\chapter{Simple statements \label{simple}} \indexii{simple}{statement} Simple statements are comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is: \begin{productionlist} \production{simple_stmt} {\token{expression_stmt} | \token{assert_stmt} | \token{assignment_stmt} | \token{augmented_assignment_stmt} | \token{pass_stmt} | \token{del_stmt} | \token{print_stmt} | \token{return_stmt} | \token{yield_stmt} | \token{raise_stmt} | \token{break_stmt} | \token{continue_stmt} | \token{import_stmt} | \token{global_stmt} | \token{exec_stmt}} \end{productionlist} \section{Expression statements \label{exprstmts}} \indexii{expression}{statement} 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}). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is: \begin{productionlist} \production{expression_stmt} {\token{expression_list}} \end{productionlist} 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 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 \code{None} are not written, so that procedure calls do not cause any output.) \ttindex{None} \indexii{string}{conversion} \index{output} \indexii{standard}{output} \indexii{writing}{values} \indexii{procedure}{call} \section{Assert statements \label{assert}} Assert statements\stindex{assert} are a convenient way to insert debugging assertions\indexii{debugging}{assertions} into a program: \begin{productionlist} \production{assert_statement} {"assert" \token{expression} ["," \token{expression}]} \end{productionlist} The simple form, \samp{assert expression}, is equivalent to \begin{verbatim} if __debug__: if not expression: raise AssertionError \end{verbatim} The extended form, \samp{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 \exception{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. Assignments to \code{__debug__} are illegal. The value for the built-in variable is determined when the interpreter starts. \section{Assignment statements \label{assignment}} Assignment statements\indexii{assignment}{statement} are used to (re)bind names to values and to modify attributes or items of mutable objects: \indexii{binding}{name} \indexii{rebinding}{name} \obindex{mutable} \indexii{attribute}{assignment} \begin{productionlist} \production{assignment_stmt} {(\token{target_list} "=")+ \token{expression_list}} \production{target_list} {\token{target} ("," \token{target})* [","]} \production{target} {\token{identifier} | "(" \token{target_list} ")" | "[" \token{target_list} "]" | \token{attributeref} | \token{subscription} | \token{slicing}} \end{productionlist} (See section~\ref{primaries} for the syntax definitions for the last three symbols.) An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right. \indexii{expression}{list} Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section~\ref{types}). \index{target} \indexii{target}{list} Assignment of an object to a target list is recursively defined as follows. \indexiii{target}{list}{assignment} \begin{itemize} \item 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 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 \samp{a, b = "xy"} is now legal as long as the string has the right length.) \end{itemize} Assignment of an object to a single target is recursively defined as follows. \begin{itemize} % nested \item If the target is an identifier (name): \begin{itemize} \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 namespace. \stindex{global} \item 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. 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 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 reference is evaluated. It should yield an object with assignable attributes; if this is not the case, \exception{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 \exception{AttributeError}). \indexii{attribute}{assignment} \item If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence 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 (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 the assigned object to its item with that index. If the index is out of range, \exception{IndexError} is raised (assignment to a subscripted sequence cannot add new items to a list). \obindex{sequence} \obindex{list} 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 pair with the same key value, or insert a new key/value pair (if no key with the same value existed). \obindex{mapping} \obindex{dictionary} \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 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 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} (In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected 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., \samp{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 \samp{[0, 2]}: \begin{verbatim} x = [0, 1] i = 0 i, x[i] = 1, 2 print x \end{verbatim} \subsection{Augmented Assignment statements \label{augassign}} Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement: \indexii{augmented}{assignment} \index{statement!assignment, augmented} \begin{productionlist} \production{augmented_assignment_stmt} {\token{target} \token{augop} \token{expression_list}} \production{augop} {"+=" | "-=" | "*=" | "/=" | "\%=" | "**=" | ">>=" | "<<=" | "\&=" | "\textasciicircum=" | "|="} \production{target} {\token{identifier} | "(" \token{target_list} ")" | "[" \token{target_list} "]" | \token{attributeref} | \token{subscription} | \token{slicing}} \end{productionlist} (See section~\ref{primaries} for the syntax definitions for the last three symbols.) An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once. An augmented assignment expression like \code{x += 1} can be rewritten as \code{x = x + 1} to achieve a similar, but not exactly equal effect. In the augmented version, \code{x} is only evaluated once. Also, when possible, the actual operation is performed \emph{in-place}, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead. With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible \emph{in-place} behavior, the binary operation performed by augmented assignment is the same as the normal binary operations. \section{The \keyword{pass} statement \label{pass}} \stindex{pass} \begin{productionlist} \production{pass_stmt} {"pass"} \end{productionlist} \keyword{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} \begin{verbatim} def f(arg): pass # a function that does nothing (yet) class C: pass # a class with no methods (yet) \end{verbatim} \section{The \keyword{del} statement \label{del}} \stindex{del} \begin{productionlist} \production{del_stmt} {"del" \token{target_list}} \end{productionlist} Deletion is recursively defined very similar to the way assignment is defined. Rather that spelling it out in full details, here are some hints. \indexii{deletion}{target} \indexiii{deletion}{target}{list} 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 namespace, depending on whether the name occurs in a \keyword{global} statement in the same code block. \stindex{global} \indexii{unbinding}{name} Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object). \indexii{attribute}{deletion} \section{The \keyword{print} statement \label{print}} \stindex{print} \begin{productionlist} \production{print_stmt} {"print" ( \optional{\token{expression} ("," \token{expression})* \optional{","}} | ">\code{>}" \token{expression} \optional{("," \token{expression})+ \optional{","}})} \end{productionlist} \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, (2) when the last character written to standard output is \character{\e 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.) \note{Objects which act like file objects but which are not the built-in file objects often do not properly emulate this aspect of the file object's behavior, so it is best not to rely on this.} \index{output} \indexii{writing}{values} A \character{\e n} character is written at the end, unless the \keyword{print} statement ends with a comma. This is the only action if the statement contains just the keyword \keyword{print}. \indexii{trailing}{comma} \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 does not have a \method{write()} method, a \exception{RuntimeError} exception is raised. \indexii{standard}{output} \refbimodindex{sys} \withsubitem{(in module sys)}{\ttindex{stdout}} \exindex{RuntimeError} \keyword{print} also has an extended\index{extended print statement} form, defined by the second portion of the syntax described above. This form is sometimes referred to as ``\keyword{print} chevron.'' In this form, the first expression after the \code{>}\code{>} must evaluate to a ``file-like'' object, specifically an object that has a \method{write()} method as described above. With this extended form, the subsequent expressions are printed to this file object. If the first expression evaluates to \code{None}, then \code{sys.stdout} is used as the file for output. \section{The \keyword{return} statement \label{return}} \stindex{return} \begin{productionlist} \production{return_stmt} {"return" [\token{expression_list}]} \end{productionlist} \keyword{return} may only occur syntactically nested in a function definition, not within a nested class definition. \indexii{function}{definition} \indexii{class}{definition} If an expression list is present, it is evaluated, else \code{None} is substituted. \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 \keyword{finally} clause, that \keyword{finally} clause is executed before really leaving the function. \kwindex{finally} In a generator function, the \keyword{return} statement is not allowed to include an \grammartoken{expression_list}. In that context, a bare \keyword{return} indicates that the generator is done and will cause \exception{StopIteration} to be raised. \section{The \keyword{yield} statement \label{yield}} \stindex{yield} \begin{productionlist} \production{yield_stmt} {"yield" \token{expression_list}} \end{productionlist} \index{generator!function} \index{generator!iterator} \index{function!generator} \exindex{StopIteration} The \keyword{yield} statement is only used when defining a generator function, and is only used in the body of the generator function. Using a \keyword{yield} statement in a function definition is sufficient to cause that definition to create a generator function instead of a normal function. When a generator function is called, it returns an iterator known as a generator iterator, or more commonly, a generator. The body of the generator function is executed by calling the generator's \method{next()} method repeatedly until it raises an exception. When a \keyword{yield} statement is executed, the state of the generator is frozen and the value of \grammartoken{expression_list} is returned to \method{next()}'s caller. By ``frozen'' we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, and the internal evaluation stack: enough information is saved so that the next time \method{next()} is invoked, the function can proceed exactly as if the \keyword{yield} statement were just another external call. The \keyword{yield} statement is not allowed in the \keyword{try} clause of a \keyword{try} ...\ \keyword{finally} construct. The difficulty is that there's no guarantee the generator will ever be resumed, hence no guarantee that the \keyword{finally} block will ever get executed. \note{In Python 2.2, the \keyword{yield} statement is only allowed when the \code{generators} feature has been enabled. It will always be enabled in Python 2.3. This \code{__future__} import statment can be used to enable the feature:} \begin{verbatim} from __future__ import generators \end{verbatim} \begin{seealso} \seepep{0255}{Simple Generators} {The proposal for adding generators and the \keyword{yield} statement to Python.} \end{seealso} \section{The \keyword{raise} statement \label{raise}} \stindex{raise} \begin{productionlist} \production{raise_stmt} {"raise" [\token{expression} ["," \token{expression} ["," \token{expression}]]]} \end{productionlist} If no expressions are present, \keyword{raise} re-raises the last expression that was raised in the current scope. Otherwise, \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 expression is a class object, then the second expression may be an instance of that class or one of its derivatives, and then that instance is raised. If the second expression is not such an instance, the given class is instantiated. The argument list for the instantiation is determined as follows: if the second expression is a tuple, it is used as the argument list; if it is \code{None}, the argument list is empty; otherwise, the argument list consists of a single argument which is the second expression. If the first expression is an instance object, the second expression must be \code{None}. \index{exception} \indexii{raising}{exception} If the first object is a string, it then raises the exception identified by the first object, with the second one (or \code{None}) as its parameter. If the first object is a class or instance, it raises the exception identified by the class of the instance determined in the previous step, with the instance as its parameter. 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 transparently in an except clause. \obindex{traceback} \section{The \keyword{break} statement \label{break}} \stindex{break} \begin{productionlist} \production{break_stmt} {"break"} \end{productionlist} \keyword{break} may only occur syntactically nested in a \keyword{for} or \keyword{while} loop, but not nested in a function or class definition within that loop. \stindex{for} \stindex{while} \indexii{loop}{statement} It terminates the nearest enclosing loop, skipping the optional \keyword{else} clause if the loop has one. \kwindex{else} If a \keyword{for} loop is terminated by \keyword{break}, the loop control target keeps its current value. \indexii{loop control}{target} When \keyword{break} passes control out of a \keyword{try} statement with a \keyword{finally} clause, that \keyword{finally} clause is executed before really leaving the loop. \kwindex{finally} \section{The \keyword{continue} statement \label{continue}} \stindex{continue} \begin{productionlist} \production{continue_stmt} {"continue"} \end{productionlist} \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{It may occur within an \keyword{except} or \keyword{else} clause. The restriction on occurring in the \keyword{try} clause is implementor'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} \section{The \keyword{import} statement \label{import}} \stindex{import} \begin{productionlist} \production{import_stmt} {"import" \token{module} ["as" \token{name}] ( "," \token{module} ["as" \token{name}] )* | "from" \token{module} "import" \token{identifier} ["as" \token{name}] ( "," \token{identifier} ["as" \token{name}] )* | "from" \token{module} "import" "*"} \production{module} {(\token{identifier} ".")* \token{identifier}} \end{productionlist} 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 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 form with \keyword{from} performs step (1) once, and then performs step (2) repeatedly. \indexii{importing}{module} \indexii{name}{binding} \kwindex{from} % XXX Need to define what ``initialize'' means here The system maintains a table of modules that have been initialized, indexed by module name. This table is 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. 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}. \withsubitem{(in module sys)}{\ttindex{modules}} \ttindex{sys.modules} \indexii{module}{name} \indexii{built-in}{module} \indexii{user-defined}{module} \refbimodindex{sys} \indexii{filename}{extension} \indexiii{module}{search}{path} 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, \exception{ImportError} is raised. If a file is found, it is parsed, yielding an executable code block. If a syntax error occurs, \exception{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). \indexii{module}{initialization} \exindex{SyntaxError} \exindex{ImportError} \index{code block} 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 namespace to the module object, and then goes on to import the next identifier, if any. If the module name is followed by \keyword{as}, the name following \keyword{as} is used as the local name for the module. To avoid confusion, you cannot import modules with dotted names \keyword{as} a different local name. So \code{import module as m} is legal, but \code{import module.submod as s} is not. The latter should be written as \code{from module import submod as s}; see below. 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 namespace to the object thus found. As with the first form of \keyword{import}, an alternate local name can be supplied by specifying "\keyword{as} localname". If a name is not found, \exception{ImportError} is raised. If the list of identifiers is replaced by a star (\character{*}), all public names defined in the module are bound in the local namespace of the \keyword{import} statement.. \indexii{name}{binding} \exindex{ImportError} The \emph{public names} defined by a module are determined by checking the module's namespace for a variable named \code{__all__}; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in \code{__all__} are all considered public and are required to exist. If \code{__all__} is not defined, the set of public names includes all names found in the module's namespace which do not begin with an underscore character (\character{_}). Names bound by \keyword{import} statements may not occur in \keyword{global} statements in the same scope. \stindex{global} The \keyword{from} form with \samp{*} may only occur in a module scope. \kwindex{from} \stindex{from} \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 \url{http://www.python.org/doc/essays/packages.html} for more details, also about how the module search works from inside a package.] The built-in function \function{__import__()} is provided to support applications that determine which modules need to be loaded dynamically; refer to \ulink{Built-in Functions}{../lib/built-in-funcs.html} in the \citetitle[../lib/lib.html]{Python Library Reference} for additional information. \bifuncindex{__import__} \section{The \keyword{global} statement \label{global}} \stindex{global} \begin{productionlist} \production{global_stmt} {"global" \token{identifier} ("," \token{identifier})*} \end{productionlist} The \keyword{global} statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals. While \emph{using} global names is automatic if they are not defined in the local scope, \emph{assigning} to global names would be impossible without \keyword{global}. \indexiii{global}{name}{binding} Names listed in a \keyword{global} statement must not be used in the same code block textually preceding that \keyword{global} statement. Names listed in a \keyword{global} statement must not be defined as formal parameters or in a \keyword{for} loop control target, \keyword{class} definition, function definition, or \keyword{import} statement. (The current implementation does not enforce the latter two restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.) \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} the \keyword{exec} statement, and code contained in an \keyword{exec} statement is unaffected by \keyword{global} statements in the code containing the \keyword{exec} statement. The same applies to the \function{eval()}, \function{execfile()} and \function{compile()} functions. \stindex{exec} \bifuncindex{eval} \bifuncindex{execfile} \bifuncindex{compile} \section{The \keyword{exec} statement \label{exec}} \stindex{exec} \begin{productionlist} \production{exec_stmt} {"exec" \token{expression} ["in" \token{expression} ["," \token{expression}]]} \end{productionlist} This statement supports dynamic execution of Python code. The first expression should evaluate to either a string, an open file object, or a code object. If it is a string, the string is parsed as a suite of Python statements which is then executed (unless a syntax error 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 \keyword{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. As a side effect, an implementation may insert additional keys into the dictionaries given besides those corresponding to variable names set by the executed code. For example, the current implementation may add a reference to the dictionary of the built-in module \module{__builtin__} under the key \code{__builtins__} (!). \ttindex{__builtins__} \refbimodindex{__builtin__} \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 for use by \keyword{exec}. \bifuncindex{eval} \bifuncindex{globals} \bifuncindex{locals} Also, in the current implementation, multi-line compound statements must end with a newline: \code{exec "for v in seq:\e{}n\e{}tprint v\e{}n"} works, but \code{exec "for v in seq:\e{}n\e{}tprint v"} fails with \exception{SyntaxError}. \exindex{SyntaxError}