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

Overview

The :mod:`io` module provides Python's main facilities for dealing for various types of I/O. There are three main types of I/O: text I/O, binary I/O, raw I/O. These are generic categories, and various backing stores can be used for each of them. Concrete objects belonging to any of these categories will often be called streams; another common term is file-like objects.

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 for the :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.

Note

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.

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:`IOError` 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:`IOError` in this case.

IOBase (and its subclasses) support 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:`readline` below.

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`, 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 members 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 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' 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.

The :meth:`read` (when called with a positive argument), :meth:`readinto` and :meth:`write` methods on this class will only make one system call.

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

Buffered Streams

In many situations, buffered I/O streams will provide higher performance (bandwidth and latency) than raw I/O streams. Their API is also more usable.

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 held into an internal buffer. The buffer will be written out to the underlying :class:`RawIOBase` object under various conditions, including:

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

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`.

A third argument, max_buffer_size, is supported, but unused and deprecated.

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

A buffered I/O object giving a combined, higher-level access to two sequential :class:`RawIOBase` objects: one readable, the other writeable. It is useful for pairs of unidirectional communication channels (pipes, for instance). 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`.

A fourth argument, max_buffer_size, is supported, but unused and deprecated.

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

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`.

A third argument, max_buffer_size, is supported, but unused and deprecated.

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

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`:

A buffered text stream over a :class:`BufferedIOBase` binary stream. It inherits :class:`TextIOBase`.

encoding gives the name of the encoding that the stream will be decoded or encoded with. It defaults to :func:`locale.getpreferredencoding`.

errors is an optional string that specifies how encoding and decoding errors are to be handled. Pass 'strict' to raise a :exc:`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 :func:`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, :data:`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, :meth:`flush` is implied when a call to write contains a newline character.

:class:`TextIOWrapper` provides one attribute in addition to those of :class:`TextIOBase` and its parents:

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.
output.close()

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