: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 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 (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 (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
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.
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:`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) 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.
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.
Raw File I/O
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.
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.
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.
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.
A third argument, max_buffer_size, is supported, but unused and deprecated.
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.
A fourth argument, max_buffer_size, is supported, but unused and deprecated.
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.
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 controls how line endings are handled. It can be None, '', '\n', '\r', and '\r\n'. It works as follows:
- When reading input from the stream, if newline is None, :term:`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 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, :data:`os.linesep`. If 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.
If line_buffering is True, :meth:`flush` is implied when a call to write contains a newline character.
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.
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()
This section discusses the performance of the provided concrete I/O implementations.
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 most always preferable to use buffered I/O rather than unbuffered I/O for binary datal
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:`TextIOWrapper.seek` are both quite slow due to the reconstruction algorithm used.
: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 renter 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.