# cpython-withatomic / Doc / ref7.tex

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  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 \chapter{Compound statements} \indexii{compound}{statement} Compound statements contain (groups of) other statements; they affect 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 handlers and/or cleanup code for a group of statements. Function and class definitions are also syntactically compound statements. Compound statements consist of one or more clauses'. A clause consists of a header and a suite'. The clause headers of a particular compound statement are all at the same indentation level. Each clause header begins with a uniquely identifying keyword and ends with a colon. A suite is a group of statements controlled by a clause. A suite can be one or more semicolon-separated simple 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 belong: \index{clause} \index{suite} \begin{verbatim} if test1: if test2: print x \end{verbatim} 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: \begin{verbatim} if x < y < z: print x; print y; print z \end{verbatim} Summarizing: \begin{verbatim} compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT 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\. \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). \indexii{dangling}{else} The formatting of the grammar rules in the following sections places each clause on a separate line for clarity. \section{The {\tt if} statement} \stindex{if} The \verb\if\ statement is used for conditional execution: \begin{verbatim} if_stmt: "if" condition ":" suite ("elif" condition ":" suite)* ["else" ":" suite] \end{verbatim} 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 present, is executed. \kwindex{elif} \kwindex{else} \section{The {\tt while} statement} \stindex{while} \indexii{loop}{statement} The \verb\while\ statement is used for repeated execution as long as a condition is true: \begin{verbatim} while_stmt: "while" condition ":" suite ["else" ":" suite] \end{verbatim} 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 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 of the suite and goes back to testing the condition. \stindex{break} \stindex{continue} \section{The {\tt for} statement} \stindex{for} \indexii{loop}{statement} The \verb\for\ statement is used to iterate over the elements of a sequence (string, tuple or list): \obindex{sequence} \begin{verbatim} for_stmt: "for" target_list "in" condition_list ":" suite ["else" ":" suite] \end{verbatim} The condition list is evaluated once; it should yield a sequence. The 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 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\ clause if there was no next item. \stindex{break} \stindex{continue} The suite may assign to the variable(s) in the target list; this does not affect the next item assigned to it. 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]\. \bifuncindex{range} \index{Pascal} {\bf Warning:} There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, i.e. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g. \index{loop!over mutable sequence} \index{mutable sequence!loop over} \begin{verbatim} for x in a[:]: if x < 0: a.remove(x) \end{verbatim} \section{The {\tt try} statement} \stindex{try} 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)+ 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. 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 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. Note that the object identities must match, i.e. it must be the same object, not just an object with the same value. \kwindex{except} 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 (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 assigned to the target specified in that except clause, if present, and the except clause's suite is executed. When the end of this suite is reached, execution continues normally after 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 handle 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 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 reason is a problem with the current implementation --- this restriction may be lifted in the future). \stindex{return} \stindex{break} \stindex{continue} \section{Function definitions} \label{function} \indexii{function}{definition} A function definition defines a user-defined function object (see section \ref{types}): \obindex{user-defined function} \obindex{function} \begin{verbatim} funcdef: "def" funcname "(" [parameter_list] ")" ":" suite parameter_list: (parameter ",")* ("*" identifier | parameter [","]) sublist: parameter ("," parameter)* [","] parameter: identifier | "(" sublist ")" funcname: identifier \end{verbatim} A function definition is an executable statement. Its execution binds the function name in the current local name space to a function object (a wrapper around the executable code for the function). This function object contains a reference to the current global name space as the global name space to be used when the function is called. \indexii{function}{name} \indexii{name}{binding} The function definition does not execute the function body; this gets executed only when the function is called. Function call semantics are described in section \ref{calls}. When a user-defined function is called, the arguments (a.k.a. actual parameters) are bound to the (formal) parameters, as follows: \indexii{function}{call} \indexiii{user-defined}{function}{call} \index{parameter} \index{argument} \indexii{parameter}{formal} \indexii{parameter}{actual} \begin{itemize} \item If there are no formal parameters, there must be no arguments. \item If the formal parameter list does not end in a star followed by an identifier, there must be exactly as many arguments as there are parameters in the formal parameter list (at the top level); the arguments are assigned to the formal parameters one by one. Note that the presence or absence of a trailing comma at the top level in either the formal or the actual parameter list makes no difference. The assignment to a formal parameter is performed as if the parameter occurs on the left hand side of an assignment statement whose right hand side's value is that of the argument. \item If the formal parameter list ends in a star followed by an identifier, preceded by zero or more comma-followed parameters, there must be at least as many arguments as there are parameters preceding the star. 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 there is just one extra argument, it is a singleton tuple. \indexii{variable length}{parameter list} \end{itemize} Note that the variable length parameter list' feature only works at the top level of the parameter list; individual parameters use a model corresponding more closely to that of ordinary assignment. While the latter model is generally preferable, because of the greater type safety it offers (wrong-sized tuples aren't silently mistreated), 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.) \section{Class definitions} \label{class} \indexii{class}{definition} A class definition defines a class object (see section \ref{types}): \obindex{class} \begin{verbatim} classdef: "class" classname [inheritance] ":" suite inheritance: "(" [condition_list] ")" classname: identifier \end{verbatim} A class definition is an executable statement. It first evaluates the inheritance list, if present. Each item in the inheritance list should evaluate to a class object. The class's suite is then executed in a new execution frame (see section \ref{execframes}), using a newly created local name space and the original global name space. (Usually, the suite contains only function definitions.) When the class's suite finishes execution, its execution frame is discarded but its local name space is saved. A class object is then created using the inheritance list for the base classes and the saved local name space for the attribute dictionary. The class name is bound to this class object in the original local name space. \index{inheritance} \indexii{class}{name} \indexii{name}{binding} \indexii{execution}{frame}