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Benjamin Peterson 2011-02-24 02:53:05 +00:00
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Doc/library/io.rst
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@ -788,31 +788,30 @@ Text I/O
Performance
-----------
This section discusses the performance of the provided concrete IO
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 is designed to hide any inefficiency in
calling and executing the operating system's unbuffered I/O routines. The
gain will vary very much depending on the OS and the kind of I/O which is
performed (for example, on some contemporary 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 will offer you 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.
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
^^^^^^^^
Text I/O over a binary storage (such as a file) is significantly slower than
binary I/O over the same storage, because it implies conversions from
unicode to binary data using a character codec. This can become noticeable
if you handle huge amounts of text data (for example very large log files).
Also, :meth:`TextIOWrapper.tell` and :meth:`TextIOWrapper.seek` are both
quite slow due to the reconstruction algorithm used.
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:`StringIO`, however, is a native in-memory unicode container and will
exhibit similar speed to :class:`BytesIO`.
@ -820,9 +819,8 @@ exhibit similar speed to :class:`BytesIO`.
Multi-threading
^^^^^^^^^^^^^^^
:class:`FileIO` objects are thread-safe to the extent that the operating
system calls (such as ``read(2)`` under Unix) they are wrapping are thread-safe
too.
: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`)
@ -837,12 +835,13 @@ Reentrancy
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 if you are doing I/O in a :mod:`signal` handler. If it is
attempted to enter a buffered object again while already being accessed
*from the same thread*, then a :exc:`RuntimeError` is raised.
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.
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.