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Changes copied from the FrameMaker version. Also documented some 

previously undocumented features.

Packages and the import mechanism in general are still left
undocumented.
This commit is contained in:
Guido van Rossum 1998-07-24 18:25:38 +00:00
parent 34116ba933
commit 56c2013d15
1 changed files with 141 additions and 83 deletions
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Doc/ref/ref6.tex
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@ -7,6 +7,7 @@ by semicolons. The syntax for simple statements is:
\begin{verbatim}
simple_stmt: expression_stmt
| assert_stmt
| assignment_stmt
| pass_stmt
| del_stmt
@ -26,22 +27,23 @@ 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
\code{None}):
\code{None}). Other uses of expression statements are allowed and
occasionally useful. The syntax for an expression statement is:
\begin{verbatim}
expression_stmt: condition_list
expression_stmt: expression_list
\end{verbatim}
An expression statement evaluates the condition list (which may be a
single condition).
An expression statement evaluates the expression list (which may be a
single expression).
\indexii{expression}{list}
In interactive mode, if the value is not \code{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.
(The exception for \code{None} is made so that procedure calls, which
are syntactically equivalent to expressions, do not cause any output.)
to a string using the built-in \function{repr()}\bifuncindex{repr}
function and the resulting string is written to standard output (see
section \ref{print}) on a line by itself. (Expression statements
yielding None are not written, so that procedure calls do not cause
any output.)
\ttindex{None}
\indexii{string}{conversion}
\index{output}
@ -49,6 +51,41 @@ are syntactically equivalent to expressions, do not cause any output.)
\indexii{writing}{values}
\indexii{procedure}{call}
\section{Assert statements}\stindex{assert}
Assert statements are a convenient way to insert debugging
assertions\indexii{debugging}{assertions} into a program:
\begin{verbatim}
assert_statement: "assert" expression ["," expression]
\end{verbatim}
The simple form, ``\code{assert expression}'', is equivalent to
\begin{verbatim}
if __debug__:
if not expression: raise AssertionError
\end{verbatim}
The extended form, ``\code{assert expression1, expression2}'', is
equivalent to
\begin{verbatim}
if __debug__:
if not expression1: raise AssertionError, expression2
\end{verbatim}
These equivalences assume that \code{__debug__}\ttindex{__debug__} and
\code{AssertionError}\exindex{AssertionError} refer to the built-in
variables with those names. In the current implementation, the
built-in variable \code{__debug__} is 1 under normal circumstances, 0
when optimization is requested (command line option -O). The current
code generator emits no code for an assert statement when optimization
is requested at compile time. Note that it is unnecessary to include
the source code for the expression that failed in the error message;
it will be displayed as part of the stack trace.
\section{Assignment statements}
\indexii{assignment}{statement}
@ -91,14 +128,17 @@ follows.
\begin{itemize}
\item
If the target list is a single target: the object is assigned to that
If the target list is a single target: The object is assigned to that
target.
\item
If the target list is a comma-separated list of targets: the object
must be a tuple with the same number of items as the list contains
targets, and the items are assigned, from left to right, to the
corresponding targets.
If the target list is a comma-separated list of targets: The object
must be a sequence with the same number of items as the there are
targets in the target list, and the items are assigned, from left to
right, to the corresponding targets. (This rule is relaxed as of
Python 1.5; in earlier versions, the object had to be a tuple. Since
strings are sequences, an assignment like ``\code{a, b = "xy"}'' is
now legal as long as the string has the right length.)
\end{itemize}
@ -114,27 +154,26 @@ If the target is an identifier (name):
\item
If the name does not occur in a \keyword{global} statement in the current
code block: the name is bound to the object in the current local name
space.
code block: the name is bound to the object in the current local
namespace.
\stindex{global}
\item
Otherwise: the name is bound to the object in the current global name
space.
Otherwise: the name is bound to the object in the current global
namespace.
\end{itemize} % nested
The name is rebound if it was already bound.
The name is rebound if it was already bound. This may cause the
reference count for the object previously bound to the name to reach
zero, causing the object to be deallocated and its
destructor\index{destructor} (if it has one) to be called.
\item
If the target is a target list enclosed in parentheses: the object is
assigned to that target list as described above.
\item
If the target is a target list enclosed in square brackets: the object
must be a list with the same number of items as the target list
contains targets, and its items are assigned, from left to right, to
the corresponding targets.
If the target is a target list enclosed in parentheses or in square
brackets: The object must be a sequence with the same number of items
as there are targets in the target list, and its items are assigned,
from left to right, to the corresponding targets.
\item
If the target is an attribute reference: The primary expression in the
@ -148,12 +187,12 @@ attribute; if it cannot perform the assignment, it raises an exception
\item
If the target is a subscription: The primary expression in the
reference is evaluated. It should yield either a mutable sequence
(list) object or a mapping (dictionary) object. Next, the subscript
expression is evaluated.
object (e.g., a list) or a mapping object (e.g., a dictionary). Next,
the subscript expression is evaluated.
\indexii{subscription}{assignment}
\obindex{mutable}
If the primary is a mutable sequence object (a list), the subscript
If the primary is a mutable sequence object (e.g., a list), the subscript
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
@ -163,7 +202,7 @@ sequence cannot add new items to a list).
\obindex{sequence}
\obindex{list}
If the primary is a mapping (dictionary) object, the subscript must
If the primary is a mapping object (e.g., a dictionary), the subscript must
have a type compatible with the mapping's key type, and the mapping is
then asked to create a key/datum pair which maps the subscript to
the assigned object. This can either replace an existing key/value
@ -174,7 +213,7 @@ 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 object (e.g. a list). 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
@ -195,10 +234,10 @@ during the code generation phase, causing less detailed error
messages.)
WARNING: Although the definition of assignment implies that overlaps
between the left-hand side and the right-hand side are `safe' (e.g.
\code{a, b = b, a} swaps two variables), overlaps within the
between the left-hand side and the right-hand side are `safe' (e.g.,
``\code{a, b = b, a}'' swaps two variables), overlaps \emph{within} the
collection of assigned-to variables are not safe! For instance, the
following program prints \code{[0, 2]}:
following program prints ``\code{[0, 2]}'':
\begin{verbatim}
x = [0, 1]
@ -243,7 +282,7 @@ Deletion of a target list recursively deletes each target, from left
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
from the local or global namespace, depending on whether the name
occurs in a \keyword{global} statement in the same code block.
\stindex{global}
\indexii{unbinding}{name}
@ -258,18 +297,18 @@ right type (but even this is determined by the sliced object).
\stindex{print}
\begin{verbatim}
print_stmt: "print" [ condition ("," condition)* [","] ]
print_stmt: "print" [ expression ("," expression)* [","] ]
\end{verbatim}
\keyword{print} evaluates each condition in turn and writes the resulting
\keyword{print} evaluates each expression 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
space is written before each object is (converted and) written, unless
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 \character{\\n}; or (3) when the last write operation on standard
line. This is the case (1) when no characters have yet been written
to standard output, (2) when the last character written to standard
output is \character{\\n}, or (3) when the last write operation on standard
output was not a \keyword{print} statement. (In some cases it may be
functional to write an empty string to standard output for this
reason.)
@ -283,10 +322,9 @@ contains just the keyword \keyword{print}.
\indexii{newline}{suppression}
Standard output is defined as the file object named \code{stdout}
in the built-in module \module{sys}. If no such object exists,
or if it is not a writable file, a \exception{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.)
in the built-in module \module{sys}. If no such object exists, or if
it does not have a \method{write()} method, a \exception{RuntimeError}
exception is raised.
\indexii{standard}{output}
\refbimodindex{sys}
\ttindex{stdout}
@ -296,7 +334,7 @@ standard output instead, but this is not safe, and should be fixed.)
\stindex{return}
\begin{verbatim}
return_stmt: "return" [condition_list]
return_stmt: "return" [expression_list]
\end{verbatim}
\keyword{return} may only occur syntactically nested in a function
@ -304,14 +342,14 @@ definition, not within a nested class definition.
\indexii{function}{definition}
\indexii{class}{definition}
If a condition list is present, it is evaluated, else \code{None}
If an expression list is present, it is evaluated, else \code{None}
is substituted.
\keyword{return} leaves the current function call with the condition
\keyword{return} leaves the current function call with the expression
list (or \code{None}) as return value.
When \keyword{return} passes control out of a \keyword{try} statement
with a finally clause, that finally clause is executed
with a \keyword{finally} clause, that \keyword{finally} clause is executed
before really leaving the function.
\kwindex{finally}
@ -319,15 +357,18 @@ before really leaving the function.
\stindex{raise}
\begin{verbatim}
raise_stmt: "raise" condition ["," condition ["," condition]]
raise_stmt: "raise" [expression ["," expression ["," expression]]]
\end{verbatim}
\keyword{raise} evaluates its first condition, which must yield
a string, class, or instance object. If there is a second condition,
If no expressions are present, \keyword{raise} re-raises the last
expression that was raised in the current scope.
Otherwose, \keyword{raise} evaluates its first expression, which must yield
a string, class, or instance object. If there is a second expression,
this is evaluated, else \code{None} is substituted. If the first
condition is a class object, then the second condition must be an
expression is a class object, then the second expression must be an
instance of that class or one of its derivatives. If the first
condition is an instance object, the second condition must be
expression is an instance object, the second expression must be
\code{None}.
\index{exception}
\indexii{raising}{exception}
@ -339,7 +380,7 @@ exception identified by the class of the object, with the instance as
its parameter (and there should be no second object, or the second
object should be \code{None}).
If a third object is present, and it it not \code{None}, it should be
If a third object is present, and it is not \code{None}, it should be
a traceback object (see section \ref{traceback}), and it is
substituted instead of the current location as the place where the
exception occurred. This is useful to re-raise an exception
@ -361,7 +402,7 @@ within that loop.
\indexii{loop}{statement}
It terminates the nearest enclosing loop, skipping the optional
else clause if the loop has one.
\keyword{else} clause if the loop has one.
\kwindex{else}
If a \keyword{for} loop is terminated by \keyword{break}, the loop control
@ -369,7 +410,7 @@ target keeps its current value.
\indexii{loop control}{target}
When \keyword{break} passes control out of a \keyword{try} statement
with a finally clause, that finally clause is executed
with a \keyword{finally} clause, that \keyword{finally} clause is executed
before really leaving the loop.
\kwindex{finally}
@ -382,45 +423,45 @@ continue_stmt: "continue"
\keyword{continue} may only occur syntactically nested in a \keyword{for} or
\keyword{while} loop, but not nested in a function or class definition or
\keyword{try} statement within that loop.\footnote{Except that it may
currently occur within an except clause.}
\keyword{try} statement within that loop.\footnote{It may
occur within an \keyword{except} or \keyword{else} clause. The
restriction on occurring in the \keyword{try} clause is implementer's
laziness and will eventually be lifted.}
It continues with the next cycle of the nearest enclosing loop.
\stindex{for}
\stindex{while}
\indexii{loop}{statement}
\kwindex{finally}
It continues with the next cycle of the nearest enclosing loop.
\section{The \keyword{import} statement} \label{import}
\stindex{import}
\begin{verbatim}
import_stmt: "import" identifier ("," identifier)*
| "from" identifier "import" identifier ("," identifier)*
| "from" identifier "import" "*"
import_stmt: "import" module ("," module)*
| "from" module "import" identifier ("," identifier)*
| "from" module "import" "*"
module: (identifier ".")* identifier
\end{verbatim}
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 \keyword{import} statement occurs).
namespace (of the scope where the \keyword{import} statement occurs).
The first form (without \keyword{from}) repeats these steps for each
identifier in the list, the \keyword{from} form performs them once, with
the first identifier specifying the module name.
identifier in the list. The form with \keyword{from} performs step
(1) once, and then performs step (2) repeatedly.
\indexii{importing}{module}
\indexii{name}{binding}
\kwindex{from}
The system maintains a table of modules that have been initialized,
indexed by module name. (The current implementation makes this table
accessible as \code{sys.modules}.) When a module name is found in
indexed by module name. This table table
accessible as \code{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 \file{.py}. (The current
implementation uses the list of strings \code{sys.path} as the search
path; it is initialized from the shell environment variable
\envvar{PYTHONPATH}, with an installation-dependent default.)
definition is started. When a module is found, it is loaded. Details
of the module searching and loading process are implementation and
platform specific. It generally involves searching for a ``built-in''
module with the given name and then searching a list of locations
given as \code{sys.path}.
\ttindex{modules}
\ttindex{sys.modules}
\indexii{module}{name}
@ -447,19 +488,19 @@ When step (1) finishes without raising an exception, step (2) can
begin.
The first form of \keyword{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 \keyword{from} from does not bind the
local namespace to the module object, and then goes on to import the
next identifier, if any. The \keyword{from} form 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,
local namespace to the object thus found. If a name is not found,
\exception{ImportError} is raised. If the list of identifiers is replaced
by a star (\code{*}), all names defined in the module are bound,
except those beginning with an underscore(\code{_}).
\indexii{name}{binding}
\exindex{ImportError}
Names bound by import statements may not occur in \keyword{global}
statements in the same scope.
Names bound by \keyword{import} statements may not occur in
\keyword{global} statements in the same scope.
\stindex{global}
The \keyword{from} form with \code{*} may only occur in a module scope.
@ -471,6 +512,21 @@ restrictions, but programs should not abuse this freedom, as future
implementations may enforce them or silently change the meaning of the
program.)
\strong{Hierarchical module names:}\indexiii{hierarchical}{module names}
when the module names contains one or more dots, the module search
path is carried out differently. The sequence of identifiers up to
the last dot is used to find a ``package''\index{packages}; the final
identifier is then searched inside the package. A package is
generally a subdirectory of a directory on \code{sys.path} that has a
file \file{__init__.py}.\ttindex{__init__.py}
%
[XXX Can't be bothered to spell this out right now; see the URL
http://www.python.org/doc/essays/packages.html for more details, also
about how the module search works from inside a package.]
[XXX Also should mention __import__().]
\bifuncindex{__import__}
\section{The \keyword{global} statement} \label{global}
\stindex{global}
@ -497,7 +553,8 @@ restrictions, but programs should not abuse this freedom, as future
implementations may enforce them or silently change the meaning of the
program.)
Note: the \keyword{global} is a directive to the parser. Therefore, it
\strong{Programmer's note:}
the \keyword{global} is a directive to the parser. It
applies only to code parsed at the same time as the \keyword{global}
statement. In particular, a \keyword{global} statement contained in an
\keyword{exec} statement does not affect the code block \emph{containing}
@ -539,7 +596,8 @@ may add a reference to the dictionary of the built-in module
\ttindex{__builtins__}
\refbimodindex{__builtin__}
Hints: dynamic evaluation of expressions is supported by the built-in
\strong{Programmer's hints:}
dynamic evaluation of expressions is supported by the built-in
function \function{eval()}. The built-in functions
\function{globals()} and \function{locals()} return the current global
and local dictionary, respectively, which may be useful to pass around