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Issue #9715: improve documentation of the io module
This commit is contained in:
Antoine Pitrou
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.. moduleauthor:: Benjamin Peterson <benjamin@python.org>
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.. sectionauthor:: Benjamin Peterson <benjamin@python.org>
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The :mod:`io` module provides the Python interfaces to stream handling. The
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built-in :func:`open` function is defined in this module.
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Overview
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--------
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At the top of the I/O hierarchy is the abstract base class :class:`IOBase`. It
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defines the basic interface to a stream. Note, however, that there is no
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separation between reading and writing to streams; implementations are allowed
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to raise an :exc:`IOError` if they do not support a given operation.
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The :mod:`io` module provides Python 3's main facilities for dealing for
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various types of I/O. Three main types of I/O are defined: *text I/O*,
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*binary I/O*, *raw I/O*. It should be noted that these are generic categories,
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and various backing stores can be used for each of them. Concrete objects
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belonging to any of these categories will often be called *streams*; another
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common term is *file-like objects*.
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Extending :class:`IOBase` is :class:`RawIOBase` which deals simply with the
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reading and writing of raw bytes to a stream. :class:`FileIO` subclasses
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:class:`RawIOBase` to provide an interface to files in the machine's
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file system.
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Independently of its category, each concrete stream object will also have
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various capabilities: it can be read-only, write-only, or read-write; it
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can also allow arbitrary random access (seeking forwards or backwards to
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any location), or only sequential access (for example in the case of a
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socket or pipe).
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:class:`BufferedIOBase` deals with buffering on a raw byte stream
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(:class:`RawIOBase`). Its subclasses, :class:`BufferedWriter`,
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:class:`BufferedReader`, and :class:`BufferedRWPair` buffer streams that are
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readable, writable, and both readable and writable.
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:class:`BufferedRandom` provides a buffered interface to random access
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streams. :class:`BytesIO` is a simple stream of in-memory bytes.
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All streams are careful about the type of data you give to them. For example
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giving a :class:`str` object to the ``write()`` method of a binary stream
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will raise a ``TypeError``. So will giving a :class:`bytes` object to the
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``write()`` method of a text stream.
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Another :class:`IOBase` subclass, :class:`TextIOBase`, deals with
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streams whose bytes represent text, and handles encoding and decoding
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from and to strings. :class:`TextIOWrapper`, which extends it, is a
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buffered text interface to a buffered raw stream
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(:class:`BufferedIOBase`). Finally, :class:`StringIO` is an in-memory
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stream for text.
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Text I/O
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^^^^^^^^
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Argument names are not part of the specification, and only the arguments of
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:func:`.open` are intended to be used as keyword arguments.
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Text I/O expects and produces :class:`str` objects. This means that,
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whenever the backing store is natively made of bytes (such as in the case
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of a file), encoding and decoding of data is made transparently, as well as,
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optionally, translation of platform-specific newline characters.
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.. seealso::
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:mod:`sys`
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contains the standard IO streams: :data:`sys.stdin`, :data:`sys.stdout`,
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and :data:`sys.stderr`.
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A way to create a text stream is to :meth:`open()` a file in text mode,
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optionally specifying an encoding::
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f = open("myfile.txt", "r", encoding="utf-8")
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In-memory text streams are also available as :class:`StringIO` objects::
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f = io.StringIO("some initial text data")
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The detailed API of text streams is described by the :class:`TextIOBase`
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class.
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.. note::
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Text I/O over a binary storage (such as a file) is significantly
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slower than binary I/O over the same storage. This can become noticeable
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if you handle huge amounts of text data (for example very large log files).
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Binary I/O
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^^^^^^^^^^
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Binary I/O (also called *buffered I/O*) expects and produces
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:class:`bytes` objects. No encoding, decoding or character translation
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is performed. This is the category of streams used for all kinds of non-text
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data, and also when manual control over the handling of text data is desired.
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A way to create a binary stream is to :meth:`open()` a file in binary mode::
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f = open("myfile.jpg", "rb")
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In-memory binary streams are also available as :class:`BytesIO` objects::
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f = io.BytesIO(b"some initial binary data: \x00\x01")
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The detailed API of binary streams is described by the :class:`BufferedIOBase`
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class.
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Other library modules may provide additional ways to create text or binary
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streams. See for example :meth:`socket.socket.makefile`.
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Raw I/O
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^^^^^^^
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Raw I/O (also called *unbuffered I/O*) is generally used as a low-level
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building-block for binary and text streams; it is rarely useful to directly
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manipulate a raw stream from user code. Nevertheless, you can for example
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create a raw stream by opening a file in binary mode with buffering disabled::
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f = open("myfile.jpg", "rb", buffering=0)
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The detailed API of raw streams is described by the :class:`RawIOBase`
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class.
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Module Interface
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----------------
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High-level Module Interface
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---------------------------
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.. data:: DEFAULT_BUFFER_SIZE
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@ -89,17 +135,22 @@ Module Interface
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not be used in new code)
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========= ===============================================================
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The default mode is ``'rt'`` (open for reading text). For binary random
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access, the mode ``'w+b'`` opens and truncates the file to 0 bytes, while
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``'r+b'`` opens the file without truncation.
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The default mode is ``'r'`` (open for reading text, synonym of ``'rt'``).
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For binary read-write access, the mode ``'w+b'`` opens and truncates the
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file to 0 bytes, while ``'r+b'`` opens the file without truncation.
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Python distinguishes between files opened in binary and text modes, even when
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the underlying operating system doesn't. Files opened in binary mode
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(including ``'b'`` in the *mode* argument) return contents as ``bytes``
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objects without any decoding. In text mode (the default, or when ``'t'`` is
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included in the *mode* argument), the contents of the file are returned as
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strings, the bytes having been first decoded using a platform-dependent
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encoding or using the specified *encoding* if given.
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As mentioned in the `overview`_, Python distinguishes between binary
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and text I/O. Files opened in binary mode (including ``'b'`` in the
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*mode* argument) return contents as :class:`bytes` objects without
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any decoding. In text mode (the default, or when ``'t'``
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is included in the *mode* argument), the contents of the file are
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returned as strings, the bytes having been first decoded using a
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platform-dependent encoding or using the specified *encoding* if given.
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.. note::
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Python doesn't depend on the underlying operating system's notion
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of text files; all the the processing is done by Python itself, and
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is therefore platform-independent.
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*buffering* is an optional integer used to set the buffering policy.
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Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
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@ -168,11 +219,6 @@ Module Interface
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:class:`BufferedRandom`. When buffering is disabled, the raw stream, a
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subclass of :class:`RawIOBase`, :class:`FileIO`, is returned.
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It is also possible to use a string or bytearray as a file for both reading
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and writing. For strings :class:`StringIO` can be used like a file opened in
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a text mode, and for bytearrays a :class:`BytesIO` can be used like a
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file opened in a binary mode.
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.. exception:: BlockingIOError
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@ -194,8 +240,67 @@ Module Interface
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when an unsupported operation is called on a stream.
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In-memory streams
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^^^^^^^^^^^^^^^^^
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It is also possible to use a :class:`str` or :class:`bytes`-like object as a
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file for both reading and writing. For strings :class:`StringIO` can be
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used like a file opened in text mode, and :class:`BytesIO` can be used like
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a file opened in binary mode. Both provide full read-write capabilities
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with random access.
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.. seealso::
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:mod:`sys`
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contains the standard IO streams: :data:`sys.stdin`, :data:`sys.stdout`,
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and :data:`sys.stderr`.
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Class hierarchy
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---------------
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The implementation of I/O streams is organized as a hierarchy of classes.
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First :term:`abstract base classes <abstract base class>` (ABCs), which are used to specify the
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various categories of streams, then concrete classes providing the standard
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stream implementations.
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.. note::
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The abstract base classes also provide default implementations of
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some methods in order to help implementation of concrete stream
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classes. For example, :class:`BufferedIOBase` provides
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unoptimized implementations of ``readinto()`` and ``readline()``.
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At the top of the I/O hierarchy is the abstract base class :class:`IOBase`. It
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defines the basic interface to a stream. Note, however, that there is no
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separation between reading and writing to streams; implementations are allowed
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to raise an :exc:`UnsupportedOperation` if they do not support a given
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operation.
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Extending :class:`IOBase` is the :class:`RawIOBase` ABC which deals simply
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with the reading and writing of raw bytes to a stream. :class:`FileIO`
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subclasses :class:`RawIOBase` to provide an interface to files in the
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machine's file system.
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The :class:`BufferedIOBase` ABC deals with buffering on a raw byte stream
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(:class:`RawIOBase`). Its subclasses, :class:`BufferedWriter`,
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:class:`BufferedReader`, and :class:`BufferedRWPair` buffer streams that are
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readable, writable, and both readable and writable.
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:class:`BufferedRandom` provides a buffered interface to random access
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streams. :class:`BytesIO` is a simple stream of in-memory bytes.
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Another :class:`IOBase` subclass, the :class:`TextIOBase` ABC, deals with
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streams whose bytes represent text, and handles encoding and decoding
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from and to strings. :class:`TextIOWrapper`, which extends it, is a
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buffered text interface to a buffered raw stream
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(:class:`BufferedIOBase`). Finally, :class:`StringIO` is an in-memory
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stream for text.
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Argument names are not part of the specification, and only the arguments of
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:func:`.open` are intended to be used as keyword arguments.
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I/O Base Classes
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----------------
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^^^^^^^^^^^^^^^^
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.. class:: IOBase
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@ -467,7 +572,7 @@ I/O Base Classes
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Raw File I/O
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------------
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^^^^^^^^^^^^
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.. class:: FileIO(name, mode='r', closefd=True)
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@ -505,7 +610,7 @@ Raw File I/O
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Buffered Streams
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----------------
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^^^^^^^^^^^^^^^^
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In many situations, buffered I/O streams will provide higher performance
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(bandwidth and latency) than raw I/O streams. Their API is also more usable.
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@ -515,7 +620,7 @@ In many situations, buffered I/O streams will provide higher performance
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A stream implementation using an in-memory bytes buffer. It inherits
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:class:`BufferedIOBase`.
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The argument *initial_bytes* is an optional initial bytearray.
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The argument *initial_bytes* contains optional initial :class:`bytes` data.
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:class:`BytesIO` provides or overrides these methods in addition to those
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from :class:`BufferedIOBase` and :class:`IOBase`:
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@ -632,7 +737,7 @@ In many situations, buffered I/O streams will provide higher performance
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Text I/O
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--------
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^^^^^^^^
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.. class:: TextIOBase
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@ -736,14 +841,14 @@ Text I/O
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.. class:: StringIO(initial_value='', newline=None)
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An in-memory stream for text. It inherits :class:`TextIOWrapper`.
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An in-memory stream for text I/O.
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The initial value of the buffer (an empty string by default) can be set by
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providing *initial_value*. The *newline* argument works like that of
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:class:`TextIOWrapper`. The default is to do no newline translation.
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:class:`StringIO` provides this method in addition to those from
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:class:`TextIOWrapper` and its parents:
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:class:`TextIOBase` and its parents:
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.. method:: getvalue()
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@ -767,6 +872,11 @@ Text I/O
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# .getvalue() will now raise an exception.
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output.close()
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.. note::
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:class:`StringIO` uses a native text storage and doesn't suffer from
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the performance issues of other text streams, such as those based on
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:class:`TextIOWrapper`.
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.. class:: IncrementalNewlineDecoder
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A helper codec that decodes newlines for universal newlines mode. It
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