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chapter on new things in 1.3

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Guido van Rossum 1995-08-10 14:18:10 +00:00
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\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}

190
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@ -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}