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Diffstat (limited to 'Doc/whatsnew/whatsnew20.tex')
| -rw-r--r-- | Doc/whatsnew/whatsnew20.tex | 105 |
1 files changed, 94 insertions, 11 deletions
diff --git a/Doc/whatsnew/whatsnew20.tex b/Doc/whatsnew/whatsnew20.tex index c35ce9e..969cdd0 100644 --- a/Doc/whatsnew/whatsnew20.tex +++ b/Doc/whatsnew/whatsnew20.tex @@ -9,9 +9,14 @@ \section{Introduction} -{\large This is a draft document; please report inaccuracies and -omissions to the authors. \\ -XXX marks locations where fact-checking or rewriting is still needed. +{\large This is a draft document; please report inaccuracies and +omissions to the authors. This document should not be treated as +definitive; features described here might be removed or changed before +Python 1.6final. \\ + +XXX marks locations in the text where fact-checking or rewriting is +still needed. + } A new release of Python, version 1.6, will be released some time this @@ -65,7 +70,7 @@ throughout a Python program. If an encoding isn't specified, the default encoding is usually 7-bit ASCII, though it can be changed for your Python installation by calling the \function{sys.setdefaultencoding(\var{encoding})} function in a -customized version of \file{site.py}. +customised version of \file{site.py}. Combining 8-bit and Unicode strings always coerces to Unicode, using the default ASCII encoding; the result of \code{'a' + u'bc'} is @@ -125,8 +130,9 @@ the given encoding and return Unicode strings. \item \var{stream_writer}, similarly, is a class that supports encoding output to a stream. \var{stream_writer(\var{file_obj})} -returns an object that supports the \method{write()} and -\method{writelines()} methods. These methods expect Unicode strings, translating them to the given encoding on output. +returns an object that supports the \method{write()} and +\method{writelines()} methods. These methods expect Unicode strings, +translating them to the given encoding on output. \end{itemize} For example, the following code writes a Unicode string into a file, @@ -365,6 +371,72 @@ For example, the number 8.1 can't be represented exactly in binary, so %into strings instead of classes, has been removed. % ====================================================================== +\section{Optional Collection of Cycles} + +The C implementation of Python uses reference counting to implement +garbage collection. Every Python object maintains a count of the +number of references pointing to itself, and adjusts the count as +references are created or destroyed. Once the reference count reaches +zero, the object is no longer accessible, since you need to have a +reference to an object to access it, and if the count is zero, no +references exist any longer. + +Reference counting has some pleasant properties: it's easy to +understand and implement, and the resulting implementation is +portable, fairly fast, and reacts well with other libraries that +implement their own memory handling schemes. The major problem with +reference counting is that it sometimes doesn't realise that objects +are no longer accessible, resulting in a memory leak. This happens +when there are cycles of references. + +Consider the simplest possible cycle, +a class instance which has a reference to itself: + +\begin{verbatim} +instance = SomeClass() +instance.myself = instance +\end{verbatim} + +After the above two lines of code have been executed, the reference +count of \code{instance} is 2; one reference is from the variable +named \samp{'instance'}, and the other is from the \samp{myself} +attribute of the instance. + +If the next line of code is \code{del instance}, what happens? The +reference count of \code{instance} is decreased by 1, so it has a +reference count of 1; the reference in the \samp{myself} attribute +still exists. Yet the instance is no longer accessible through Python +code, and it could be deleted. Several objects can participate in a +cycle if they have references to each other, causing all of the +objects to be leaked. + +An experimental step has been made toward fixing this problem. When +compiling Python, the \code{--with-cycle-gc} (XXX correct option +flag?) option can be specified. This causes a cycle detection +algorithm to be periodically executed, which looks for inaccessible +cycles and deletes the objects involved. + +Why isn't this enabled by default? Running the cycle detection +algorithm takes some time, and some tuning will be required to +minimize the overhead cost. It's not yet obvious how much performance +is lost, because benchmarking this is tricky and depends sensitively +on how often the program creates and destroys objects. XXX is this +actually the correct reason? Or is it fear of breaking software that +runs happily while leaving garbage? + +Several people worked on this problem. Early versions were written by +XXX1, XXX2. (I vaguely remember several people writing first cuts at this. +Anyone recall who?) +The implementation that's in Python 1.6 is a rewritten version, this +time done by Neil Schemenauer. Lots of other people offered +suggestions along the way, such as (in alphabetical order) +Marc-Andr\'e Lemburg, Tim Peters, Greg Stein, Eric Tiedemann. The +March 2000 archives of the python-dev mailing list contain most of the +relevant discussion, especially in the threads titled ``Reference +cycle collection for Python'' and ``Finalization again''. + + +% ====================================================================== \section{Core Changes} Various minor changes have been made to Python's syntax and built-in @@ -488,7 +560,7 @@ This means you no longer have to remember to write code such as The \file{Python/importdl.c} file, which was a mass of \#ifdefs to support dynamic loading on many different platforms, was cleaned up -and reorganized by Greg Stein. \file{importdl.c} is now quite small, +and reorganised by Greg Stein. \file{importdl.c} is now quite small, and platform-specific code has been moved into a bunch of \file{Python/dynload_*.c} files. @@ -535,6 +607,12 @@ which takes a socket object and returns an SSL socket. The support ``https://'' URLs, though no one has implemented FTP or SMTP over SSL. +The \module{httplib} module has been rewritten by Greg Stein to +support HTTP/1.1. Backward compatibility with the 1.5 version of +\module{httplib} is provided, though using HTTP/1.1 features such as +pipelining will require rewriting code to use a different set of +interfaces. + The \module{Tkinter} module now supports Tcl/Tk version 8.1, 8.2, or 8.3, and support for the older 7.x versions has been dropped. The Tkinter module also supports displaying Unicode strings in Tk @@ -543,10 +621,10 @@ widgets. The \module{curses} module has been greatly extended, starting from Oliver Andrich's enhanced version, to provide many additional functions from ncurses and SYSV curses, such as colour, alternative -character set support, pads, and other new features. This means the -module is no longer compatible with operating systems that only have -BSD curses, but there don't seem to be any currently maintained OSes -that fall into this category. +character set support, pads, and mouse support. This means the module +is no longer compatible with operating systems that only have BSD +curses, but there don't seem to be any currently maintained OSes that +fall into this category. As mentioned in the earlier discussion of 1.6's Unicode support, the underlying implementation of the regular expressions provided by the @@ -609,6 +687,11 @@ DOS/Windows or \program{zip} on Unix, not to be confused with module) (Contributed by James C. Ahlstrom.) +\item{\module{imputil}:} A module that provides a simpler way for +writing customised import hooks, in comparison to the existing +\module{ihooks} module. (Implemented by Greg Stein, with much +discussion on python-dev along the way.) + \end{itemize} % ====================================================================== |