chapter on new things in 1.3
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Guido van Rossum
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Doc/tut.tex
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Doc/tut.tex
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@ -3529,4 +3529,194 @@ the buffer in bytes.
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\end{itemize}
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\end{itemize}
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\chapter{New in Release 1.3}
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This chapter describes yet more recent additions to the Python
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language and library.
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\section{New Keyword Arguments}
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Functions and methods written in Python can now be called using
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keyword arguments of the form \code{\var{keyword} = \var{value}}. For
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instance, the following function:
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\begin{verbatim}
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def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):
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print "-- This parrot wouldn't", action,
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print "if you put", voltage, "Volts through it."
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print "-- Lovely plumage, the", type
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print "-- It's", state, "!"
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\end{verbatim}
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could be called in any of the following ways:
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\begin{verbatim}
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parrot(1000)
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parrot(action = 'VOOOOOM', voltage = 1000000)
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parrot('a thousand', state = 'pushing up the daisies')
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parrot('a million', 'bereft of life', 'jump')
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\end{verbatim}
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but the following calls would all be invalid:
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\begin{verbatim}
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parrot() # required argument missing
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parrot(voltage=5.0, 'dead') # non-keyword argument following keyword
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parrot(110, voltage=220) # duplicate value for argument
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parrot(actor='John Cleese') # unknown keyword
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\end{verbatim}
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In general, an argument list must have the form: zero or more
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positional arguments followed by zero or more keyword arguments, where
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the keywords must be chosen from the formal parameter names. It's not
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important whether a formal parameter has a default value or not. No
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argument must receive a value more than once -- formal parameter names
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corresponding to positional arguments cannot be used as keywords in
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the same calls.
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|
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Note that no special syntax is required to allow a function to be
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called with keyword arguments. The additional costs incurred by
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keyword arguments are only present when a call uses them.
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|
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(As far as I know, these rules are exactly the same as used by
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Modula-3, even if they are enforced by totally different means. This
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is intentional.)
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When a final formal parameter of the form \code{**\var{name}} is
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present, it receives a dictionary containing all keyword arguments
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whose keyword doesn't correspond to a formal parameter. This may be
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combined with a formal parameter of the form \code{*\var{name}} which
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receives a tuple containing the positional arguments beyond the formal
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parameter list. (\code{*\var{name}} must occur before
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\code{**\var{name}}.) For example, if we define a function like this:
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\begin{verbatim}
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def cheeseshop(kind, *arguments, **keywords):
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print "-- Do you have any", kind, '?'
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print "-- I'm sorry, we're all out of", kind
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for arg in arguments: print arg
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print '-'*40
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for kw in keywords.keys(): print kw, ':', keywords[kw]
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\end{verbatim}
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It could be called like this:
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\begin{verbatim}
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cheeseshop('Limburger', "It's very runny, sir.",
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"It's really very, VERY runny, sir.",
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client='John Cleese',
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shopkeeper='Michael Palin',
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sketch='Cheese Shop Sketch')
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\end{verbatim}
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and of course it would print:
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\begin{verbatim}
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-- Do you have any Limburger ?
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-- I'm sorry, we're all out of Limburger
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It's very runny, sir.
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It's really very, VERY runny, sir.
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----------------------------------------
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client : John Cleese
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shopkeeper : Michael Palin
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sketch : Cheese Shop Sketch
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\end{verbatim}
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Side effects of this change include:
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\begin{itemize}
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\item
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In the effort of implementing keyword arguments, function and
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|
especially method calls have been sped up significantly -- for a
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|
method with ten formal parameters, the call overhead has been cut in
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|
half; for a function with one formal parameters, the overhead has been
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|
reduced by a third.
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\item
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The format of \code{.pyc} files has changed (again).
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\end{itemize}
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\section{Minor Changes}
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\begin{itemize}
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\item
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For file objects, \code{\var{f}.read(0)} and
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\code{\var{f}.readline(0)} now return an empty string rather than
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reading an unlimited number of bytes. For the latter, omit the
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argument altogether or pass a negative value.
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|
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\item
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A new system variable, \code{sys.platform}, has been added. It
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specifies the current platform, e.g. \code{sunos5} or \code{linux1}.
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|
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\item
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The built-in functions \code{input()} and \code{raw_input()} now use
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|
the GNU readline library when it has been configured (formerly, only
|
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|
interactive input to the interpreter itself was read using GNU
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|
readline). The GNU readline library provides elaborate line editing
|
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|
and history. The Python debugger (\code{pdb}) is the first
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|
beneficiary of this change.
|
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|
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|
\item
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|
Two new built-in functions, \code{globals()} and \code{locals()},
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|
provide access to dictionaries containming current global and local
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|
variables, respectively. (These augment rather than replace
|
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|
\code{vars()}, which returns the current local variables when called
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|
without an argument, and a module's global variables when called with
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|
an argument of type module.)
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|
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\item
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|
The optional built-in modules \code{dbm} and \code{gdbm} are more
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|
coordinated --- their \code{open()} functions now take the same values
|
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|
for their \var{flag} argument, and the \var{flag} and \var{mode}
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|
argument have default values (to open the database for reading only,
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|
and to create the database with mode \code{0666} minuse the umask,
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|
respectively).
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|
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\item
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|
A new dbm-like module, \code{dbhash}, has been added, which uses the
|
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|
BSD DB package's hash method.
|
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|
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\item
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The \code{raise} statement now takes an optional argument which
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|
specifies the traceback to be used when printing the exception's stack
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|
trace. This must be a traceback object, such as found in
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|
\code{sys.exc_traceback}. When omitted or given as \code{None}, the
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|
old behavior (to generate a stack trace entry for the current stack
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|
frame) is used.
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|
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\item
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The built-in function \code{compile()} now takes a third possible
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value for the kind of code to be compiled: specifying \code{'single'}
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generates code for a single interactive statement, which prints the
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|
output of expression statements that evaluate to something else than
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\code{None}.
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\item
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|
The tokenizer is now more tolerant of alien whitespace. Control-L in
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the leading whitespace of a line resets the column number to zero,
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|
while Control-R just before the end of the line is ignored.
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|
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\item
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|
The dynamic module loader recognizes the fact that different filenames
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|
point to the same shared library and loads the library only once, so
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|
you can have a single shared library that defines multiple modules.
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|
(SunOS / SVR4 style shared libraries only.)
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|
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\item
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|
Jim Fulton's ``abstract object interface'' has been incorporated into
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|
the run-time API. For more detailes, read the files
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\code{Include/abstract.h} and \code{Objects/abstract.c}.
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\item
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Numerous things I have forgotten or that are so obscure no-one will
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notice them anyway :-)
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\end{itemize}
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\end{document}
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\end{document}
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190
Doc/tut/tut.tex
190
Doc/tut/tut.tex
|
|
@ -3529,4 +3529,194 @@ the buffer in bytes.
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||||||
\end{itemize}
|
\end{itemize}
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|
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\chapter{New in Release 1.3}
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|
|
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|
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|
This chapter describes yet more recent additions to the Python
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|
language and library.
|
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|
|
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|
|
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|
\section{New Keyword Arguments}
|
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|
|
||||||
|
Functions and methods written in Python can now be called using
|
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|
keyword arguments of the form \code{\var{keyword} = \var{value}}. For
|
||||||
|
instance, the following function:
|
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|
|
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|
\begin{verbatim}
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|
def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):
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|
print "-- This parrot wouldn't", action,
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|
print "if you put", voltage, "Volts through it."
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|
print "-- Lovely plumage, the", type
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|
print "-- It's", state, "!"
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\end{verbatim}
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|
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could be called in any of the following ways:
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\begin{verbatim}
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parrot(1000)
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parrot(action = 'VOOOOOM', voltage = 1000000)
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parrot('a thousand', state = 'pushing up the daisies')
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parrot('a million', 'bereft of life', 'jump')
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\end{verbatim}
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|
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|
but the following calls would all be invalid:
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|
|
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\begin{verbatim}
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|
parrot() # required argument missing
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|
parrot(voltage=5.0, 'dead') # non-keyword argument following keyword
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|
parrot(110, voltage=220) # duplicate value for argument
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|
parrot(actor='John Cleese') # unknown keyword
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|
\end{verbatim}
|
||||||
|
|
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|
In general, an argument list must have the form: zero or more
|
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|
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
|
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|
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:
|
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|
|
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|
\begin{verbatim}
|
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|
def cheeseshop(kind, *arguments, **keywords):
|
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|
print "-- Do you have any", kind, '?'
|
||||||
|
print "-- I'm sorry, we're all out of", kind
|
||||||
|
for arg in arguments: print arg
|
||||||
|
print '-'*40
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|
for kw in keywords.keys(): print kw, ':', keywords[kw]
|
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|
\end{verbatim}
|
||||||
|
|
||||||
|
It could be called like this:
|
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|
|
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|
\begin{verbatim}
|
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|
cheeseshop('Limburger', "It's very runny, sir.",
|
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|
"It's really very, VERY runny, sir.",
|
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|
client='John Cleese',
|
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|
shopkeeper='Michael Palin',
|
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|
sketch='Cheese Shop Sketch')
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|
\end{verbatim}
|
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|
|
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|
and of course it would print:
|
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|
|
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|
\begin{verbatim}
|
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|
-- 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.
|
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|
----------------------------------------
|
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|
client : John Cleese
|
||||||
|
shopkeeper : Michael Palin
|
||||||
|
sketch : Cheese Shop Sketch
|
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|
\end{verbatim}
|
||||||
|
|
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|
Side effects of this change include:
|
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|
|
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|
\begin{itemize}
|
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|
|
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|
\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}
|
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|
|
||||||
|
\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
|
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|
notice them anyway :-)
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\end{itemize}
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\end{document}
|
\end{document}
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|
|
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Loading…
Reference in New Issue