% Format this file with latex. \documentstyle[myformat]{report} \title{\bf Python Reference Manual \\ {\em Incomplete Draft} } \author{ Guido van Rossum \\ Dept. CST, CWI, Kruislaan 413 \\ 1098 SJ Amsterdam, The Netherlands \\ E-mail: {\tt guido@cwi.nl} } \begin{document} \pagenumbering{roman} \maketitle \begin{abstract} \noindent Python is a simple, yet powerful programming language that bridges the gap between C and shell programming, and is thus ideally suited for ``throw-away programming'' and rapid prototyping. Its syntax is put together from constructs borrowed from a variety of other languages; most prominent are influences from ABC, C, Modula-3 and Icon. The Python interpreter is easily extended with new functions and data types implemented in C. Python is also suitable as an extension language for highly customizable C applications such as editors or window managers. Python is available for various operating systems, amongst which several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S., and MS-DOS. This reference manual describes the syntax and ``core semantics'' of the language. It is terse, but exact and complete. The semantics of non-essential built-in object types and of the built-in functions and modules are described in the {\em Library Reference} document. For an informal introduction to the language, see the {\em Tutorial} document. \end{abstract} \pagebreak \tableofcontents \pagebreak \pagenumbering{arabic} \chapter{Introduction} This reference manual describes the Python programming language. It is not intended as a tutorial. \chapter{Lexical analysis} A Python program is read by a {\em parser}. Input to the parser is a stream of {\em tokens}, generated by the {\em lexical analyzer}. \section{Line structure} A Python program is divided in a number of logical lines. Statements may not straddle logical line boundaries except where explicitly allowed by the syntax. To this purpose, the end of a logical line is represented by the token NEWLINE. \subsection{Comments} A comment starts with a hash character (\verb/#/) and ends at the end of the physical line. Comments are ignored by the syntax. A hash character in a string literal does not start a comment. \subsection{Line joining} Physical lines may be joined into logical lines using backslash characters (\verb/\/), as follows. If a physical line ends in a backslash that is not part of a string literal or comment, it is joined with the following forming a single logical line, deleting the backslash and the following end-of-line character. More than two physical lines may be joined together in this way. \subsection{Blank lines} A physical line that is not the continuation of the previous line and contains only spaces, tabs and possibly a comment, is ignored (i.e., no NEWLINE token is generated), except that during interactive input of statements, an empty physical line terminates a multi-line statement. \subsection{Indentation} Spaces and tabs at the beginning of a line are used to compute the indentation level of the line, which in turn is used to determine the grouping of statements. First, each tab is replaced by one to eight spaces such that the column number of the next character is a multiple of eight (counting from zero). The column number of the first non-space character then defines the line's indentation. Indentation cannot be split over multiple physical lines using backslashes. The indentation levels of consecutive lines are used to generate INDENT and DEDENT tokens, using a stack, as follows. Before the first line of the file is read, a single zero is pushed on the stack; this will never be popped off again. The numbers pushed on the stack will always be strictly increasing from bottom to top. At the beginning of each logical line, the line's indentation level is compared to the top of the stack. If it is equal, nothing happens. If it larger, it is pushed on the stack, and one INDENT token is generated. If it is smaller, it {\em must} be one of the numbers occurring on the stack; all numbers on the stack that are larger are popped off, and for each number popped off a DEDENT token is generated. At the end of the file, a DEDENT token is generated for each number remaining on the stack that is larger than zero. \section{Other tokens} Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist: identifiers, keywords, literals, operators, and delimiters. Spaces and tabs are not tokens, but serve to delimit tokens. Where ambiguity exists, a token comprises the longest possible string that forms a legal token, when reading from left to right. Tokens are described using an extended regular expression notation. This is similar to the extended BNF notation used later, except that the notation <...> is used to give an informal description of a character, and that spaces and tabs are not to be ignored. \section{Identifiers} Identifiers are described by the following regular expressions: \begin{verbatim} identifier: (letter|'_') (letter|digit|'_')* letter: lowercase | uppercase lowercase: 'a'|'b'|...|'z' uppercase: 'A'|'B'|...|'Z' digit: '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9' \end{verbatim} Identifiers are unlimited in length. Upper and lower case letters are different. \section{Keywords} The following tokens are used as reserved words, or keywords of the language, and may not be used as ordinary identifiers. They must be spelled exactly as written here: {\tt and break class continue def del elif else except finally for from if import in is not or pass print raise return try while } \section{Literals} \subsection{String literals} String literals are described by the following regular expressions: \begin{verbatim} stringliteral: '\'' stringitem* '\'' stringitem: stringchar | escapeseq stringchar: escapeseq: '\\' \end{verbatim} String literals cannot span physical line boundaries. Escape sequences in strings are actually interpreted according to almost the same rules as used by Standard C (XXX which should be made explicit here), except that \verb/\E/ is equivalent to \verb/\033/, \verb/\"/ is not recognized, newline characters cannot be escaped, and {\em all unrecognized escape sequences are left in the string unchanged}. (The latter rule is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken. It also helps somewhat for string literals used as regular expressions or otherwise passed to other modules that do their own escape handling.) \subsection{Numeric literals} There are three types of numeric literals: integers, long integers, and floating point numbers. Integers and long integers are described by the following regular expressions: \begin{verbatim} longinteger: integer ('l'|'L') integer: decimalinteger | octinteger | hexinteger decimalinteger: nonzerodigit digit* | '0' octinteger: '0' octdigit+ hexinteger: '0' ('x'|'X') hexdigit+ nonzerodigit: '1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9' octdigit: '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7' hexdigit: digit|'a'|'b'|'c'|'d'|'e'|'f'|'A'|'B'|'C'|'D'|'E'|'F' \end{verbatim} Floating point numbers are described by the following regular expressions: \begin{verbatim} floatnumber: [intpart] fraction [exponent] | intpart ['.'] exponent intpart: digit+ fraction: '.' digit+ exponent: ('e'|'E') ['+'|'-'] digit+ \end{verbatim} \section{Operators} The following tokens are operators: \begin{verbatim} + - * / % << >> & | ^ ~ < = == > <= <> != >= \end{verbatim} \section{Delimiters} The following tokens are delimiters: \begin{verbatim} ( ) [ ] { } ; , : . ` \end{verbatim} The following printing ASCII characters are currently not used; their occurrence is an unconditional error: \begin{verbatim} ! @ $ " ? \end{verbatim} \chapter{Execution model} (XXX This chapter should explain the general model of the execution of Python code and the evaluation of expressions. It should introduce objects, values, code blocks, scopes, name spaces, name binding, types, sequences, numbers, mappings, exceptions, and other technical terms needed to make the following chapters concise and exact.) \chapter{Expressions and conditions} (From now on, extended BNF notation will be used to describe syntax, not lexical analysis.) (XXX Explain the notation.) This chapter explains the meaning of the elements of expressions and conditions. Conditions are a superset of expressions, and a condition may be used where an expression is required by enclosing it in parentheses. The only place where an unparenthesized condition is not allowed is on the right-hand side of the assignment operator, because this operator is the same token (\verb/'='/) as used for compasisons. The comma plays a somewhat special role in Python's syntax. It is an operator with a lower precedence than all others, but occasionally serves other purposes as well (e.g., it has special semantics in print statements). When a comma is accepted by the syntax, one of the syntactic categories \verb/expression_list/ or \verb/condition_list/ is always used. When (one alternative of) a syntax rule has the form \begin{verbatim} name: othername \end{verbatim} and no semantics are given, the semantics of this form of \verb/name/ are the same as for \verb/othername/. \section{Arithmetic conversions} When a description of an arithmetic operator below uses the phrase ``the numeric arguments are converted to a common type'', this both means that if either argument is not a number, a {\tt TypeError} exception is raised, and that otherwise the following conversions are applied: \begin{itemize} \item First, if either argument is a floating point number, the other is converted to floating point; \item else, if either argument is a long integer, the other is converted to long integer; \item otherwise, both must be short integers and no conversion is necessary. \end{itemize} (Note: ``short integers'' in Python are at least 32 bits in size; ``long integers'' are arbitrary precision integers.) \section{Atoms} Atoms are the most basic elements of expressions. Forms enclosed in reverse quotes or various types of parentheses or braces are also categorized syntactically as atoms. Syntax rules for atoms: \begin{verbatim} atom: identifier | literal | parenth_form | string_conversion literal: stringliteral | integer | longinteger | floatnumber parenth_form: enclosure | list_display | dict_display enclosure: '(' [condition_list] ')' list_display: '[' [condition_list] ']' dict_display: '{' [key_datum (',' key_datum)* [','] '}' key_datum: condition ':' condition string_conversion:'`' condition_list '`' \end{verbatim} \subsection{Identifiers (Names)} An identifier occurring as an atom is a reference to a local, global or built-in name binding. If a name can be assigned to anywhere in a code block, it refers to a local name throughout that code block. Otherwise, it refers to a global name if one exists, else to a built-in name. When the name is bound to an object, evaluation of the atom yields that object. When it is not bound, a {\tt NameError} exception is raised, with the identifier as string parameter. \subsection{Literals} Evaluation of a literal yields an object of the given type (string, integer, long integer, floating point number) with the given value. The value may be approximated in the case of floating point literals. All literals correspond to immutable data types, and hence the object's identity is less important than its value. Multiple evaluations of the same literal (either the same occurrence in the program text or a different occurrence) may obtain the same object or a different object with the same value. (In the original implementation, all literals in the same code block with the same type and value yield the same object.) \subsection{Enclosures} An empty enclosure yields an empty tuple object. An enclosed condition list yields whatever that condition list yields. (Note that, except for empty tuples, tuples are not formed by enclosure in parentheses, but rather by use of the comma operator.) \subsection{List displays} A list display yields a new list object. If it has no condition list, the list object has no items. Otherwise, the elements of the condition list are evaluated from left to right and inserted in the list object in that order. \subsection{Dictionary displays} A dictionary display yields a new dictionary object. The key/datum pairs are evaluated from left to right to define the entries of the dictionary: each key object is used as a key into the dictionary to store the corresponding datum pair. Key objects must be strings, otherwise a {\tt TypeError} exception is raised. Clashes between keys are not detected; the last datum stored for a given key value prevails. \subsection{String conversions} A string conversion evaluates the contained condition list and converts the resulting object into a string according to rules specific to its type. If the object is a string, a number, \verb/None/, or a tuple, list or dictionary containing only objects whose type is in this list, the resulting string is a valid Python expression which can be passed to the built-in function \verb/eval()/ to yield an expression with the same value (or an approximation, if floating point numbers are involved). (In particular, converting a string adds quotes around it and converts ``funny'' characters to escape sequences that are safe to print.) It is illegal to attempt to convert recursive objects (e.g., lists or dictionaries that -- directly or indirectly -- contain a reference to themselves.) \section{Primaries} Primaries represent the most tightly bound operations of the language. Their syntax is: \begin{verbatim} primary: atom | attributeref | call | subscription | slicing attributeref: primary '.' identifier call: primary '(' [condition_list] ')' subscription: primary '[' condition ']' slicing: primary '[' [condition] ':' [condition] ']' \end{verbatim} \subsection{Attribute references} \subsection{Calls} \subsection{Subscriptions} \subsection{Slicings} \section{Factors} Factors represent the unary numeric operators. Their syntax is: \begin{verbatim} factor: primary | '-' factor | '+' factor | '~' factor \end{verbatim} The unary \verb/'-'/ operator yields the negative of its numeric argument. The unary \verb/'+'/ operator yields its numeric argument unchanged. The unary \verb/'~'/ operator yields the bit-wise negation of its integral numerical argument. In all three cases, if the argument does not have the proper type, a {\tt TypeError} exception is raised. \section{Terms} Terms represent the most tightly binding binary operators: \begin{verbatim} term: factor | term '*' factor | term '/' factor | term '%' factor \end{verbatim} The \verb/'*'/ operator yields the product of its arguments. The arguments must either both be numbers, or one argument must be a (short) integer and the other must be a string. In the former case, the numbers are converted to a common type and then multiplied together. In the latter case, string repetition is performed; a negative repetition factor yields the empty string. The \verb|'/'| operator yields the quotient of its arguments. The numeric arguments are first converted to a common type. (Short or long) integer division yields an integer of the same type, truncating towards zero. Division by zero raises a {\tt RuntimeError} exception. The \verb|'%'| operator yields the remainder from the division of the first argument by the second. The numeric arguments are first converted to a common type. The outcome of $x % y$ is defined as $x - y*trunc(x/y)$. A zero right argument raises a {\tt RuntimeError} exception. The arguments may be floating point numbers, e.g., $3.14 % 0.7$ equals $0.34$. \section{Arithmetic expressions} \begin{verbatim} arith_expr: term | arith_expr '+' term | arith_expr '-' term \end{verbatim} The \verb|'+'| operator yields the sum of its arguments. The arguments must either both be numbers, or both strings. In the former case, the numbers are converted to a common type and then added together. In the latter case, the strings are concatenated directly, without inserting a space. The \verb|'-'| operator yields the difference of its arguments. The numeric arguments are first converted to a common type. \section{Shift expressions} \begin{verbatim} shift_expr: arith_expr | shift_expr '<<' arith_expr | shift_expr '>>' arith_expr \end{verbatim} These operators accept short integers as arguments only. They shift their left argument to the left or right by the number of bits given by the right argument. Shifts are ``logical'', e.g., bits shifted out on one end are lost, and bits shifted in are zero; negative numbers are shifted as if they were unsigned in C. Negative shift counts and shift counts greater than {\em or equal to} the word size yield undefined results. \section{Bitwise AND expressions} \begin{verbatim} and_expr: shift_expr | and_expr '&' shift_expr \end{verbatim} This operator yields the bitwise AND of its arguments, which must be short integers. \section{Bitwise XOR expressions} \begin{verbatim} xor_expr: and_expr | xor_expr '^' and_expr \end{verbatim} This operator yields the bitwise exclusive OR of its arguments, which must be short integers. \section{Bitwise OR expressions} \begin{verbatim} or_expr: xor_expr | or_expr '|' xor_expr \end{verbatim} This operator yields the bitwise OR of its arguments, which must be short integers. \section{Expressions and expression lists} \begin{verbatim} expression: or_expression expr_list: expression (',' expression)* [','] \end{verbatim} An expression list containing at least one comma yields a new tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right. The trailing comma is required only to create a single tuple; it is optional in all other cases (a single expression without a trailing comma doesn't create a tuple, but rather yields the value of that expression). To create an empty tuple, use an empty pair of parentheses: \verb/()/. \section{Comparisons} \begin{verbatim} comparison: expression (comp_operator expression)* comp_operator: '<'|'>'|'='|'=='|'>='|'<='|'<>'|'!='|['not'] 'in'|is' ['not'] \end{verbatim} Comparisons yield integer value: 1 for true, 0 for false. Comparisons can be chained arbitrarily, e.g., $x < y <= z$ is equivalent to $x < y$ {\tt and} $y <= z$, except that $y$ is evaluated only once (but in both cases $z$ is not evaluated at all when $x < y$ is found to be false). Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to $e_0 op_1 e_1$ {\tt and} $e_1 op_2 e_2$ {\tt and} ... {\tt and} $e_{n-1} op_n e_n$, except that each expression is evaluated at most once. Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison between $e_0$ and $e_2$, e.g., $x < y > z$ is perfectly legal. For the benefit of C programmers, the comparison operators \verb/=/ and \verb/==/ are equivalent, and so are \verb/<>/ and \verb/!=/. Use of the C variants is discouraged. The operators {\tt '<', '>', '=', '>=', '<='}, and {\tt '<>'} compare the values of two objects. The objects needn't have the same type. If both are numbers, they are compared to a common type. Otherwise, objects of different types {\em always} compare unequal, and are ordered consistently but arbitrarily, except that the value \verb\None\ compares smaller than the values of any other type. (This unusual definition of comparison is done to simplify the definition of operations like sorting and the \verb/in/ and \verb/not in/ operators.) Comparison of objects of the same type depends on the type: \begin{itemize} \item Numbers are compared arithmetically. \item Strings are compared lexicographically using the numeric equivalents (the result of the built-in function ord()) of their characters. \item Tuples and lists are compared lexicographically using comparison of corresponding items. \item Dictionaries compare unequal unless they are the same object; the choice whether one dictionary object is considered smaller or larger than another one is made arbitrarily but consistently within one execution of a program. \item The latter rule is also used for most other built-in types. \end{itemize} The operators \verb\in\ and \verb\not in\ test for sequence membership: if $y$ is a sequence, $x {\tt in} y$ is true if and only if there exists an index $i$ such that $x = y_i$. $x {\tt not in} y$ yields the inverse truth value. The exception {\tt TypeError} is raised when $y$ is not a sequence, or when $y$ is a string and $x$ is not a string of length one. The operators \verb\is\ and \verb\is not\ compare object identity: $x {\tt is} y$ is true if and only if $x$ and $y$ are the same object. $x {\tt is not} y$ yields the inverse truth value. \section{Boolean operators} \begin{verbatim} condition: or_test or_test: and_test | or_test 'or' and_test and_test: not_test | and_test 'and' not_test not_test: comparison | 'not' not_test \end{verbatim} In the context of Boolean operators, and also when conditions are used by control flow statements, the following values are interpreted as false: None, numeric zero of all types, empty sequences (strings, tuples and lists), and empty mappings (dictionaries). All other values are interpreted as true. The operator \verb\not\ yields 1 if its argument is false, 0 otherwise. The condition $x {\tt and} y$ first evaluates $x$; if $x$ is false, $x$ is returned; otherwise, $y$ is evaluated and returned. The condition $x {\tt or} y$ first evaluates $x$; if $x$ is true, $x$ is returned; otherwise, $y$ is evaluated and returned. (Note that \verb\and\ and \verb\or\ do not restrict the value and type they return to 0 and 1, but rather return the last evaluated argument. This is sometimes useful, e.g., if $s$ is a string, which should be replaced by a default value if it is empty, $s {\tt or} 'foo'$ returns the desired value. Because \verb\not\ has to invent a value anyway, it does not bother to return a value of the same type as its argument, so \verb\not 'foo'\ yields $0$, not $''$.) \chapter{Simple statements} Simple statements are comprised within a single logical line. Several simple statements may occor on a single line separated by semicolons. The syntax for simple statements is: \begin{verbatim} stmt_list: simple_stmt (';' simple_stmt)* [';'] simple_stmt: expression_stmt | assignment | pass_stmt | del_stmt | print_stmt | return_stmt | raise_stmt | break_stmt | continue_stmt | import_stmt \end{verbatim} \section{Expression statements} \begin{verbatim} expression_stmt: expression_list \end{verbatim} An expression statement evaluates the expression list (which may be a single expression). If the value is not \verb\None\, it is converted to a string using the rules for string conversions, and the resulting string is written to standard output on a line by itself. (The exception for \verb\None\ is made so that procedure calls, which are syntactically equivalent to expressions, do not cause any output.) \section{Assignments} \begin{verbatim} assignment: target_list ('=' target_list)* '=' expression_list target_list: target (',' target)* [','] target: identifier | '(' target_list ')' | '[' target_list ']' | attributeref | subscription | slicing \end{verbatim} (See the section on primaries for the definition of the last three symbols.) An assignment 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. Assignment is defined recursively depending on the type of the target. Where assignment is to part of a mutable object (through an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, raising 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 (some of which are defined in the library reference). Assignment of an object to a target list is recursively defined as follows. \begin{itemize} \item If the target list contains no commas (except in nested constructs): the object is assigned to the single target contained in the list. \item If the target list contains commas (that are not in nested constructs): the object must be a tuple with as many items as the list contains targets, and the items are assigned, from left to right, to the corresponding targets. \end{itemize} Assignment of an object to a (non-list) target is recursively defined as follows. \begin{itemize} \item If the target is an identifier (name): the object is bound to that name in the current local scope. Any previous binding of the same name is undone. \item If the target is a target list enclosed in parentheses: the object is assigned to that target list. \item If the target is a target list enclosed in square brackets: the object must be a list with as many items as the target list contains targets, and the list's 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, a {\tt TypeError} exception 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. \item If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object or a mapping (dictionary) object. Next, the subscript expression is evaluated. If the primary is a sequence object, the subscript must yield a nonnegative integer smaller than the sequence's length, and the sequence is asked to assign the assigned object to its item with that index. If the primary is a mapping object, the subscript must have a type compatible with the mapping's key type, and the mapping is then asked to to create a key/datum pair which maps the subscript to the assigned object. Various exceptions can be raised. \item If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (currently only lists). 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 (once). The resulting bounds are clipped to lie between zero and the sequence's length, inclusive. (XXX Shouldn't this description be with expressions?) Finally, the sequence object is asked to replace the items indicated by the slice with the items of the assigned sequence. This may change the sequence's length, if it allows it. \end{itemize} (In the original 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.) \section{The {\tt pass} statement} \begin{verbatim} pass_stmt: 'pass' \end{verbatim} {\tt pass} is a null operation -- when it is executed, nothing happens. \section{The {\tt del} statement} \begin{verbatim} del_stmt: 'del' target_list \end{verbatim} Deletion is recursively defined similar to assignment. (XXX Rather that spelling it out in full details, here are some hints.) 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 scope. 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). \section{The {\tt print} statement} \begin{verbatim} print_stmt: 'print' [ condition (',' condition)* [','] ] \end{verbatim} {\tt print} evaluates each condition in turn and writes the resulting object to standard output (see below). If an object is not a string, it is first converted to a string using the rules for string conversions. The (resulting or original) string is then written. A 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 been written to standard output; or (2) when the last character written to standard output is \verb/'\n'/; or (3) when the last I/O operation on standard output was not a \verb\print\ statement. Finally, a \verb/'\n'/ character is written at the end, unless the \verb\print\ statement ends with a comma. This is the only action if the statement contains just the keyword \verb\print\. Standard output is defined as the file object named \verb\stdout\ in the built-in module \verb\sys\. If no such object exists, or if it is not a writable file, a {\tt 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.) \section{The {\tt return} statement} \begin{verbatim} return_stmt: 'return' [condition_list] \end{verbatim} \verb\return\ may only occur syntactically nested in a function definition, not within a nested class definition. If a condition list is present, it is evaluated, else \verb\None\ is substituted. \verb\return\ leaves the current function call with the condition list (or \verb\None\) as return value. When \verb\return\ passes control out of a \verb\try\ statement with a \verb\finally\ clause, that finally clause is executed before really leaving the function. (XXX This should be made more exact, a la Modula-3.) \section{The {\tt raise} statement} \begin{verbatim} raise_stmt: 'raise' condition [',' condition] \end{verbatim} \verb\raise\ evaluates its first condition, which must yield a string object. If there is a second condition, this is evaluated, else \verb\None\ is substituted. It then raises the exception identified by the first object, with the second one (or \verb\None\) as its parameter. \section{The {\tt break} statement} \begin{verbatim} break_stmt: 'break' \end{verbatim} \verb\break\ may only occur syntactically nested in a \verb\for\ or \verb\while\ loop, not nested in a function or class definition. It terminates the neares enclosing loop, skipping the optional \verb\else\ clause if the loop has one. If a \verb\for\ loop is terminated by \verb\break\, the loop control target (list) keeps its current value. When \verb\break\ passes control out of a \verb\try\ statement with a \verb\finally\ clause, that finally clause is executed before really leaving the loop. \section{The {\tt continue} statement} \begin{verbatim} continue_stmt: 'continue' \end{verbatim} \verb\continue\ may only occur syntactically nested in a \verb\for\ or \verb\while\ loop, not nested in a function or class definition, and {\em not nested in a \verb\try\ statement with a \verb\finally\ clause}. It continues with the next cycle of the nearest enclosing loop. \section{The {\tt import} statement} \begin{verbatim} import_stmt: 'import' identifier (',' identifier)* | 'from' identifier 'import' identifier (',' identifier)* | 'from' identifier 'import' '*' \end{verbatim} (XXX To be done.) \chapter{Compound statements} (XXX The semantic definitions of this chapter are still to be done.) \begin{verbatim} statement: stmt_list NEWLINE | compound_stmt compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef suite: statement | NEWLINE INDENT statement+ DEDENT \end{verbatim} \section{The {\tt if} statement} \begin{verbatim} if_stmt: 'if' condition ':' suite ('elif' condition ':' suite)* ['else' ':' suite] \end{verbatim} \section{The {\tt while} statement} \begin{verbatim} while_stmt: 'while' condition ':' suite ['else' ':' suite] \end{verbatim} \section{The {\tt for} statement} \begin{verbatim} for_stmt: 'for' target_list 'in' condition_list ':' suite ['else' ':' suite] \end{verbatim} \section{The {\tt try} statement} \begin{verbatim} try_stmt: 'try' ':' suite ('except' condition [',' condition] ':' suite)* ['finally' ':' suite] \end{verbatim} \section{Function definitions} \begin{verbatim} funcdef: 'def' identifier '(' [parameter_list] ')' ':' suite parameter_list: parameter (',' parameter)* parameter: identifier | '(' parameter_list ')' \end{verbatim} \section{Class definitions} \begin{verbatim} classdef: 'class' identifier '(' ')' [inheritance] ':' suite inheritance: '=' identifier '(' ')' (',' identifier '(' ')')* \end{verbatim} XXX Syntax for scripts, modules XXX Syntax for interactive input, eval, exec, input \end{document}