an existing OS-level file descriptor to which the resulting :class:`FileIO` object will give access.

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. :exc:`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 :meth:`read` (when called with a positive argument), :meth:`readinto` and :meth:`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 :mod:`` as opener results in functionality similar to passing None).

See the :func:`open` built-in function for examples on using the opener parameter.

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

Buffered Streams

Buffered I/O streams provide a higher-level interface to an I/O device than raw I/O does.

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

The argument initial_bytes contains optional initial :class:`bytes` data.

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

A buffer providing higher-level access to a readable, sequential :class:`RawIOBase` object. It inherits :class:`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 :class:`BufferedReader` for the given readable raw stream and buffer_size. If buffer_size is omitted, :data:`DEFAULT_BUFFER_SIZE` is used.

:class:`BufferedReader` provides or overrides these methods in addition to those from :class:`BufferedIOBase` and :class:`IOBase`:

A buffer providing higher-level access to a writeable, sequential :class:`RawIOBase` object. It inherits :class:`BufferedIOBase`. When writing to this object, data is normally placed into an internal buffer. The buffer will be written out to the underlying :class:`RawIOBase` object under various conditions, including:

The constructor creates a :class:`BufferedWriter` for the given writeable raw stream. If the buffer_size is not given, it defaults to :data:`DEFAULT_BUFFER_SIZE`.

:class:`BufferedWriter` provides or overrides these methods in addition to those from :class:`BufferedIOBase` and :class:`IOBase`:

A buffered interface to random access streams. It inherits :class:`BufferedReader` and :class:`BufferedWriter`, and further supports :meth:`seek` and :meth:`tell` functionality.

The constructor creates a reader and writer for a seekable raw stream, given in the first argument. If the buffer_size is omitted it defaults to :data:`DEFAULT_BUFFER_SIZE`.

:class:`BufferedRandom` is capable of anything :class:`BufferedReader` or :class:`BufferedWriter` can do.

A buffered I/O object combining two unidirectional :class:`RawIOBase` objects -- one readable, the other writeable -- into a single bidirectional endpoint. It inherits :class:`BufferedIOBase`.

reader and writer are :class:`RawIOBase` objects that are readable and writeable respectively. If the buffer_size is omitted it defaults to :data:`DEFAULT_BUFFER_SIZE`.

:class:`BufferedRWPair` implements all of :class:`BufferedIOBase`'s methods except for :meth:`~BufferedIOBase.detach`, which raises :exc:`UnsupportedOperation`.


:class:`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 :class:`BufferedRandom` instead.

Text I/O

Base class for text streams. This class provides a character and line based interface to stream I/O. There is no :meth:`readinto` method because Python's character strings are immutable. It inherits :class:`IOBase`. There is no public constructor.

:class:`TextIOBase` provides or overrides these data attributes and methods in addition to those from :class:`IOBase`:

An in-memory stream for text I/O.

The initial value of the buffer (an empty string by default) can be set by providing initial_value. The newline argument works like that of :class:`TextIOWrapper`. The default is to do no newline translation.

:class:`StringIO` provides this method in addition to those from :class:`TextIOBase` and its parents:

Example usage:

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.

A helper codec that decodes newlines for :term:`universal newlines` mode. It inherits :class:`codecs.IncrementalDecoder`.


This section discusses the performance of the provided concrete I/O implementations.

Binary 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.

Text 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, :meth:`TextIOWrapper.tell` and :meth:`` are both quite slow due to the reconstruction algorithm used.

:class:`StringIO`, however, is a native in-memory unicode container and will exhibit similar speed to :class:`BytesIO`.


:class:`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 :class:`BufferedReader`, :class:`BufferedWriter`, :class:`BufferedRandom` and :class:`BufferedRWPair`) protect their internal structures using a lock; it is therefore safe to call them from multiple threads at once.

:class:`TextIOWrapper` objects are not thread-safe.


Binary buffered objects (instances of :class:`BufferedReader`, :class:`BufferedWriter`, :class:`BufferedRandom` and :class:`BufferedRWPair`) are not reentrant. While reentrant calls will not happen in normal situations, they can arise from doing I/O in a :mod:`signal` handler. If a thread tries to re-enter a buffered object which it is already accessing, a :exc:`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 :func:`open()` function will wrap a buffered object inside a :class:`TextIOWrapper`. This includes standard streams and therefore affects the built-in function :func:`print()` as well.