[Python-checkins] r74879 - python/branches/py3k/Doc/library/io.rst
antoine.pitrou
python-checkins at python.org
Thu Sep 17 19:18:01 CEST 2009
Author: antoine.pitrou
Date: Thu Sep 17 19:18:01 2009
New Revision: 74879
Log:
Issue #6929: fix a couple of statements and clarify a lot of things in the IO docs.
Modified:
python/branches/py3k/Doc/library/io.rst
Modified: python/branches/py3k/Doc/library/io.rst
==============================================================================
--- python/branches/py3k/Doc/library/io.rst (original)
+++ python/branches/py3k/Doc/library/io.rst Thu Sep 17 19:18:01 2009
@@ -208,6 +208,9 @@
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
@@ -314,15 +317,20 @@
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:
.. method:: read(n=-1)
- 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.
+ Read and return up to *n* bytes from the stream. As a convenience, if
+ *n* is unspecified or -1, :meth:`readall` is called. Otherwise,
+ 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.
.. method:: readall()
@@ -337,27 +345,34 @@
.. method:: 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 :exc:`IOError` will be raised).
+ stream and return the number of bytes written. This can be less than
+ ``len(b)``, depending on specifics of the underlying raw stream, and
+ especially if it is in non-blocking mode. ``None`` is returned if the
+ raw stream is set not to block and no single byte could be readily
+ written to it.
.. class:: BufferedIOBase
- Base class for streams that support buffering. It inherits :class:`IOBase`.
- There is no public constructor.
+ 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``.
- The main difference with :class:`RawIOBase` is that the :meth:`read` method
- supports omitting the *size* argument, and does not have a default
+ Besides, the :meth:`read` method does not have a default
implementation that defers to :meth:`readinto`.
- In addition, :meth:`read`, :meth:`readinto`, and :meth:`write` may raise
- :exc:`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 :class:`RawIOBase`
- implementation, but wrap one like :class:`BufferedWriter` and
- :class:`BufferedReader`.
+ 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`:
@@ -393,13 +408,15 @@
one raw read will be issued, and a short result does not imply that EOF is
imminent.
- A :exc:`BlockingIOError` is raised if the underlying raw stream has no
- data at the moment.
+ A :exc:`BlockingIOError` is raised if the underlying raw stream is in
+ non blocking-mode, and has no data available at the moment.
.. method:: read1(n=-1)
Read and return up to *n* bytes, with at most one call to the underlying
- raw stream's :meth:`~RawIOBase.read` method.
+ raw stream's :meth:`~RawIOBase.read` method. This can be useful if you
+ are implementing your own buffering on top of a :class:`BufferedIOBase`
+ object.
.. method:: readinto(b)
@@ -407,19 +424,22 @@
read.
Like :meth:`read`, multiple reads may be issued to the underlying raw
- stream, unless the latter is 'interactive.'
+ stream, unless the latter is 'interactive'.
- A :exc:`BlockingIOError` is raised if the underlying raw stream has no
- data at the moment.
+ A :exc:`BlockingIOError` is raised if the underlying raw stream is in
+ non blocking-mode, and has no data available at the moment.
.. method:: 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 :exc:`IOError` will be raised).
-
- A :exc:`BlockingIOError` is raised if the buffer is full, and the
- underlying raw stream cannot accept more data at the moment.
+ Write the given bytes or bytearray object, *b* and return the number
+ of bytes written (never less than ``len(b)``, since if the write fails
+ an :exc:`IOError` will be raised). Depending on the actual
+ implementation, these bytes may be readily written to the underlying
+ stream, or held in a buffer for performance and latency reasons.
+
+ When in non-blocking mode, a :exc:`BlockingIOError` is raised if the
+ data needed to be written to the raw stream but it couldn't accept
+ all the data without blocking.
Raw File I/O
@@ -427,15 +447,25 @@
.. class:: FileIO(name, mode='r', closefd=True)
- :class:`FileIO` represents a file containing bytes data. It implements
- the :class:`RawIOBase` interface (and therefore the :class:`IOBase`
- interface, too).
+ :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:
@@ -449,29 +479,13 @@
The file name. This is the file descriptor of the file when no name is
given in the constructor.
- .. method:: read(n=-1)
-
- 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 :func:`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.)
-
- .. method:: 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.
-
- .. method:: 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.
-
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.
+
.. class:: BytesIO([initial_bytes])
A stream implementation using an in-memory bytes buffer. It inherits
@@ -498,8 +512,11 @@
.. class:: BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)
- A buffer for a readable, sequential :class:`RawIOBase` object. It inherits
- :class:`BufferedIOBase`.
+ 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,
@@ -528,8 +545,16 @@
.. class:: BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)
- A buffer for a writeable sequential RawIO object. It inherits
- :class:`BufferedIOBase`.
+ 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
@@ -547,17 +572,17 @@
.. method:: write(b)
- Write the bytes or bytearray object, *b*, onto the raw stream and return
- the number of bytes written. A :exc:`BlockingIOError` is raised when the
- raw stream blocks.
+ Write the bytes or bytearray object, *b* and return the number of bytes
+ written. When in non-blocking mode, a :exc:`BlockingIOError` is raised
+ if the buffer needs to be written out but the raw stream blocks.
-.. class:: BufferedRWPair(reader, writer, buffer_size, max_buffer_size=DEFAULT_BUFFER_SIZE)
+.. class:: BufferedRWPair(reader, writer, buffer_size=DEFAULT_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 :meth:`read`, :meth:`write`,
- and their variants. This is useful for sockets and two-way pipes.
- It inherits :class:`BufferedIOBase`.
+ 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
@@ -574,7 +599,8 @@
.. class:: BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)
A buffered interface to random access streams. It inherits
- :class:`BufferedReader` and :class:`BufferedWriter`.
+ :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
@@ -611,7 +637,8 @@
.. attribute:: newlines
A string, a tuple of strings, or ``None``, indicating the newlines
- translated so far.
+ translated so far. Depending on the implementation and the initial
+ constructor flags, this may not be available.
.. attribute:: buffer
@@ -621,7 +648,8 @@
.. method:: detach()
- Separate the underlying buffer from the :class:`TextIOBase` and return it.
+ Separate the underlying binary buffer from the :class:`TextIOBase` and
+ return it.
After the underlying buffer has been detached, the :class:`TextIOBase` is
in an unusable state.
@@ -635,7 +663,7 @@
.. method:: read(n)
Read and return at most *n* characters from the stream as a single
- :class:`str`. If *n* is negative or ``None``, reads to EOF.
+ :class:`str`. If *n* is negative or ``None``, reads until EOF.
.. method:: readline()
@@ -650,7 +678,7 @@
.. class:: TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False)
- A buffered text stream over a :class:`BufferedIOBase` raw stream, *buffer*.
+ 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
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