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Guido van Rossum 1994-08-01 12:38:14 +00:00
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Doc/ref/ref6.tex
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@ -27,14 +27,14 @@ simple_stmt: expression_stmt
Expression statements are used (mostly interactively) to compute and
write a value, or (usually) to call a procedure (a function that
returns no meaningful result; in Python, procedures return the value
\verb\None\):
\verb@None@):
\begin{verbatim}
expression_stmt: expression_list
\end{verbatim}
An expression statement evaluates the expression list (which may be a
single expression). If the value is not \verb\None\, it is converted
single expression). If the value is not \verb@None@, it is converted
to a string using the rules for string conversions (expressions in
reverse quotes), and the resulting string is written to standard
output (see section \ref{print}) on a line by itself.
@ -45,9 +45,9 @@ output (see section \ref{print}) on a line by itself.
\indexii{standard}{output}
\indexii{writing}{values}
(The exception for \verb\None\ is made so that procedure calls, which
(The exception for \verb@None@ is made so that procedure calls, which
are syntactically equivalent to expressions, do not cause any output.
A tuple with only \verb\None\ items is written normally.)
A tuple with only \verb@None@ items is written normally.)
\indexii{procedure}{call}
\section{Assignment statements}
@ -114,7 +114,7 @@ If the target is an identifier (name):
\begin{itemize}
\item
If the name does not occur in a \verb\global\ statement in the current
If the name does not occur in a \verb@global@ statement in the current
code block: the name is bound to the object in the current local name
space.
\stindex{global}
@ -140,10 +140,10 @@ the corresponding targets.
\item
If the target is an attribute reference: The primary expression in the
reference is evaluated. It should yield an object with assignable
attributes; if this is not the case, \verb\TypeError\ is raised. That
attributes; if this is not the case, \verb@TypeError@ is raised. That
object is then asked to assign the assigned object to the given
attribute; if it cannot perform the assignment, it raises an exception
(usually but not necessarily \verb\AttributeError\).
(usually but not necessarily \verb@AttributeError@).
\indexii{attribute}{assignment}
\item
@ -159,7 +159,7 @@ must yield a plain integer. If it is negative, the sequence's length
is added to it. The resulting value must be a nonnegative integer
less than the sequence's length, and the sequence is asked to assign
the assigned object to its item with that index. If the index is out
of range, \verb\IndexError\ is raised (assignment to a subscripted
of range, \verb@IndexError@ is raised (assignment to a subscripted
sequence cannot add new items to a list).
\obindex{sequence}
\obindex{list}
@ -175,16 +175,17 @@ key with the same value existed).
\item
If the target is a slicing: The primary expression in the reference is
evaluated. It should yield a mutable sequence (list) object. The
evaluated. It should yield a mutable sequence object (e.g. a list). The
assigned object should be a sequence object of the same type. Next,
the lower and upper bound expressions are evaluated, insofar they are
present; defaults are zero and the sequence's length. The bounds
should evaluate to (small) integers. If either bound is negative, the
sequence's length is added to it. The resulting bounds are clipped to
lie between zero and the sequence's length, inclusive. Finally, the
sequence object is asked to replace the items indicated by the slice
with the items of the assigned sequence. This may change the
sequence's length, if it allows it.
sequence object is asked to replace the slice with the items of the
assigned sequence. The length of the slice may be different from the
length of the assigned sequence, thus changing the length of the
target sequence, if the object allows it.
\indexii{slicing}{assignment}
\end{itemize}
@ -201,7 +202,7 @@ messages.)
pass_stmt: "pass"
\end{verbatim}
\verb\pass\ is a null operation --- when it is executed, nothing
\verb@pass@ is a null operation --- when it is executed, nothing
happens. It is useful as a placeholder when a statement is
required syntactically, but no code needs to be executed, for example:
\indexii{null}{operation}
@ -230,7 +231,7 @@ to right.
Deletion of a name removes the binding of that name (which must exist)
from the local or global name space, depending on whether the name
occurs in a \verb\global\ statement in the same code block.
occurs in a \verb@global@ statement in the same code block.
\stindex{global}
\indexii{unbinding}{name}
@ -247,7 +248,7 @@ right type (but even this is determined by the sliced object).
print_stmt: "print" [ condition ("," condition)* [","] ]
\end{verbatim}
\verb\print\ evaluates each condition in turn and writes the resulting
\verb@print@ evaluates each condition in turn and writes the resulting
object to standard output (see below). If an object is not a string,
it is first converted to a string using the rules for string
conversions. The (resulting or original) string is then written. A
@ -256,21 +257,21 @@ the output system believes it is positioned at the beginning of a
line. This is the case: (1) when no characters have yet been written
to standard output; or (2) when the last character written to standard
output is \verb/\n/; or (3) when the last write operation on standard
output was not a \verb\print\ statement. (In some cases it may be
output was not a \verb@print@ statement. (In some cases it may be
functional to write an empty string to standard output for this
reason.)
\index{output}
\indexii{writing}{values}
A \verb/"\n"/ character is written at the end, unless the \verb\print\
A \verb/"\n"/ character is written at the end, unless the \verb@print@
statement ends with a comma. This is the only action if the statement
contains just the keyword \verb\print\.
contains just the keyword \verb@print@.
\indexii{trailing}{comma}
\indexii{newline}{suppression}
Standard output is defined as the file object named \verb\stdout\
in the built-in module \verb\sys\. If no such object exists,
or if it is not a writable file, a \verb\RuntimeError\ exception is raised.
Standard output is defined as the file object named \verb@stdout@
in the built-in module \verb@sys@. If no such object exists,
or if it is not a writable file, a \verb@RuntimeError@ exception is raised.
(The original implementation attempts to write to the system's original
standard output instead, but this is not safe, and should be fixed.)
\indexii{standard}{output}
@ -285,19 +286,19 @@ standard output instead, but this is not safe, and should be fixed.)
return_stmt: "return" [condition_list]
\end{verbatim}
\verb\return\ may only occur syntactically nested in a function
\verb@return@ may only occur syntactically nested in a function
definition, not within a nested class definition.
\indexii{function}{definition}
\indexii{class}{definition}
If a condition list is present, it is evaluated, else \verb\None\
If a condition list is present, it is evaluated, else \verb@None@
is substituted.
\verb\return\ leaves the current function call with the condition
list (or \verb\None\) as return value.
\verb@return@ leaves the current function call with the condition
list (or \verb@None@) as return value.
When \verb\return\ passes control out of a \verb\try\ statement
with a \verb\finally\ clause, that finally clause is executed
When \verb@return@ passes control out of a \verb@try@ statement
with a \verb@finally@ clause, that finally clause is executed
before really leaving the function.
\kwindex{finally}
@ -308,14 +309,14 @@ before really leaving the function.
raise_stmt: "raise" condition ["," condition]
\end{verbatim}
\verb\raise\ evaluates its first condition, which must yield
\verb@raise@ evaluates its first condition, which must yield
a string object. If there is a second condition, this is evaluated,
else \verb\None\ is substituted.
else \verb@None@ is substituted.
\index{exception}
\indexii{raising}{exception}
It then raises the exception identified by the first object,
with the second one (or \verb\None\) as its parameter.
with the second one (or \verb@None@) as its parameter.
\section{The {\tt break} statement}
\stindex{break}
@ -324,22 +325,23 @@ with the second one (or \verb\None\) as its parameter.
break_stmt: "break"
\end{verbatim}
\verb\break\ may only occur syntactically nested in a \verb\for\
or \verb\while\ loop, not nested in a function or class definition.
\verb@break@ may only occur syntactically nested in a \verb@for@
or \verb@while@ loop, but not nested in a function or class definition
within that loop.
\stindex{for}
\stindex{while}
\indexii{loop}{statement}
It terminates the neares enclosing loop, skipping the optional
\verb\else\ clause if the loop has one.
It terminates the nearest enclosing loop, skipping the optional
\verb@else@ clause if the loop has one.
\kwindex{else}
If a \verb\for\ loop is terminated by \verb\break\, the loop control
If a \verb@for@ loop is terminated by \verb@break@, the loop control
target keeps its current value.
\indexii{loop control}{target}
When \verb\break\ passes control out of a \verb\try\ statement
with a \verb\finally\ clause, that finally clause is executed
When \verb@break@ passes control out of a \verb@try@ statement
with a \verb@finally@ clause, that finally clause is executed
before really leaving the loop.
\kwindex{finally}
@ -350,11 +352,10 @@ before really leaving the loop.
continue_stmt: "continue"
\end{verbatim}
\verb\continue\ may only occur syntactically nested in a \verb\for\ or
\verb\while\ loop, not nested in a function or class definition, and
not nested in the \verb\try\ clause of a \verb\try\ statement with a
\verb\finally\ clause (it may occur nested in a \verb\except\ or
\verb\finally\ clause of a \verb\try\ statement though).
\verb@continue@ may only occur syntactically nested in a \verb@for@ or
\verb@while@ loop, but not nested in a function or class definition or
\verb@try@ statement within that loop.\footnote{Except that it may
currently occur within an \verb@except@ clause.}
\stindex{for}
\stindex{while}
\indexii{loop}{statement}
@ -373,9 +374,9 @@ import_stmt: "import" identifier ("," identifier)*
Import statements are executed in two steps: (1) find a module, and
initialize it if necessary; (2) define a name or names in the local
name space (of the scope where the \verb\import\ statement occurs).
The first form (without \verb\from\) repeats these steps for each
identifier in the list, the \verb\from\ form performs them once, with
name space (of the scope where the \verb@import@ statement occurs).
The first form (without \verb@from@) repeats these steps for each
identifier in the list, the \verb@from@ form performs them once, with
the first identifier specifying the module name.
\indexii{importing}{module}
\indexii{name}{binding}
@ -383,15 +384,15 @@ the first identifier specifying the module name.
The system maintains a table of modules that have been initialized,
indexed by module name. (The current implementation makes this table
accessible as \verb\sys.modules\.) When a module name is found in
accessible as \verb@sys.modules@.) When a module name is found in
this table, step (1) is finished. If not, a search for a module
definition is started. This first looks for a built-in module
definition, and if no built-in module if the given name is found, it
searches a user-specified list of directories for a file whose name is
the module name with extension \verb\".py"\. (The current
implementation uses the list of strings \verb\sys.path\ as the search
the module name with extension \verb@".py"@. (The current
implementation uses the list of strings \verb@sys.path@ as the search
path; it is initialized from the shell environment variable
\verb\$PYTHONPATH\, with an installation-dependent default.)
\verb@$PYTHONPATH@, with an installation-dependent default.)
\ttindex{modules}
\ttindex{sys.modules}
\indexii{module}{name}
@ -404,9 +405,9 @@ path; it is initialized from the shell environment variable
If a built-in module is found, its built-in initialization code is
executed and step (1) is finished. If no matching file is found,
\verb\ImportError\ is raised. If a file is found, it is parsed,
\verb@ImportError@ is raised. If a file is found, it is parsed,
yielding an executable code block. If a syntax error occurs,
\verb\SyntaxError\ is raised. Otherwise, an empty module of the given
\verb@SyntaxError@ is raised. Otherwise, an empty module of the given
name is created and inserted in the module table, and then the code
block is executed in the context of this module. Exceptions during
this execution terminate step (1).
@ -418,23 +419,23 @@ this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can
begin.
The first form of \verb\import\ statement binds the module name in the
The first form of \verb@import@ statement binds the module name in the
local name space to the module object, and then goes on to import the
next identifier, if any. The \verb\from\ from does not bind the
next identifier, if any. The \verb@from@ from does not bind the
module name: it goes through the list of identifiers, looks each one
of them up in the module found in step (1), and binds the name in the
local name space to the object thus found. If a name is not found,
\verb\ImportError\ is raised. If the list of identifiers is replaced
by a star (\verb\*\), all names defined in the module are bound,
except those beginning with an underscore(\verb\_\).
\verb@ImportError@ is raised. If the list of identifiers is replaced
by a star (\verb@*@), all names defined in the module are bound,
except those beginning with an underscore(\verb@_@).
\indexii{name}{binding}
\exindex{ImportError}
Names bound by import statements may not occur in \verb\global\
Names bound by import statements may not occur in \verb@global@
statements in the same scope.
\stindex{global}
The \verb\from\ form with \verb\*\ may only occur in a module scope.
The \verb@from@ form with \verb@*@ may only occur in a module scope.
\kwindex{from}
\ttindex{from ... import *}
@ -450,19 +451,19 @@ program.)
global_stmt: "global" identifier ("," identifier)*
\end{verbatim}
The \verb\global\ statement is a declaration which holds for the
The \verb@global@ statement is a declaration which holds for the
entire current scope. It means that the listed identifiers are to be
interpreted as globals. While {\em using} global names is automatic
if they are not defined in the local scope, {\em assigning} to global
names would be impossible without \verb\global\.
names would be impossible without \verb@global@.
\indexiii{global}{name}{binding}
Names listed in a \verb\global\ statement must not be used in the same
scope before that \verb\global\ statement is executed.
Names listed in a \verb@global@ statement must not be used in the same
scope before that \verb@global@ statement is executed.
Names listed in a \verb\global\ statement must not be defined as formal
parameters or in a \verb\for\ loop control target, \verb\class\
definition, function definition, or \verb\import\ statement.
Names listed in a \verb@global@ statement must not be defined as formal
parameters or in a \verb@for@ loop control target, \verb@class@
definition, function definition, or \verb@import@ statement.
(The current implementation does not enforce the latter two
restrictions, but programs should not abuse this freedom, as future
@ -478,7 +479,7 @@ access_stmt: "access" ...
This statement will be used in the future to control access to
instance and class variables. Currently its syntax and effects are
undefined; however the keyword \verb\access\ is a reserved word for
undefined; however the keyword \verb@access@ is a reserved word for
the parser.
\section{The {\tt exec} statement} \label{exec}
@ -496,12 +497,12 @@ occurs). If it is an open file, the file is parsed until EOF and
executed. If it is a code object, it is simply executed.
In all cases, if the optional parts are omitted, the code is executed
in the current scope. If only the first expression after \verb\in\ is
in the current scope. If only the first expression after \verb@in@ is
specified, it should be a dictionary, which will be used for both the
global and the local variables. If two expressions are given, both
must be dictionaries and they are used for the global and local
variables, respectively.
Note: dynamic evaluation of expressions is supported by the built-in
function \verb\eval\.
function \verb@eval@.

132
Doc/ref/ref7.tex
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@ -6,8 +6,8 @@ or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb\if\, \verb\while\ and \verb\for\ statements implement
traditional control flow constructs. \verb\try\ specifies exception
The \verb@if@, \verb@while@ and \verb@for@ statements implement
traditional control flow constructs. \verb@try@ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
@ -21,7 +21,7 @@ statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb\if\ clause a following \verb\else\ clause would
clear to which \verb@if@ clause a following \verb@else@ clause would
belong:
\index{clause}
\index{suite}
@ -32,7 +32,7 @@ if test1: if test2: print x
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb\print\ statements are executed:
\verb@print@ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
@ -48,15 +48,15 @@ statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb\NEWLINE\ possibly followed
by a \verb\DEDENT\.
Note that statements always end in a \verb@NEWLINE@ possibly followed
by a \verb@DEDENT@.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb\else\' problem is solved in Python by requiring
nested \verb\if\ statements to be indented).
(the `dangling \verb@else@' problem is solved in Python by requiring
nested \verb@if@ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
@ -65,7 +65,7 @@ each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb\if\ statement is used for conditional execution:
The \verb@if@ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
@ -76,8 +76,8 @@ if_stmt: "if" condition ":" suite
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb\if\ statement is executed or evaluated). If
all conditions are false, the suite of the \verb\else\ clause, if
other part of the \verb@if@ statement is executed or evaluated). If
all conditions are false, the suite of the \verb@else@ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
@ -86,7 +86,7 @@ present, is executed.
\stindex{while}
\indexii{loop}{statement}
The \verb\while\ statement is used for repeated execution as long as a
The \verb@while@ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
@ -96,13 +96,13 @@ while_stmt: "while" condition ":" suite
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb\else\ clause, if present, is
is tested) the suite of the \verb@else@ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
@ -111,7 +111,7 @@ of the suite and goes back to testing the condition.
\stindex{for}
\indexii{loop}{statement}
The \verb\for\ statement is used to iterate over the elements of a
The \verb@for@ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
@ -125,16 +125,16 @@ suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb\else\ clause, if
when the sequence is empty), the suite in the \verb@else@ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb\else\
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb@else@
clause if there was no next item.
\stindex{break}
\stindex{continue}
@ -146,9 +146,10 @@ The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb\range()\ returns a sequence of
integers suitable to emulate the effect of Pascal's \verb\for i := a
to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
Hint: the built-in function \verb@range()@ returns a sequence of
integers suitable to emulate the effect of Pascal's
\verb@for i := a to b do@;
e.g. \verb@range(3)@ returns the list \verb@[0, 1, 2]@.
\bifuncindex{range}
\index{Pascal}
@ -175,24 +176,25 @@ for x in a[:]:
\section{The {\tt try} statement} \label{try}
\stindex{try}
The \verb\try\ statement specifies exception handlers and/or cleanup
The \verb@try@ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
["else" ":" suite]
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb\try\ statement: \verb\try...except\ and
\verb\try...finally\. These forms cannot be mixed.
There are two forms of \verb@try@ statement: \verb@try...except@ and
\verb@try...finally@. These forms cannot be mixed.
The \verb\try...except\ form specifies one or more exception handlers
(the \verb\except\ clauses). When no exception occurs in the
\verb\try\ clause, no exception handler is executed. When an
exception occurs in the \verb\try\ suite, a search for an exception
The \verb@try...except@ form specifies one or more exception handlers
(the \verb@except@ clauses). When no exception occurs in the
\verb@try@ clause, no exception handler is executed. When an
exception occurs in the \verb@try@ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
@ -211,7 +213,7 @@ handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb\try\ statement
on the call stack (it is treated as if the entire \verb@try@ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
@ -223,10 +225,10 @@ exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the \verb\sys\ module:
\verb\sys.exc_type\ receives the object identifying the exception;
\verb\sys.exc_value\ receives the exception's parameter;
\verb\sys.exc_traceback\ receives a traceback object (see section
exception are assigned to three variables in the \verb@sys@ module:
\verb@sys.exc_type@ receives the object identifying the exception;
\verb@sys.exc_value@ receives the exception's parameter;
\verb@sys.exc_traceback@ receives a traceback object (see section
\ref{traceback}) identifying the point in the program where the
exception occurred.
\bimodindex{sys}
@ -235,20 +237,25 @@ exception occurred.
\ttindex{exc_traceback}
\obindex{traceback}
The \verb\try...finally\ form specifies a `cleanup' handler. The
\verb\try\ clause is executed. When no exception occurs, the
\verb\finally\ clause is executed. When an exception occurs in the
\verb\try\ clause, the exception is temporarily saved, the
\verb\finally\ clause is executed, and then the saved exception is
re-raised. If the \verb\finally\ clause raises another exception or
executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
The optional \verb@else@ clause is executed when no exception occurs
in the \verb@try@ clause. Exceptions in the \verb@else@ clause are
not handled by the preceding \verb@except@ clauses.
\kwindex{else}
The \verb@try...finally@ form specifies a `cleanup' handler. The
\verb@try@ clause is executed. When no exception occurs, the
\verb@finally@ clause is executed. When an exception occurs in the
\verb@try@ clause, the exception is temporarily saved, the
\verb@finally@ clause is executed, and then the saved exception is
re-raised. If the \verb@finally@ clause raises another exception or
executes a \verb@return@, \verb@break@ or \verb@continue@ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try\ suite of a \verb\try...finally\ statement, the
\verb\finally\ clause is also executed `on the way out'. A
\verb\continue\ statement is illegal in the \verb\try\ clause. (The
When a \verb@return@ or \verb@break@ statement is executed in the
\verb@try@ suite of a \verb@try...finally@ statement, the
\verb@finally@ clause is also executed `on the way out'. A
\verb@continue@ statement is illegal in the \verb@try@ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
@ -265,7 +272,8 @@ section \ref{types}):
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (parameter ",")* ("*" identifier | parameter [","])
parameter_list: (defparameter ",")* ("*" identifier | defparameter [","])
defparameter: parameter ["=" condition]
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
@ -282,8 +290,21 @@ as the global name space to be used when the function is called.
The function definition does not execute the function body; this gets
executed only when the function is called.
When one or more top-level parameters have the form {\em parameter =
condition}, the function is said to have ``default parameter values''.
Default parameter values are evaluated when the function definition is
executed. For a parameter with a default value, the correponding
argument may be omitted from a call, in which case the parameter's
default value is substituted. If a parameter has a default value, all
following parameters must also have a default value --- this is a
syntactic restriction that is not expressed by the grammar.%
\footnote{Currently this is not checked; instead,
\verb@def f(a=1,b)@ is interpreted as \verb@def f(a=1,b=None)@.}
\indexiii{default}{parameter}{value}
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, the arguments (a.k.a. actual
user-defined function is called, first missing arguments for which a
default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
@ -292,10 +313,6 @@ parameters) are bound to the (formal) parameters, as follows:
\indexii{parameter}{formal}
\indexii{parameter}{actual}
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\begin{itemize}
\item
@ -320,7 +337,7 @@ Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb\()\, if
tuple: if there are no extra arguments, its value is \verb@()@, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
@ -335,6 +352,11 @@ variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\indexii{lambda}{form}
\section{Class definitions} \label{class}
\indexii{class}{definition}

16
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@ -13,9 +13,9 @@ While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb\sys\
(various system services), \verb\__builtin__\ (built-in functions,
exceptions and \verb\None\) and \verb\__main__\. The latter is used
available, but none have been initialized, except for \verb@sys@
(various system services), \verb@__builtin__@ (built-in functions,
exceptions and \verb@None@) and \verb@__main__@. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
@ -29,7 +29,7 @@ The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb\__main__\.
in the name space of \verb@__main__@.
\index{interactive mode}
Under {\UNIX}, a complete program can be passed to the interpreter in
@ -58,7 +58,7 @@ This syntax is used in the following situations:
\item when parsing a module;
\item when parsing a string passed to the \verb\exec\ statement;
\item when parsing a string passed to the \verb@exec@ statement;
\end{itemize}
@ -80,14 +80,14 @@ end of the input.
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb\eval()\ must have the following form:
The string argument to \verb@eval()@ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb\input()\ must have the following form:
The input line read by \verb@input()@ must have the following form:
\bifuncindex{input}
\begin{verbatim}
@ -95,7 +95,7 @@ input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb\raw_input()\ or the \verb\readline()\ method
built-in function \verb@raw_input()@ or the \verb@readline()@ method
of file objects.
\obindex{file}
\index{input!raw}

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@ -27,14 +27,14 @@ simple_stmt: expression_stmt
Expression statements are used (mostly interactively) to compute and
write a value, or (usually) to call a procedure (a function that
returns no meaningful result; in Python, procedures return the value
\verb\None\):
\verb@None@):
\begin{verbatim}
expression_stmt: expression_list
\end{verbatim}
An expression statement evaluates the expression list (which may be a
single expression). If the value is not \verb\None\, it is converted
single expression). If the value is not \verb@None@, it is converted
to a string using the rules for string conversions (expressions in
reverse quotes), and the resulting string is written to standard
output (see section \ref{print}) on a line by itself.
@ -45,9 +45,9 @@ output (see section \ref{print}) on a line by itself.
\indexii{standard}{output}
\indexii{writing}{values}
(The exception for \verb\None\ is made so that procedure calls, which
(The exception for \verb@None@ is made so that procedure calls, which
are syntactically equivalent to expressions, do not cause any output.
A tuple with only \verb\None\ items is written normally.)
A tuple with only \verb@None@ items is written normally.)
\indexii{procedure}{call}
\section{Assignment statements}
@ -114,7 +114,7 @@ If the target is an identifier (name):
\begin{itemize}
\item
If the name does not occur in a \verb\global\ statement in the current
If the name does not occur in a \verb@global@ statement in the current
code block: the name is bound to the object in the current local name
space.
\stindex{global}
@ -140,10 +140,10 @@ the corresponding targets.
\item
If the target is an attribute reference: The primary expression in the
reference is evaluated. It should yield an object with assignable
attributes; if this is not the case, \verb\TypeError\ is raised. That
attributes; if this is not the case, \verb@TypeError@ is raised. That
object is then asked to assign the assigned object to the given
attribute; if it cannot perform the assignment, it raises an exception
(usually but not necessarily \verb\AttributeError\).
(usually but not necessarily \verb@AttributeError@).
\indexii{attribute}{assignment}
\item
@ -159,7 +159,7 @@ must yield a plain integer. If it is negative, the sequence's length
is added to it. The resulting value must be a nonnegative integer
less than the sequence's length, and the sequence is asked to assign
the assigned object to its item with that index. If the index is out
of range, \verb\IndexError\ is raised (assignment to a subscripted
of range, \verb@IndexError@ is raised (assignment to a subscripted
sequence cannot add new items to a list).
\obindex{sequence}
\obindex{list}
@ -175,16 +175,17 @@ key with the same value existed).
\item
If the target is a slicing: The primary expression in the reference is
evaluated. It should yield a mutable sequence (list) object. The
evaluated. It should yield a mutable sequence object (e.g. a list). The
assigned object should be a sequence object of the same type. Next,
the lower and upper bound expressions are evaluated, insofar they are
present; defaults are zero and the sequence's length. The bounds
should evaluate to (small) integers. If either bound is negative, the
sequence's length is added to it. The resulting bounds are clipped to
lie between zero and the sequence's length, inclusive. Finally, the
sequence object is asked to replace the items indicated by the slice
with the items of the assigned sequence. This may change the
sequence's length, if it allows it.
sequence object is asked to replace the slice with the items of the
assigned sequence. The length of the slice may be different from the
length of the assigned sequence, thus changing the length of the
target sequence, if the object allows it.
\indexii{slicing}{assignment}
\end{itemize}
@ -201,7 +202,7 @@ messages.)
pass_stmt: "pass"
\end{verbatim}
\verb\pass\ is a null operation --- when it is executed, nothing
\verb@pass@ is a null operation --- when it is executed, nothing
happens. It is useful as a placeholder when a statement is
required syntactically, but no code needs to be executed, for example:
\indexii{null}{operation}
@ -230,7 +231,7 @@ to right.
Deletion of a name removes the binding of that name (which must exist)
from the local or global name space, depending on whether the name
occurs in a \verb\global\ statement in the same code block.
occurs in a \verb@global@ statement in the same code block.
\stindex{global}
\indexii{unbinding}{name}
@ -247,7 +248,7 @@ right type (but even this is determined by the sliced object).
print_stmt: "print" [ condition ("," condition)* [","] ]
\end{verbatim}
\verb\print\ evaluates each condition in turn and writes the resulting
\verb@print@ evaluates each condition in turn and writes the resulting
object to standard output (see below). If an object is not a string,
it is first converted to a string using the rules for string
conversions. The (resulting or original) string is then written. A
@ -256,21 +257,21 @@ the output system believes it is positioned at the beginning of a
line. This is the case: (1) when no characters have yet been written
to standard output; or (2) when the last character written to standard
output is \verb/\n/; or (3) when the last write operation on standard
output was not a \verb\print\ statement. (In some cases it may be
output was not a \verb@print@ statement. (In some cases it may be
functional to write an empty string to standard output for this
reason.)
\index{output}
\indexii{writing}{values}
A \verb/"\n"/ character is written at the end, unless the \verb\print\
A \verb/"\n"/ character is written at the end, unless the \verb@print@
statement ends with a comma. This is the only action if the statement
contains just the keyword \verb\print\.
contains just the keyword \verb@print@.
\indexii{trailing}{comma}
\indexii{newline}{suppression}
Standard output is defined as the file object named \verb\stdout\
in the built-in module \verb\sys\. If no such object exists,
or if it is not a writable file, a \verb\RuntimeError\ exception is raised.
Standard output is defined as the file object named \verb@stdout@
in the built-in module \verb@sys@. If no such object exists,
or if it is not a writable file, a \verb@RuntimeError@ exception is raised.
(The original implementation attempts to write to the system's original
standard output instead, but this is not safe, and should be fixed.)
\indexii{standard}{output}
@ -285,19 +286,19 @@ standard output instead, but this is not safe, and should be fixed.)
return_stmt: "return" [condition_list]
\end{verbatim}
\verb\return\ may only occur syntactically nested in a function
\verb@return@ may only occur syntactically nested in a function
definition, not within a nested class definition.
\indexii{function}{definition}
\indexii{class}{definition}
If a condition list is present, it is evaluated, else \verb\None\
If a condition list is present, it is evaluated, else \verb@None@
is substituted.
\verb\return\ leaves the current function call with the condition
list (or \verb\None\) as return value.
\verb@return@ leaves the current function call with the condition
list (or \verb@None@) as return value.
When \verb\return\ passes control out of a \verb\try\ statement
with a \verb\finally\ clause, that finally clause is executed
When \verb@return@ passes control out of a \verb@try@ statement
with a \verb@finally@ clause, that finally clause is executed
before really leaving the function.
\kwindex{finally}
@ -308,14 +309,14 @@ before really leaving the function.
raise_stmt: "raise" condition ["," condition]
\end{verbatim}
\verb\raise\ evaluates its first condition, which must yield
\verb@raise@ evaluates its first condition, which must yield
a string object. If there is a second condition, this is evaluated,
else \verb\None\ is substituted.
else \verb@None@ is substituted.
\index{exception}
\indexii{raising}{exception}
It then raises the exception identified by the first object,
with the second one (or \verb\None\) as its parameter.
with the second one (or \verb@None@) as its parameter.
\section{The {\tt break} statement}
\stindex{break}
@ -324,22 +325,23 @@ with the second one (or \verb\None\) as its parameter.
break_stmt: "break"
\end{verbatim}
\verb\break\ may only occur syntactically nested in a \verb\for\
or \verb\while\ loop, not nested in a function or class definition.
\verb@break@ may only occur syntactically nested in a \verb@for@
or \verb@while@ loop, but not nested in a function or class definition
within that loop.
\stindex{for}
\stindex{while}
\indexii{loop}{statement}
It terminates the neares enclosing loop, skipping the optional
\verb\else\ clause if the loop has one.
It terminates the nearest enclosing loop, skipping the optional
\verb@else@ clause if the loop has one.
\kwindex{else}
If a \verb\for\ loop is terminated by \verb\break\, the loop control
If a \verb@for@ loop is terminated by \verb@break@, the loop control
target keeps its current value.
\indexii{loop control}{target}
When \verb\break\ passes control out of a \verb\try\ statement
with a \verb\finally\ clause, that finally clause is executed
When \verb@break@ passes control out of a \verb@try@ statement
with a \verb@finally@ clause, that finally clause is executed
before really leaving the loop.
\kwindex{finally}
@ -350,11 +352,10 @@ before really leaving the loop.
continue_stmt: "continue"
\end{verbatim}
\verb\continue\ may only occur syntactically nested in a \verb\for\ or
\verb\while\ loop, not nested in a function or class definition, and
not nested in the \verb\try\ clause of a \verb\try\ statement with a
\verb\finally\ clause (it may occur nested in a \verb\except\ or
\verb\finally\ clause of a \verb\try\ statement though).
\verb@continue@ may only occur syntactically nested in a \verb@for@ or
\verb@while@ loop, but not nested in a function or class definition or
\verb@try@ statement within that loop.\footnote{Except that it may
currently occur within an \verb@except@ clause.}
\stindex{for}
\stindex{while}
\indexii{loop}{statement}
@ -373,9 +374,9 @@ import_stmt: "import" identifier ("," identifier)*
Import statements are executed in two steps: (1) find a module, and
initialize it if necessary; (2) define a name or names in the local
name space (of the scope where the \verb\import\ statement occurs).
The first form (without \verb\from\) repeats these steps for each
identifier in the list, the \verb\from\ form performs them once, with
name space (of the scope where the \verb@import@ statement occurs).
The first form (without \verb@from@) repeats these steps for each
identifier in the list, the \verb@from@ form performs them once, with
the first identifier specifying the module name.
\indexii{importing}{module}
\indexii{name}{binding}
@ -383,15 +384,15 @@ the first identifier specifying the module name.
The system maintains a table of modules that have been initialized,
indexed by module name. (The current implementation makes this table
accessible as \verb\sys.modules\.) When a module name is found in
accessible as \verb@sys.modules@.) When a module name is found in
this table, step (1) is finished. If not, a search for a module
definition is started. This first looks for a built-in module
definition, and if no built-in module if the given name is found, it
searches a user-specified list of directories for a file whose name is
the module name with extension \verb\".py"\. (The current
implementation uses the list of strings \verb\sys.path\ as the search
the module name with extension \verb@".py"@. (The current
implementation uses the list of strings \verb@sys.path@ as the search
path; it is initialized from the shell environment variable
\verb\$PYTHONPATH\, with an installation-dependent default.)
\verb@$PYTHONPATH@, with an installation-dependent default.)
\ttindex{modules}
\ttindex{sys.modules}
\indexii{module}{name}
@ -404,9 +405,9 @@ path; it is initialized from the shell environment variable
If a built-in module is found, its built-in initialization code is
executed and step (1) is finished. If no matching file is found,
\verb\ImportError\ is raised. If a file is found, it is parsed,
\verb@ImportError@ is raised. If a file is found, it is parsed,
yielding an executable code block. If a syntax error occurs,
\verb\SyntaxError\ is raised. Otherwise, an empty module of the given
\verb@SyntaxError@ is raised. Otherwise, an empty module of the given
name is created and inserted in the module table, and then the code
block is executed in the context of this module. Exceptions during
this execution terminate step (1).
@ -418,23 +419,23 @@ this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can
begin.
The first form of \verb\import\ statement binds the module name in the
The first form of \verb@import@ statement binds the module name in the
local name space to the module object, and then goes on to import the
next identifier, if any. The \verb\from\ from does not bind the
next identifier, if any. The \verb@from@ from does not bind the
module name: it goes through the list of identifiers, looks each one
of them up in the module found in step (1), and binds the name in the
local name space to the object thus found. If a name is not found,
\verb\ImportError\ is raised. If the list of identifiers is replaced
by a star (\verb\*\), all names defined in the module are bound,
except those beginning with an underscore(\verb\_\).
\verb@ImportError@ is raised. If the list of identifiers is replaced
by a star (\verb@*@), all names defined in the module are bound,
except those beginning with an underscore(\verb@_@).
\indexii{name}{binding}
\exindex{ImportError}
Names bound by import statements may not occur in \verb\global\
Names bound by import statements may not occur in \verb@global@
statements in the same scope.
\stindex{global}
The \verb\from\ form with \verb\*\ may only occur in a module scope.
The \verb@from@ form with \verb@*@ may only occur in a module scope.
\kwindex{from}
\ttindex{from ... import *}
@ -450,19 +451,19 @@ program.)
global_stmt: "global" identifier ("," identifier)*
\end{verbatim}
The \verb\global\ statement is a declaration which holds for the
The \verb@global@ statement is a declaration which holds for the
entire current scope. It means that the listed identifiers are to be
interpreted as globals. While {\em using} global names is automatic
if they are not defined in the local scope, {\em assigning} to global
names would be impossible without \verb\global\.
names would be impossible without \verb@global@.
\indexiii{global}{name}{binding}
Names listed in a \verb\global\ statement must not be used in the same
scope before that \verb\global\ statement is executed.
Names listed in a \verb@global@ statement must not be used in the same
scope before that \verb@global@ statement is executed.
Names listed in a \verb\global\ statement must not be defined as formal
parameters or in a \verb\for\ loop control target, \verb\class\
definition, function definition, or \verb\import\ statement.
Names listed in a \verb@global@ statement must not be defined as formal
parameters or in a \verb@for@ loop control target, \verb@class@
definition, function definition, or \verb@import@ statement.
(The current implementation does not enforce the latter two
restrictions, but programs should not abuse this freedom, as future
@ -478,7 +479,7 @@ access_stmt: "access" ...
This statement will be used in the future to control access to
instance and class variables. Currently its syntax and effects are
undefined; however the keyword \verb\access\ is a reserved word for
undefined; however the keyword \verb@access@ is a reserved word for
the parser.
\section{The {\tt exec} statement} \label{exec}
@ -496,12 +497,12 @@ occurs). If it is an open file, the file is parsed until EOF and
executed. If it is a code object, it is simply executed.
In all cases, if the optional parts are omitted, the code is executed
in the current scope. If only the first expression after \verb\in\ is
in the current scope. If only the first expression after \verb@in@ is
specified, it should be a dictionary, which will be used for both the
global and the local variables. If two expressions are given, both
must be dictionaries and they are used for the global and local
variables, respectively.
Note: dynamic evaluation of expressions is supported by the built-in
function \verb\eval\.
function \verb@eval@.

132
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@ -6,8 +6,8 @@ or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb\if\, \verb\while\ and \verb\for\ statements implement
traditional control flow constructs. \verb\try\ specifies exception
The \verb@if@, \verb@while@ and \verb@for@ statements implement
traditional control flow constructs. \verb@try@ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
@ -21,7 +21,7 @@ statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb\if\ clause a following \verb\else\ clause would
clear to which \verb@if@ clause a following \verb@else@ clause would
belong:
\index{clause}
\index{suite}
@ -32,7 +32,7 @@ if test1: if test2: print x
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb\print\ statements are executed:
\verb@print@ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
@ -48,15 +48,15 @@ statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb\NEWLINE\ possibly followed
by a \verb\DEDENT\.
Note that statements always end in a \verb@NEWLINE@ possibly followed
by a \verb@DEDENT@.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb\else\' problem is solved in Python by requiring
nested \verb\if\ statements to be indented).
(the `dangling \verb@else@' problem is solved in Python by requiring
nested \verb@if@ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
@ -65,7 +65,7 @@ each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb\if\ statement is used for conditional execution:
The \verb@if@ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
@ -76,8 +76,8 @@ if_stmt: "if" condition ":" suite
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb\if\ statement is executed or evaluated). If
all conditions are false, the suite of the \verb\else\ clause, if
other part of the \verb@if@ statement is executed or evaluated). If
all conditions are false, the suite of the \verb@else@ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
@ -86,7 +86,7 @@ present, is executed.
\stindex{while}
\indexii{loop}{statement}
The \verb\while\ statement is used for repeated execution as long as a
The \verb@while@ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
@ -96,13 +96,13 @@ while_stmt: "while" condition ":" suite
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb\else\ clause, if present, is
is tested) the suite of the \verb@else@ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
@ -111,7 +111,7 @@ of the suite and goes back to testing the condition.
\stindex{for}
\indexii{loop}{statement}
The \verb\for\ statement is used to iterate over the elements of a
The \verb@for@ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
@ -125,16 +125,16 @@ suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb\else\ clause, if
when the sequence is empty), the suite in the \verb@else@ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb\else\
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb@else@
clause if there was no next item.
\stindex{break}
\stindex{continue}
@ -146,9 +146,10 @@ The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb\range()\ returns a sequence of
integers suitable to emulate the effect of Pascal's \verb\for i := a
to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
Hint: the built-in function \verb@range()@ returns a sequence of
integers suitable to emulate the effect of Pascal's
\verb@for i := a to b do@;
e.g. \verb@range(3)@ returns the list \verb@[0, 1, 2]@.
\bifuncindex{range}
\index{Pascal}
@ -175,24 +176,25 @@ for x in a[:]:
\section{The {\tt try} statement} \label{try}
\stindex{try}
The \verb\try\ statement specifies exception handlers and/or cleanup
The \verb@try@ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
["else" ":" suite]
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb\try\ statement: \verb\try...except\ and
\verb\try...finally\. These forms cannot be mixed.
There are two forms of \verb@try@ statement: \verb@try...except@ and
\verb@try...finally@. These forms cannot be mixed.
The \verb\try...except\ form specifies one or more exception handlers
(the \verb\except\ clauses). When no exception occurs in the
\verb\try\ clause, no exception handler is executed. When an
exception occurs in the \verb\try\ suite, a search for an exception
The \verb@try...except@ form specifies one or more exception handlers
(the \verb@except@ clauses). When no exception occurs in the
\verb@try@ clause, no exception handler is executed. When an
exception occurs in the \verb@try@ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
@ -211,7 +213,7 @@ handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb\try\ statement
on the call stack (it is treated as if the entire \verb@try@ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
@ -223,10 +225,10 @@ exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the \verb\sys\ module:
\verb\sys.exc_type\ receives the object identifying the exception;
\verb\sys.exc_value\ receives the exception's parameter;
\verb\sys.exc_traceback\ receives a traceback object (see section
exception are assigned to three variables in the \verb@sys@ module:
\verb@sys.exc_type@ receives the object identifying the exception;
\verb@sys.exc_value@ receives the exception's parameter;
\verb@sys.exc_traceback@ receives a traceback object (see section
\ref{traceback}) identifying the point in the program where the
exception occurred.
\bimodindex{sys}
@ -235,20 +237,25 @@ exception occurred.
\ttindex{exc_traceback}
\obindex{traceback}
The \verb\try...finally\ form specifies a `cleanup' handler. The
\verb\try\ clause is executed. When no exception occurs, the
\verb\finally\ clause is executed. When an exception occurs in the
\verb\try\ clause, the exception is temporarily saved, the
\verb\finally\ clause is executed, and then the saved exception is
re-raised. If the \verb\finally\ clause raises another exception or
executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
The optional \verb@else@ clause is executed when no exception occurs
in the \verb@try@ clause. Exceptions in the \verb@else@ clause are
not handled by the preceding \verb@except@ clauses.
\kwindex{else}
The \verb@try...finally@ form specifies a `cleanup' handler. The
\verb@try@ clause is executed. When no exception occurs, the
\verb@finally@ clause is executed. When an exception occurs in the
\verb@try@ clause, the exception is temporarily saved, the
\verb@finally@ clause is executed, and then the saved exception is
re-raised. If the \verb@finally@ clause raises another exception or
executes a \verb@return@, \verb@break@ or \verb@continue@ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try\ suite of a \verb\try...finally\ statement, the
\verb\finally\ clause is also executed `on the way out'. A
\verb\continue\ statement is illegal in the \verb\try\ clause. (The
When a \verb@return@ or \verb@break@ statement is executed in the
\verb@try@ suite of a \verb@try...finally@ statement, the
\verb@finally@ clause is also executed `on the way out'. A
\verb@continue@ statement is illegal in the \verb@try@ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
@ -265,7 +272,8 @@ section \ref{types}):
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (parameter ",")* ("*" identifier | parameter [","])
parameter_list: (defparameter ",")* ("*" identifier | defparameter [","])
defparameter: parameter ["=" condition]
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
@ -282,8 +290,21 @@ as the global name space to be used when the function is called.
The function definition does not execute the function body; this gets
executed only when the function is called.
When one or more top-level parameters have the form {\em parameter =
condition}, the function is said to have ``default parameter values''.
Default parameter values are evaluated when the function definition is
executed. For a parameter with a default value, the correponding
argument may be omitted from a call, in which case the parameter's
default value is substituted. If a parameter has a default value, all
following parameters must also have a default value --- this is a
syntactic restriction that is not expressed by the grammar.%
\footnote{Currently this is not checked; instead,
\verb@def f(a=1,b)@ is interpreted as \verb@def f(a=1,b=None)@.}
\indexiii{default}{parameter}{value}
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, the arguments (a.k.a. actual
user-defined function is called, first missing arguments for which a
default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
@ -292,10 +313,6 @@ parameters) are bound to the (formal) parameters, as follows:
\indexii{parameter}{formal}
\indexii{parameter}{actual}
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\begin{itemize}
\item
@ -320,7 +337,7 @@ Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb\()\, if
tuple: if there are no extra arguments, its value is \verb@()@, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
@ -335,6 +352,11 @@ variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\indexii{lambda}{form}
\section{Class definitions} \label{class}
\indexii{class}{definition}

16
Doc/ref8.tex
View File

@ -13,9 +13,9 @@ While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb\sys\
(various system services), \verb\__builtin__\ (built-in functions,
exceptions and \verb\None\) and \verb\__main__\. The latter is used
available, but none have been initialized, except for \verb@sys@
(various system services), \verb@__builtin__@ (built-in functions,
exceptions and \verb@None@) and \verb@__main__@. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
@ -29,7 +29,7 @@ The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb\__main__\.
in the name space of \verb@__main__@.
\index{interactive mode}
Under {\UNIX}, a complete program can be passed to the interpreter in
@ -58,7 +58,7 @@ This syntax is used in the following situations:
\item when parsing a module;
\item when parsing a string passed to the \verb\exec\ statement;
\item when parsing a string passed to the \verb@exec@ statement;
\end{itemize}
@ -80,14 +80,14 @@ end of the input.
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb\eval()\ must have the following form:
The string argument to \verb@eval()@ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb\input()\ must have the following form:
The input line read by \verb@input()@ must have the following form:
\bifuncindex{input}
\begin{verbatim}
@ -95,7 +95,7 @@ input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb\raw_input()\ or the \verb\readline()\ method
built-in function \verb@raw_input()@ or the \verb@readline()@ method
of file objects.
\obindex{file}
\index{input!raw}