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diff --git a/Doc/ref/ref.tex b/Doc/ref/ref.tex index d6d4f56..f659569 100644 --- a/Doc/ref/ref.tex +++ b/Doc/ref/ref.tex @@ -1,5 +1,5 @@ % Format this file with latex. - + \documentstyle[myformat]{report} \title{\bf @@ -65,17 +65,18 @@ rather than formal specifications for everything except syntax and lexical analysis. This should make the document better understandable to the average reader, but will leave room for ambiguities. Consequently, if you were coming from Mars and tried to re-implement -Python from this document alone, you might in fact be implementing -quite a different language. On the other hand, if you are using +Python from this document alone, you might have to guess things and in +fact you would be implementing quite a different language. +On the other hand, if you are using Python and wonder what the precise rules about a particular area of -the language are, you should be able to find it here. +the language are, you should definitely be able to find it here. It is dangerous to add too many implementation details to a language reference document -- the implementation may change, and other implementations of the same language may work differently. On the other hand, there is currently only one Python implementation, and -particular quirks of it are sometimes worth mentioning, especially -where it differs from the ``ideal'' specification. +its particular quirks are sometimes worth being mentioned, especially +where the implementation imposes additional limitations. Every Python implementation comes with a number of built-in and standard modules. These are not documented here, but in the separate @@ -93,20 +94,20 @@ name: lcletter (lcletter | "_")* lcletter: "a"..."z" \end{verbatim} -The first line says that a \verb\name\ is a \verb\lcletter\ followed by -a sequence of zero or more \verb\lcletter\s and underscores. A +The first line says that a \verb\name\ is an \verb\lcletter\ followed by +a sequence of zero or more \verb\lcletter\s and underscores. An \verb\lcletter\ in turn is any of the single characters `a' through `z'. (This rule is actually adhered to for the names defined in syntax and grammar rules in this document.) Each rule begins with a name (which is the name defined by the rule) -followed by a colon. Each rule is wholly contained on one line. A -vertical bar (\verb\|\) is used to separate alternatives, it is the -least binding operator in this notation. A star (\verb\*\) means zero -or more repetitions of the preceding item; likewise, a plus (\verb\+\) -means one or more repetitions and a question mark (\verb\?\) zero or -one (in other words, the preceding item is optional). These three -operators bind as tight as possible; parentheses are used for +and a colon, and is wholly contained on one line. A vertical bar +(\verb\|\) is used to separate alternatives; it is the least binding +operator in this notation. A star (\verb\*\) means zero or more +repetitions of the preceding item; likewise, a plus (\verb\+\) means +one or more repetitions, and a question mark (\verb\?\) zero or one +(in other words, the preceding item is optional). These three +operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in double quotes. White space is only meaningful to separate tokens. @@ -117,7 +118,7 @@ characters. A phrase between angular brackets (\verb\<...>\) gives an informal description of the symbol defined; e.g., this could be used to describe the notion of `control character' if needed. -Although the notation used is almost the same, there is a big +Even though the notation used is almost the same, there is a big difference between the meaning of lexical and syntactic definitions: a lexical definition operates on the individual characters of the input source, while a syntax definition operates on the stream of @@ -131,22 +132,22 @@ chapter describes how the lexical analyzer breaks a file into tokens. \section{Line structure} -A Python program is divided in a number of logical lines. Statements -do not straddle logical line boundaries except where explicitly -indicated by the syntax (i.e., for compound statements). To this -purpose, the end of a logical line is represented by the token -NEWLINE. +A Python program is divided in a number of logical lines. The end of +a logical line is represented by the token NEWLINE. Statements cannot +cross logical line boundaries except where NEWLINE is allowed by the +syntax (e.g., between statements in compound statements). \subsection{Comments} A comment starts with a hash character (\verb\#\) that is not part of -a string literal, and ends at the end of the physical line. Comments -are ignored by the syntax. +a string literal, and ends at the end of the physical line. A comment +always signifies the end of the logical line. Comments are ignored by +the syntax. \subsection{Line joining} Two or more physical lines may be joined into logical lines using -backslash characters (\verb/\/), as follows: When physical line ends +backslash characters (\verb/\/), as follows: when 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. @@ -160,13 +161,14 @@ terminates a multi-line statement. \subsection{Indentation} -Spaces and tabs at the beginning of a logical line are used to compute -the indentation level of the line, which in turn is used to determine -the grouping of statements. +Leading whitespace (spaces and tabs) at the beginning of a logical +line is 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 total -number of spaces up to that point is a multiple of eight. The total -number of spaces preceding the first non-blank character then +First, tabs are replaced (from left to right) by one to eight spaces +such that the total number of characters up to there is a multiple of +eight (this is intended to be the same rule as used by UNIX). The +total number of spaces preceding the first non-blank character then determines the line's indentation. Indentation cannot be split over multiple physical lines using backslashes. @@ -185,6 +187,38 @@ 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. +Here is an example of a correctly (though confusingly) indented piece +of Python code: + +\begin{verbatim} +def perm(l): + if len(l) <= 1: + return [l] + r = [] + for i in range(len(l)): + s = l[:i] + l[i+1:] + p = perm(s) + for x in p: + r.append(l[i:i+1] + x) + return r +\end{verbatim} + +The following example shows various indentation errors: + +\begin{verbatim} + def perm(l): # error: first line indented + for i in range(len(l)): # error: not indented + s = l[:i] + l[i+1:] + p = perm(l[:i] + l[i+1:]) # error: unexpected indent + for x in p: + r.append(l[i:i+1] + x) + return r # error: inconsistent indent +\end{verbatim} + +(Actually, the first three errors are detected by the parser; only the +last error is found by the lexical analyzer -- the indentation of +\verb\return r\ does not match a level popped off the stack.) + \section{Other tokens} Besides NEWLINE, INDENT and DEDENT, the following categories of tokens @@ -205,12 +239,13 @@ uppercase: "A"..."Z" digit: "0"..."9" \end{verbatim} -Identifiers are unlimited in length. Case is significant. +Identifiers are unlimited in length. Case is significant. Keywords +are not identifiers. \section{Keywords} The following identifiers are used as reserved words, or {\em -keywords} of the language, and may not be used as ordinary +keywords} of the language, and cannot be used as ordinary identifiers. They must be spelled exactly as written here: \begin{verbatim} @@ -260,7 +295,7 @@ are: \verb/\'/ & Single quote (\verb/'/) \\ \verb/\a/ & ASCII Bell (BEL) \\ \verb/\b/ & ASCII Backspace (BS) \\ -\verb/\E/ & ASCII Escape (ESC) \\ +%\verb/\E/ & ASCII Escape (ESC) \\ \verb/\f/ & ASCII Formfeed (FF) \\ \verb/\n/ & ASCII Linefeed (LF) \\ \verb/\r/ & ASCII Carriage Return (CR) \\ @@ -272,13 +307,13 @@ are: \end{tabular} \end{center} -For compatibility with in Standard C, up to three octal digits are +In strict compatibility with in Standard C, up to three octal digits are accepted, but an unlimited number of hex digits is taken to be part of the hex escape (and then the lower 8 bits of the resulting hex number -are used...). +are used in all current implementations...). -All unrecognized escape sequences are left in the string {\em -unchanged}, i.e., the backslash is left in the string. (This rule is +All unrecognized escape sequences are left in the string unchanged, +i.e., {\em the backslash is left in the string.} (This rule is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken. It also helps a great deal for string literals used as regular expressions or @@ -313,6 +348,18 @@ fraction: "." digit+ exponent: ("e"|"E") ["+"|"-"] digit+ \end{verbatim} +Some examples of numeric literals: + +\begin{verbatim} +1 1234567890 0177777 0x80000 + + +\end{verbatim} + +Note that the definitions for literals do not include a sign; a phrase +like \verb\-1\ is actually an expression composed of the operator +\verb\-\ and the literal \verb\1\. + \section{Operators} The following tokens are operators: @@ -336,13 +383,16 @@ meaning: ; , : . ` = \end{verbatim} -The following printing ASCII characters are currently not used; -their occurrence is an unconditional error: +The following printing ASCII characters are not used in Python (except +in string literals and in comments). Their occurrence is an +unconditional error: \begin{verbatim} ! @ $ " ? \end{verbatim} +They may be used by future versions of the language though! + \chapter{Execution model} (XXX This chapter should explain the general model of the execution of |