mirror of https://github.com/python/cpython
1021 lines
33 KiB
TeX
1021 lines
33 KiB
TeX
% Format this file with latex.
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\documentstyle[myformat]{report}
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\title{\bf
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Python Reference Manual \\
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{\em Incomplete Draft}
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}
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\author{
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Guido van Rossum \\
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Dept. CST, CWI, Kruislaan 413 \\
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1098 SJ Amsterdam, The Netherlands \\
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E-mail: {\tt guido@cwi.nl}
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}
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\begin{document}
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\pagenumbering{roman}
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\maketitle
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\begin{abstract}
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\noindent
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Python is a simple, yet powerful programming language that bridges the
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gap between C and shell programming, and is thus ideally suited for
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``throw-away programming''
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and rapid prototyping. Its syntax is put
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together from constructs borrowed from a variety of other languages;
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most prominent are influences from ABC, C, Modula-3 and Icon.
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The Python interpreter is easily extended with new functions and data
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types implemented in C. Python is also suitable as an extension
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language for highly customizable C applications such as editors or
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window managers.
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Python is available for various operating systems, amongst which
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several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,
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and MS-DOS.
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This reference manual describes the syntax and ``core semantics'' of
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the language. It is terse, but exact and complete. The semantics of
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non-essential built-in object types and of the built-in functions and
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modules are described in the {\em Library Reference} document. For an
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informal introduction to the language, see the {\em Tutorial}
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document.
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\end{abstract}
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\pagebreak
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\tableofcontents
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\pagebreak
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\pagenumbering{arabic}
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\chapter{Introduction}
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This reference manual describes the Python programming language.
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It is not intended as a tutorial.
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\chapter{Lexical analysis}
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A Python program is read by a {\em parser}.
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Input to the parser is a stream of {\em tokens}, generated
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by the {\em lexical analyzer}.
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\section{Line structure}
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A Python program is divided in a number of logical lines.
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Statements may not straddle logical line boundaries except where
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explicitly allowed by the syntax.
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To this purpose, the end of a logical line
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is represented by the token NEWLINE.
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\subsection{Comments}
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A comment starts with a hash character (\verb/#/) and ends at the end
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of the physical line. Comments are ignored by the syntax.
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A hash character in a string literal does not start a comment.
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\subsection{Line joining}
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Physical lines may be joined into logical lines using backslash
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characters (\verb/\/), as follows.
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If a physical line ends in a backslash that is not part of a string
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literal or comment, it is joined with
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the following forming a single logical line, deleting the backslash
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and the following end-of-line character. More than two physical
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lines may be joined together in this way.
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\subsection{Blank lines}
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A physical line that is not the continuation of the previous line
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and contains only spaces, tabs and possibly a comment, is ignored
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(i.e., no NEWLINE token is generated),
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except that during interactive input of statements, an empty
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physical line terminates a multi-line statement.
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\subsection{Indentation}
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Spaces and tabs at the beginning of a line are used to compute
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the indentation level of the line, which in turn is used to determine
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the grouping of statements.
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First, each tab is replaced by one to eight spaces such that the column number
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of the next character is a multiple of eight (counting from zero).
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The column number of the first non-space character then defines the
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line's indentation.
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Indentation cannot be split over multiple physical lines using
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backslashes.
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The indentation levels of consecutive lines are used to generate
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INDENT and DEDENT tokens, using a stack, as follows.
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Before the first line of the file is read, a single zero is pushed on
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the stack; this will never be popped off again. The numbers pushed
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on the stack will always be strictly increasing from bottom to top.
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At the beginning of each logical line, the line's indentation level
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is compared to the top of the stack.
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If it is equal, nothing happens.
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If it larger, it is pushed on the stack, and one INDENT token is generated.
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If it is smaller, it {\em must} be one of the numbers occurring on the
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stack; all numbers on the stack that are larger are popped off,
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and for each number popped off a DEDENT token is generated.
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At the end of the file, a DEDENT token is generated for each number
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remaining on the stack that is larger than zero.
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\section{Other tokens}
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Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
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exist: identifiers, keywords, literals, operators, and delimiters.
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Spaces and tabs are not tokens, but serve to delimit tokens.
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Where ambiguity exists, a token comprises the longest possible
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string that forms a legal token, when reading from left to right.
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Tokens are described using an extended regular expression notation.
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This is similar to the extended BNF notation used later, except that
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the notation <...> is used to give an informal description of a character,
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and that spaces and tabs are not to be ignored.
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\section{Identifiers}
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Identifiers are described by the following regular expressions:
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\begin{verbatim}
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identifier: (letter|'_') (letter|digit|'_')*
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letter: lowercase | uppercase
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lowercase: 'a'|'b'|...|'z'
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uppercase: 'A'|'B'|...|'Z'
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digit: '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
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\end{verbatim}
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Identifiers are unlimited in length.
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Upper and lower case letters are different.
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\section{Keywords}
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The following tokens are used as reserved words,
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or keywords of the language,
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and may not be used as ordinary identifiers.
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They must be spelled exactly as written here:
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{\tt
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and
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break
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class
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continue
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def
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del
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elif
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else
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except
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finally
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for
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from
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if
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import
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in
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is
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not
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or
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pass
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print
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raise
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return
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try
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while
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}
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\section{Literals}
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\subsection{String literals}
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String literals are described by the following regular expressions:
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\begin{verbatim}
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stringliteral: '\'' stringitem* '\''
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stringitem: stringchar | escapeseq
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stringchar: <any character except newline or '\\' or '\''>
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escapeseq: '\\' <any character except newline>
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\end{verbatim}
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String literals cannot span physical line boundaries.
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Escape sequences in strings are actually interpreted according to almost the
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same rules as used by Standard C
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(XXX which should be made explicit here),
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except that \verb/\E/ is equivalent to \verb/\033/,
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\verb/\"/ is not recognized,
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newline characters cannot be escaped, and
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{\em all unrecognized escape sequences are left in the string unchanged}.
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(The latter rule is useful when debugging: if an escape sequence is
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mistyped, the resulting output is more easily recognized as broken.
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It also helps somewhat for string literals used as regular expressions
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or otherwise passed to other modules that do their own escape handling.)
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\subsection{Numeric literals}
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There are three types of numeric literals: integers, long integers,
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and floating point numbers.
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Integers and long integers are described by the following regular expressions:
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\begin{verbatim}
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longinteger: integer ('l'|'L')
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integer: decimalinteger | octinteger | hexinteger
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decimalinteger: nonzerodigit digit* | '0'
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octinteger: '0' octdigit+
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hexinteger: '0' ('x'|'X') hexdigit+
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nonzerodigit: '1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
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octdigit: '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'
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hexdigit: digit|'a'|'b'|'c'|'d'|'e'|'f'|'A'|'B'|'C'|'D'|'E'|'F'
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\end{verbatim}
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Floating point numbers are described by the following regular expressions:
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\begin{verbatim}
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floatnumber: [intpart] fraction [exponent] | intpart ['.'] exponent
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intpart: digit+
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fraction: '.' digit+
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exponent: ('e'|'E') ['+'|'-'] digit+
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\end{verbatim}
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\section{Operators}
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The following tokens are operators:
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\begin{verbatim}
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+ - * / %
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<< >> & | ^ ~
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< = == > <= <> != >=
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\end{verbatim}
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\section{Delimiters}
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The following tokens are delimiters:
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\begin{verbatim}
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( ) [ ] { }
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; , : . `
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\end{verbatim}
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The following printing ASCII characters are currently not used;
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their occurrence is an unconditional error:
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\begin{verbatim}
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! @ $ " ?
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\end{verbatim}
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\chapter{Execution model}
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(XXX This chapter should explain the general model
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of the execution of Python code and
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the evaluation of expressions.
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It should introduce objects, values, code blocks, scopes, name spaces,
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name binding,
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types, sequences, numbers, mappings,
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exceptions, and other technical terms needed to make the following
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chapters concise and exact.)
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\chapter{Expressions and conditions}
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(From now on, extended BNF notation will be used to describe
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syntax, not lexical analysis.)
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(XXX Explain the notation.)
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This chapter explains the meaning of the elements of expressions and
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conditions. Conditions are a superset of expressions, and a condition
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may be used where an expression is required by enclosing it in
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parentheses. The only place where an unparenthesized condition
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is not allowed is on the right-hand side of the assignment operator,
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because this operator is the same token (\verb/'='/) as used for
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compasisons.
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The comma plays a somewhat special role in Python's syntax.
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It is an operator with a lower precedence than all others, but
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occasionally serves other purposes as well (e.g., it has special
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semantics in print statements). When a comma is accepted by the
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syntax, one of the syntactic categories \verb/expression_list/
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or \verb/condition_list/ is always used.
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When (one alternative of) a syntax rule has the form
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\begin{verbatim}
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name: othername
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\end{verbatim}
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and no semantics are given, the semantics of this form of \verb/name/
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are the same as for \verb/othername/.
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\section{Arithmetic conversions}
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When a description of an arithmetic operator below uses the phrase
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``the numeric arguments are converted to a common type'',
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this both means that if either argument is not a number, a
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{\tt TypeError} exception is raised, and that otherwise
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the following conversions are applied:
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\begin{itemize}
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\item First, if either argument is a floating point number,
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the other is converted to floating point;
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\item else, if either argument is a long integer,
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the other is converted to long integer;
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\item otherwise, both must be short integers and no conversion
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is necessary.
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\end{itemize}
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(Note: ``short integers'' in Python are at least 32 bits in size;
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``long integers'' are arbitrary precision integers.)
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\section{Atoms}
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Atoms are the most basic elements of expressions.
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Forms enclosed in reverse quotes or various types of parentheses
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or braces are also categorized syntactically as atoms.
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Syntax rules for atoms:
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\begin{verbatim}
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atom: identifier | literal | parenth_form | string_conversion
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literal: stringliteral | integer | longinteger | floatnumber
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parenth_form: enclosure | list_display | dict_display
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enclosure: '(' [condition_list] ')'
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list_display: '[' [condition_list] ']'
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dict_display: '{' [key_datum (',' key_datum)* [','] '}'
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key_datum: condition ':' condition
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string_conversion:'`' condition_list '`'
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\end{verbatim}
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\subsection{Identifiers (Names)}
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An identifier occurring as an atom is a reference to a local, global
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or built-in name binding. If a name can be assigned to anywhere in a code
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block, it refers to a local name throughout that code block.
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Otherwise, it refers to a global name if one exists, else to a
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built-in name.
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When the name is bound to an object, evaluation of the atom
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yields that object.
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When it is not bound, a {\tt NameError} exception
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is raised, with the identifier as string parameter.
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\subsection{Literals}
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Evaluation of a literal yields an object of the given type
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(string, integer, long integer, floating point number)
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with the given value.
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The value may be approximated in the case of floating point literals.
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All literals correspond to immutable data types, and hence the object's
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identity is less important than its value.
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Multiple evaluations of the same literal (either the same occurrence
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in the program text or a different occurrence) may
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obtain the same object or a different object with the same value.
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(In the original implementation, all literals in the same code block
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with the same type and value yield the same object.)
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\subsection{Enclosures}
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An empty enclosure yields an empty tuple object.
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An enclosed condition list yields whatever that condition list yields.
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(Note that, except for empty tuples, tuples are not formed by
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enclosure in parentheses, but rather by use of the comma operator.)
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\subsection{List displays}
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A list display yields a new list object.
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If it has no condition list, the list object has no items.
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Otherwise, the elements of the condition list are evaluated
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from left to right and inserted in the list object in that order.
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\subsection{Dictionary displays}
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A dictionary display yields a new dictionary object.
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The key/datum pairs are evaluated from left to right to
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define the entries of the dictionary:
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each key object is used as a key into the dictionary to store
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the corresponding datum pair.
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Key objects must be strings, otherwise a {\tt TypeError}
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exception is raised.
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Clashes between keys are not detected; the last datum stored for a given
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key value prevails.
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\subsection{String conversions}
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A string conversion evaluates the contained condition list and converts the
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resulting object into a string according to rules specific to its type.
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If the object is a string, a number, \verb/None/, or a tuple, list or
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dictionary containing only objects whose type is in this list,
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the resulting
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string is a valid Python expression which can be passed to the
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built-in function \verb/eval()/ to yield an expression with the
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same value (or an approximation, if floating point numbers are
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involved).
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(In particular, converting a string adds quotes around it and converts
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``funny'' characters to escape sequences that are safe to print.)
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It is illegal to attempt to convert recursive objects (e.g.,
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lists or dictionaries that -- directly or indirectly -- contain a reference
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to themselves.)
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\section{Primaries}
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Primaries represent the most tightly bound operations of the language.
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Their syntax is:
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\begin{verbatim}
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primary: atom | attributeref | call | subscription | slicing
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attributeref: primary '.' identifier
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call: primary '(' [condition_list] ')'
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subscription: primary '[' condition ']'
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slicing: primary '[' [condition] ':' [condition] ']'
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\end{verbatim}
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\subsection{Attribute references}
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\subsection{Calls}
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\subsection{Subscriptions}
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\subsection{Slicings}
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\section{Factors}
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Factors represent the unary numeric operators.
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Their syntax is:
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\begin{verbatim}
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factor: primary | '-' factor | '+' factor | '~' factor
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\end{verbatim}
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The unary \verb/'-'/ operator yields the negative of its numeric argument.
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The unary \verb/'+'/ operator yields its numeric argument unchanged.
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The unary \verb/'~'/ operator yields the bit-wise negation of its
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integral numerical argument.
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In all three cases, if the argument does not have the proper type,
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a {\tt TypeError} exception is raised.
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\section{Terms}
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Terms represent the most tightly binding binary operators:
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\begin{verbatim}
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term: factor | term '*' factor | term '/' factor | term '%' factor
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\end{verbatim}
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The \verb/'*'/ operator yields the product of its arguments.
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The arguments must either both be numbers, or one argument must be
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a (short) integer and the other must be a string.
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In the former case, the numbers are converted to a common type
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and then multiplied together.
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In the latter case, string repetition is performed; a negative
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repetition factor yields the empty string.
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The \verb|'/'| operator yields the quotient of its arguments.
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The numeric arguments are first converted to a common type.
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(Short or long) integer division yields an integer of the same type,
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truncating towards zero.
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Division by zero raises a {\tt RuntimeError} exception.
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The \verb|'%'| operator yields the remainder from the division
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of the first argument by the second.
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The numeric arguments are first converted to a common type.
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The outcome of $x % y$ is defined as $x - y*trunc(x/y)$.
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A zero right argument raises a {\tt RuntimeError} exception.
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The arguments may be floating point numbers, e.g.,
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$3.14 % 0.7$ equals $0.34$.
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\section{Arithmetic expressions}
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\begin{verbatim}
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arith_expr: term | arith_expr '+' term | arith_expr '-' term
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\end{verbatim}
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The \verb|'+'| operator yields the sum of its arguments.
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The arguments must either both be numbers, or both strings.
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In the former case, the numbers are converted to a common type
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and then added together.
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In the latter case, the strings are concatenated directly,
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without inserting a space.
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The \verb|'-'| operator yields the difference of its arguments.
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The numeric arguments are first converted to a common type.
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\section{Shift expressions}
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\begin{verbatim}
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shift_expr: arith_expr | shift_expr '<<' arith_expr | shift_expr '>>' arith_expr
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\end{verbatim}
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These operators accept short integers as arguments only.
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They shift their left argument to the left or right by the number of bits
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given by the right argument. Shifts are ``logical'', e.g., bits shifted
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out on one end are lost, and bits shifted in are zero;
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negative numbers are shifted as if they were unsigned in C.
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Negative shift counts and shift counts greater than {\em or equal to}
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the word size yield undefined results.
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\section{Bitwise AND expressions}
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\begin{verbatim}
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and_expr: shift_expr | and_expr '&' shift_expr
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\end{verbatim}
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This operator yields the bitwise AND of its arguments,
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which must be short integers.
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\section{Bitwise XOR expressions}
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\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}
|