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Completed execution model and try statement.

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Guido van Rossum 1992-03-02 16:13:50 +00:00
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Doc/ref.tex
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@ -431,9 +431,9 @@ some properties of objects that are important to know about.
Every object has an identity, a type and a value. An object's {\em
identity} never changes once it has been created; think of it as the
object's (permanent) address. An object's {\em type} determines the
operations that an object supports (e.g., does it have a length?) and
also defines the ``meaning'' of the object's value. The type also
object's address in memory. An object's {\em type} determines the
operations that an object supports (e.g., ``does it have a length?'')
and also defines the ``meaning'' of the object's value. The type also
never changes. The {\em value} of some objects can change; whether
this is possible is a property of its type.
@ -450,12 +450,13 @@ references.)
Note that the use of the implementation's tracing or debugging
facilities may keep objects alive that would normally be collectable.
(Some objects contain references to ``external'' resources such as
open files. It is understood that these resources are freed when the
object is garbage-collected, but since garbage collection is not
guaranteed, such objects also provide an explicit way to release the
external resource (e.g., a \verb\close\ method). Programs are strongly
recommended to use this.)
Some objects contain references to ``external'' resources such as open
files or windows. It is understood that these resources are freed
when the object is garbage-collected, but since garbage collection is
not guaranteed to happen, such objects also provide an explicit way to
release the external resource, usually a \verb\close\ method.
Programs are strongly recommended to always explicitly close such
objects.
Some objects contain references to other objects. These references
are part of the object's value; in most cases, when such a
@ -463,23 +464,135 @@ are part of the object's value; in most cases, when such a
comparison applies to the {\em values} of the referenced objects (not
their identities).
Types affect almost all aspects of objects.
Even object identity is affected in some sense: for immutable
types, operations that compute new values may actually return a
reference to any existing object with the same type and value, while
for mutable objects this is not allowed. E.g., after
Types affect almost all aspects of an object's life. Even the meaning
of object identity is affected in some sense: for immutable types,
operations that compute new values may actually return a reference to
any existing object with the same type and value, while for mutable
objects this is not allowed. E.g., after
\begin{verbatim}
a = 1; b = 1; c = []; d = []
\end{verbatim}
\verb\a\ and \verb\b\ may or may not refer to the same object, but
\verb\c\ and \verb\d\ are guaranteed to refer to two different, unique,
newly created lists.
\verb\a\ and \verb\b\ may or may not refer to the same object with the
value one, depending on the implementation, but \verb\c\ and \verb\d\
are guaranteed to refer to two different, unique, newly created empty
lists.
\section{Execution frames, name spaces, and scopes}
\section{Code blocks, execution frames, and name spaces}
XXX code blocks, scopes, name spaces, name binding, exceptions
A ``code block'' is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code block may
textually contain other code blocks. Code blocks may invoke other
code blocks (that aren't textually contained) as part of their
execution.
Each command typed interactively is a separate code block; a script
file is a code block; the string argument passed to the built-in
functions \verb\eval\ and \verb\exec\ are code blocks; the expression
read and evaluated by the built-in function \verb\input\ is a code
block.
A code block is executed in an ``execution frame''. An execution
frame contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
``name spaces'' that affect execution of the code block.
A name space is a mapping from names (identifiers) to objects. A
particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called ``binding'' a name (to an object); changing the mapping of a
name is called ``rebinding''; removing a name from the name space is
called ``unbinding''. Name spaces are functionally equivalent to
dictionaries (described below).
The ``local name space'' of an execution frame determines the default
place where names are defined and searched. The ``global name
space'' determines the place where names listed in \verb\global\
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
Whether a name is local or global in a code block is determined by
static inspection of the source text for the code block: in the
absence of \verb\global\ statements, a name that is bound anywhere in
the code block is local in the entire code block; all other names are
considered global. The \verb\global\ statement forces global
interpretation of selected names throughout the code block. The
following constructs bind names: formal parameters, \verb\import\
statements, class and function definitions (these bind the class or
function name), and targets that are identifiers if occurring in an
assignment, \verb\for\ loop header, or \verb\except\ clause header.
(A target occurring in a \verb\del\ statement does not bind a name.)
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (this is actually the
global name space of the module \verb\builtin\). When a name is not
found at all, the \verb\NameError\ exception is raised.
The following table lists the meaning of the local and global name
space for various types of code blocks. The name space for a
particular module is automatically created when the module is first
referenced.
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
Code block type & Global name space & Local name space & Notes \\
\hline
Module & n.s. for this module & same as global & \\
Script & n.s. for \verb\__main__\ & same as global & \\
Interactive command & n.s. for \verb\__main__\ & same as global & \\
Class definition & global n.s. of containing block & new n.s. & \\
Function body & global n.s. of containing block & new n.s. & \\
String passed to \verb\exec\ or \verb\eval\
& global n.s. of caller & local n.s. of caller & (1) \\
File read by \verb\execfile\
& global n.s. of caller & local n.s. of caller & (1) \\
Expression read by \verb\input\
& global n.s. of caller & local n.s. of caller & \\
\hline
\end{tabular}
\end{center}
Notes:
\begin{description}
\item[(1)] The global and local name space for these functions can be
overridden with optional extra arguments.
\end{description}
\section{Exceptions}
Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors (or other exceptional
conditions). An exception is ``raised'' at the point where the error
is detected; it may be ``handled'' by the surrounding code block or by any
code block that directly or indirectly invoked the code block where
the error occurred.
The Python interpreter raises an exception when it detects an run-time
error (such as division by zero). A Python program can also
explicitly raise an exception with the \verb\raise\ statement.
Exception handlers are specified with the \verb\try...except\ statement.
Python uses the ``termination'' model of error handling: a handler can
find out what happened and continue execution at an outer level, but
it cannot repair the cause of the error and retry the failing
operation (except by re-entering the the offending piece of code from
the top).
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
Exceptions are identified by string objects. Two different string
objects with the same value identify different exceptions.
When an exception is raised, an object (maybe \verb\None\) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
exception handler as additional information.
\chapter{The standard type hierarchy}
@ -700,7 +813,7 @@ call a class object with one or more arguments.
Modules are imported by the \verb\import\ statement (see section
\ref{import}). A module object is a container for a module's name
space, which is a dictionary (the same dictionary as referenced by the
\ver\func_globals\ attribute of functions defined in the module).
\verb\func_globals\ attribute of functions defined in the module).
Module attribute references are translated to lookups in this
dictionary. A module object does not contain the code object used to
initialize the module (since it isn't needed once the initialization
@ -1069,9 +1182,32 @@ call: primary "(" [condition_list] ")"
The primary must evaluate to a callable object (user-defined
functions, built-in functions, methods of built-in objects, class
objects, and methods of class instances are callable). If it is a
class, the argument list must be empty.
class, the argument list must be empty; otherwise, the arguments are
evaluated.
XXX explain what happens on function call
A call always returns some value, possibly \verb\None\, unless it
raises an exception. How this value is computed depends on the type
of the callable object. If it is:
\begin{description}
\item[a user-defined function:] the code block for the function is
executed, passing it the argument list. The first thing the code
block will do is bind the formal parameters to the arguments. When
the code block executes a \verb\return\ statement, this specifies the
return value of the function call.
\item[a built-in function or method:] the result is up to the
interpreter; see the library reference manual for the descriptions of
built-in functions and methods.
\item[a class object:] a new instance of that class is returned.
\item[a class instance method:] the corresponding user-defined
function is called, with an argument list that is one longer than the
argument list of the call: the instance becomes the first argument.
\end{description}
\section{Factors}
@ -1501,9 +1637,9 @@ happens. It is useful as a placeholder when a statement is
required syntactically, but no code needs to be executed, for example:
\begin{verbatim}
def f(arg): pass # a no-op function
def f(arg): pass # a function that does nothing (yet)
class C: pass # an empty class
class C: pass # an class with no methods (yet)
\end{verbatim}
\section{The {\tt del} statement}
@ -1844,7 +1980,7 @@ code for a group of statements:
\begin{verbatim}
try_stmt: "try" ":" suite
("except" condition ["," condition] ":" suite)*
("except" condition ["," target] ":" suite)*
["except" ":" suite]
["finally" ":" suite]
\end{verbatim}
@ -1859,11 +1995,36 @@ clause just executes the suite of statements in its \verb\try\ clause.
The \verb\try...except\ form specifies one or more exception handlers.
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 HIRO
suite, a search for an exception handler is started. This inspects
the except clauses (exception handlers) in turn until one is found
that matches the exception. A condition-less except clause (which
must be last) matches any exception. For except clause with a
condition, that condition is evaluated, and the clause matches the
exception if the resulting object is ``compatible'' with the
exception. An object is compatible with an exception if it is either
the object that identifies the exception or it is a tuple containing
an item that is compatible with the exception.
If no except clause matches the exception, the search for an exception
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.
When a matching except clause is found in a try statement, the
exception's parameter is assigned to the target specified in the
except clause (if present), and the except clause's suite is executed.
When the end of this suite is reached, execution continues normally
at the point following the entire try statement. (This means that if
two nested handlers exist for the same exception, and the exception
occurs in the try clause of the inner handler, the outer handler will
not notice the exception.)
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 on 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
@ -1871,7 +2032,7 @@ executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
the saved exception is lost.
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try suite of a \verb\try...finally\ statement, 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
@ -1895,7 +2056,7 @@ XXX
\begin{verbatim}
classdef: "class" identifier [inheritance] ":" suite
inheritance: "(" expression ("," expression)* ")"
inheritance: "(" condition_list ")"
\end{verbatim}
XXX
@ -1903,7 +2064,7 @@ XXX
\section{P.M.}
XXX Syntax for scripts, modules
XXX Syntax for interactive input, eval, exec, input
XXX Syntax for interactive input, eval, exec, execfile, input
XXX New definition of expressions (as conditions)
\end{document}

221
Doc/ref/ref.tex
View File

@ -431,9 +431,9 @@ some properties of objects that are important to know about.
Every object has an identity, a type and a value. An object's {\em
identity} never changes once it has been created; think of it as the
object's (permanent) address. An object's {\em type} determines the
operations that an object supports (e.g., does it have a length?) and
also defines the ``meaning'' of the object's value. The type also
object's address in memory. An object's {\em type} determines the
operations that an object supports (e.g., ``does it have a length?'')
and also defines the ``meaning'' of the object's value. The type also
never changes. The {\em value} of some objects can change; whether
this is possible is a property of its type.
@ -450,12 +450,13 @@ references.)
Note that the use of the implementation's tracing or debugging
facilities may keep objects alive that would normally be collectable.
(Some objects contain references to ``external'' resources such as
open files. It is understood that these resources are freed when the
object is garbage-collected, but since garbage collection is not
guaranteed, such objects also provide an explicit way to release the
external resource (e.g., a \verb\close\ method). Programs are strongly
recommended to use this.)
Some objects contain references to ``external'' resources such as open
files or windows. It is understood that these resources are freed
when the object is garbage-collected, but since garbage collection is
not guaranteed to happen, such objects also provide an explicit way to
release the external resource, usually a \verb\close\ method.
Programs are strongly recommended to always explicitly close such
objects.
Some objects contain references to other objects. These references
are part of the object's value; in most cases, when such a
@ -463,23 +464,135 @@ are part of the object's value; in most cases, when such a
comparison applies to the {\em values} of the referenced objects (not
their identities).
Types affect almost all aspects of objects.
Even object identity is affected in some sense: for immutable
types, operations that compute new values may actually return a
reference to any existing object with the same type and value, while
for mutable objects this is not allowed. E.g., after
Types affect almost all aspects of an object's life. Even the meaning
of object identity is affected in some sense: for immutable types,
operations that compute new values may actually return a reference to
any existing object with the same type and value, while for mutable
objects this is not allowed. E.g., after
\begin{verbatim}
a = 1; b = 1; c = []; d = []
\end{verbatim}
\verb\a\ and \verb\b\ may or may not refer to the same object, but
\verb\c\ and \verb\d\ are guaranteed to refer to two different, unique,
newly created lists.
\verb\a\ and \verb\b\ may or may not refer to the same object with the
value one, depending on the implementation, but \verb\c\ and \verb\d\
are guaranteed to refer to two different, unique, newly created empty
lists.
\section{Execution frames, name spaces, and scopes}
\section{Code blocks, execution frames, and name spaces}
XXX code blocks, scopes, name spaces, name binding, exceptions
A ``code block'' is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code block may
textually contain other code blocks. Code blocks may invoke other
code blocks (that aren't textually contained) as part of their
execution.
Each command typed interactively is a separate code block; a script
file is a code block; the string argument passed to the built-in
functions \verb\eval\ and \verb\exec\ are code blocks; the expression
read and evaluated by the built-in function \verb\input\ is a code
block.
A code block is executed in an ``execution frame''. An execution
frame contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
``name spaces'' that affect execution of the code block.
A name space is a mapping from names (identifiers) to objects. A
particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called ``binding'' a name (to an object); changing the mapping of a
name is called ``rebinding''; removing a name from the name space is
called ``unbinding''. Name spaces are functionally equivalent to
dictionaries (described below).
The ``local name space'' of an execution frame determines the default
place where names are defined and searched. The ``global name
space'' determines the place where names listed in \verb\global\
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
Whether a name is local or global in a code block is determined by
static inspection of the source text for the code block: in the
absence of \verb\global\ statements, a name that is bound anywhere in
the code block is local in the entire code block; all other names are
considered global. The \verb\global\ statement forces global
interpretation of selected names throughout the code block. The
following constructs bind names: formal parameters, \verb\import\
statements, class and function definitions (these bind the class or
function name), and targets that are identifiers if occurring in an
assignment, \verb\for\ loop header, or \verb\except\ clause header.
(A target occurring in a \verb\del\ statement does not bind a name.)
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (this is actually the
global name space of the module \verb\builtin\). When a name is not
found at all, the \verb\NameError\ exception is raised.
The following table lists the meaning of the local and global name
space for various types of code blocks. The name space for a
particular module is automatically created when the module is first
referenced.
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
Code block type & Global name space & Local name space & Notes \\
\hline
Module & n.s. for this module & same as global & \\
Script & n.s. for \verb\__main__\ & same as global & \\
Interactive command & n.s. for \verb\__main__\ & same as global & \\
Class definition & global n.s. of containing block & new n.s. & \\
Function body & global n.s. of containing block & new n.s. & \\
String passed to \verb\exec\ or \verb\eval\
& global n.s. of caller & local n.s. of caller & (1) \\
File read by \verb\execfile\
& global n.s. of caller & local n.s. of caller & (1) \\
Expression read by \verb\input\
& global n.s. of caller & local n.s. of caller & \\
\hline
\end{tabular}
\end{center}
Notes:
\begin{description}
\item[(1)] The global and local name space for these functions can be
overridden with optional extra arguments.
\end{description}
\section{Exceptions}
Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors (or other exceptional
conditions). An exception is ``raised'' at the point where the error
is detected; it may be ``handled'' by the surrounding code block or by any
code block that directly or indirectly invoked the code block where
the error occurred.
The Python interpreter raises an exception when it detects an run-time
error (such as division by zero). A Python program can also
explicitly raise an exception with the \verb\raise\ statement.
Exception handlers are specified with the \verb\try...except\ statement.
Python uses the ``termination'' model of error handling: a handler can
find out what happened and continue execution at an outer level, but
it cannot repair the cause of the error and retry the failing
operation (except by re-entering the the offending piece of code from
the top).
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
Exceptions are identified by string objects. Two different string
objects with the same value identify different exceptions.
When an exception is raised, an object (maybe \verb\None\) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
exception handler as additional information.
\chapter{The standard type hierarchy}
@ -700,7 +813,7 @@ call a class object with one or more arguments.
Modules are imported by the \verb\import\ statement (see section
\ref{import}). A module object is a container for a module's name
space, which is a dictionary (the same dictionary as referenced by the
\ver\func_globals\ attribute of functions defined in the module).
\verb\func_globals\ attribute of functions defined in the module).
Module attribute references are translated to lookups in this
dictionary. A module object does not contain the code object used to
initialize the module (since it isn't needed once the initialization
@ -1069,9 +1182,32 @@ call: primary "(" [condition_list] ")"
The primary must evaluate to a callable object (user-defined
functions, built-in functions, methods of built-in objects, class
objects, and methods of class instances are callable). If it is a
class, the argument list must be empty.
class, the argument list must be empty; otherwise, the arguments are
evaluated.
XXX explain what happens on function call
A call always returns some value, possibly \verb\None\, unless it
raises an exception. How this value is computed depends on the type
of the callable object. If it is:
\begin{description}
\item[a user-defined function:] the code block for the function is
executed, passing it the argument list. The first thing the code
block will do is bind the formal parameters to the arguments. When
the code block executes a \verb\return\ statement, this specifies the
return value of the function call.
\item[a built-in function or method:] the result is up to the
interpreter; see the library reference manual for the descriptions of
built-in functions and methods.
\item[a class object:] a new instance of that class is returned.
\item[a class instance method:] the corresponding user-defined
function is called, with an argument list that is one longer than the
argument list of the call: the instance becomes the first argument.
\end{description}
\section{Factors}
@ -1501,9 +1637,9 @@ happens. It is useful as a placeholder when a statement is
required syntactically, but no code needs to be executed, for example:
\begin{verbatim}
def f(arg): pass # a no-op function
def f(arg): pass # a function that does nothing (yet)
class C: pass # an empty class
class C: pass # an class with no methods (yet)
\end{verbatim}
\section{The {\tt del} statement}
@ -1844,7 +1980,7 @@ code for a group of statements:
\begin{verbatim}
try_stmt: "try" ":" suite
("except" condition ["," condition] ":" suite)*
("except" condition ["," target] ":" suite)*
["except" ":" suite]
["finally" ":" suite]
\end{verbatim}
@ -1859,11 +1995,36 @@ clause just executes the suite of statements in its \verb\try\ clause.
The \verb\try...except\ form specifies one or more exception handlers.
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 HIRO
suite, a search for an exception handler is started. This inspects
the except clauses (exception handlers) in turn until one is found
that matches the exception. A condition-less except clause (which
must be last) matches any exception. For except clause with a
condition, that condition is evaluated, and the clause matches the
exception if the resulting object is ``compatible'' with the
exception. An object is compatible with an exception if it is either
the object that identifies the exception or it is a tuple containing
an item that is compatible with the exception.
If no except clause matches the exception, the search for an exception
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.
When a matching except clause is found in a try statement, the
exception's parameter is assigned to the target specified in the
except clause (if present), and the except clause's suite is executed.
When the end of this suite is reached, execution continues normally
at the point following the entire try statement. (This means that if
two nested handlers exist for the same exception, and the exception
occurs in the try clause of the inner handler, the outer handler will
not notice the exception.)
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 on 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
@ -1871,7 +2032,7 @@ executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
the saved exception is lost.
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try suite of a \verb\try...finally\ statement, 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
@ -1895,7 +2056,7 @@ XXX
\begin{verbatim}
classdef: "class" identifier [inheritance] ":" suite
inheritance: "(" expression ("," expression)* ")"
inheritance: "(" condition_list ")"
\end{verbatim}
XXX
@ -1903,7 +2064,7 @@ XXX
\section{P.M.}
XXX Syntax for scripts, modules
XXX Syntax for interactive input, eval, exec, input
XXX Syntax for interactive input, eval, exec, execfile, input
XXX New definition of expressions (as conditions)
\end{document}