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diff --git a/Doc/tut.tex b/Doc/tut.tex index cf884c1..fb54a3c 100644 --- a/Doc/tut.tex +++ b/Doc/tut.tex @@ -3529,4 +3529,194 @@ the buffer in bytes. \end{itemize} +\chapter{New in Release 1.3} + + +This chapter describes yet more recent additions to the Python +language and library. + + +\section{New Keyword Arguments} + +Functions and methods written in Python can now be called using +keyword arguments of the form \code{\var{keyword} = \var{value}}. For +instance, the following function: + +\begin{verbatim} +def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'): + print "-- This parrot wouldn't", action, + print "if you put", voltage, "Volts through it." + print "-- Lovely plumage, the", type + print "-- It's", state, "!" +\end{verbatim} + +could be called in any of the following ways: + +\begin{verbatim} +parrot(1000) +parrot(action = 'VOOOOOM', voltage = 1000000) +parrot('a thousand', state = 'pushing up the daisies') +parrot('a million', 'bereft of life', 'jump') +\end{verbatim} + +but the following calls would all be invalid: + +\begin{verbatim} +parrot() # required argument missing +parrot(voltage=5.0, 'dead') # non-keyword argument following keyword +parrot(110, voltage=220) # duplicate value for argument +parrot(actor='John Cleese') # unknown keyword +\end{verbatim} + +In general, an argument list must have the form: zero or more +positional arguments followed by zero or more keyword arguments, where +the keywords must be chosen from the formal parameter names. It's not +important whether a formal parameter has a default value or not. No +argument must receive a value more than once -- formal parameter names +corresponding to positional arguments cannot be used as keywords in +the same calls. + +Note that no special syntax is required to allow a function to be +called with keyword arguments. The additional costs incurred by +keyword arguments are only present when a call uses them. + +(As far as I know, these rules are exactly the same as used by +Modula-3, even if they are enforced by totally different means. This +is intentional.) + +When a final formal parameter of the form \code{**\var{name}} is +present, it receives a dictionary containing all keyword arguments +whose keyword doesn't correspond to a formal parameter. This may be +combined with a formal parameter of the form \code{*\var{name}} which +receives a tuple containing the positional arguments beyond the formal +parameter list. (\code{*\var{name}} must occur before +\code{**\var{name}}.) For example, if we define a function like this: + +\begin{verbatim} +def cheeseshop(kind, *arguments, **keywords): + print "-- Do you have any", kind, '?' + print "-- I'm sorry, we're all out of", kind + for arg in arguments: print arg + print '-'*40 + for kw in keywords.keys(): print kw, ':', keywords[kw] +\end{verbatim} + +It could be called like this: + +\begin{verbatim} +cheeseshop('Limburger', "It's very runny, sir.", + "It's really very, VERY runny, sir.", + client='John Cleese', + shopkeeper='Michael Palin', + sketch='Cheese Shop Sketch') +\end{verbatim} + +and of course it would print: + +\begin{verbatim} +-- Do you have any Limburger ? +-- I'm sorry, we're all out of Limburger +It's very runny, sir. +It's really very, VERY runny, sir. +---------------------------------------- +client : John Cleese +shopkeeper : Michael Palin +sketch : Cheese Shop Sketch +\end{verbatim} + +Side effects of this change include: + +\begin{itemize} + +\item +In the effort of implementing keyword arguments, function and +especially method calls have been sped up significantly -- for a +method with ten formal parameters, the call overhead has been cut in +half; for a function with one formal parameters, the overhead has been +reduced by a third. + +\item +The format of \code{.pyc} files has changed (again). + +\end{itemize} + +\section{Minor Changes} + +\begin{itemize} + +\item +For file objects, \code{\var{f}.read(0)} and +\code{\var{f}.readline(0)} now return an empty string rather than +reading an unlimited number of bytes. For the latter, omit the +argument altogether or pass a negative value. + +\item +A new system variable, \code{sys.platform}, has been added. It +specifies the current platform, e.g. \code{sunos5} or \code{linux1}. + +\item +The built-in functions \code{input()} and \code{raw_input()} now use +the GNU readline library when it has been configured (formerly, only +interactive input to the interpreter itself was read using GNU +readline). The GNU readline library provides elaborate line editing +and history. The Python debugger (\code{pdb}) is the first +beneficiary of this change. + +\item +Two new built-in functions, \code{globals()} and \code{locals()}, +provide access to dictionaries containming current global and local +variables, respectively. (These augment rather than replace +\code{vars()}, which returns the current local variables when called +without an argument, and a module's global variables when called with +an argument of type module.) + +\item +The optional built-in modules \code{dbm} and \code{gdbm} are more +coordinated --- their \code{open()} functions now take the same values +for their \var{flag} argument, and the \var{flag} and \var{mode} +argument have default values (to open the database for reading only, +and to create the database with mode \code{0666} minuse the umask, +respectively). + +\item +A new dbm-like module, \code{dbhash}, has been added, which uses the +BSD DB package's hash method. + +\item +The \code{raise} statement now takes an optional argument which +specifies the traceback to be used when printing the exception's stack +trace. This must be a traceback object, such as found in +\code{sys.exc_traceback}. When omitted or given as \code{None}, the +old behavior (to generate a stack trace entry for the current stack +frame) is used. + +\item +The built-in function \code{compile()} now takes a third possible +value for the kind of code to be compiled: specifying \code{'single'} +generates code for a single interactive statement, which prints the +output of expression statements that evaluate to something else than +\code{None}. + +\item +The tokenizer is now more tolerant of alien whitespace. Control-L in +the leading whitespace of a line resets the column number to zero, +while Control-R just before the end of the line is ignored. + +\item +The dynamic module loader recognizes the fact that different filenames +point to the same shared library and loads the library only once, so +you can have a single shared library that defines multiple modules. +(SunOS / SVR4 style shared libraries only.) + +\item +Jim Fulton's ``abstract object interface'' has been incorporated into +the run-time API. For more detailes, read the files +\code{Include/abstract.h} and \code{Objects/abstract.c}. + +\item +Numerous things I have forgotten or that are so obscure no-one will +notice them anyway :-) + +\end{itemize} + \end{document} |