373 lines
12 KiB
TeX
373 lines
12 KiB
TeX
\chapter{Lexical analysis}
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A Python program is read by a \emph{parser}. Input to the parser is a
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stream of \emph{tokens}, generated by the \emph{lexical analyzer}. This
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chapter describes how the lexical analyzer breaks a file into tokens.
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\index{lexical analysis}
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\index{parser}
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\index{token}
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\section{Line structure}
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A Python program is divided in a number of logical lines. The end of
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a logical line is represented by the token NEWLINE. Statements cannot
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cross logical line boundaries except where NEWLINE is allowed by the
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syntax (e.g. between statements in compound statements).
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\index{line structure}
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\index{logical line}
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\index{NEWLINE token}
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\subsection{Comments}
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A comment starts with a hash character (\code{\#}) that is not part of
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a string literal, and ends at the end of the physical line. A comment
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always signifies the end of the logical line. Comments are ignored by
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the syntax.
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\index{comment}
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\index{logical line}
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\index{physical line}
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\index{hash character}
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\subsection{Explicit line joining}
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Two or more physical lines may be joined into logical lines using
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backslash characters (\code{\e}), as follows: when a physical line ends
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in a backslash that is not part of a string literal or comment, it is
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joined with the following forming a single logical line, deleting the
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backslash and the following end-of-line character. For example:
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\index{physical line}
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\index{line joining}
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\index{line continuation}
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\index{backslash character}
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%
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\begin{verbatim}
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if 1900 < year < 2100 and 1 <= month <= 12 \
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and 1 <= day <= 31 and 0 <= hour < 24 \
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and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
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return 1
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\end{verbatim}
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A line ending in a backslash cannot carry a comment; a backslash does
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not continue a comment (but it does continue a string literal, see
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below).
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\subsection{Implicit line joining}
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Expressions in parentheses, square brackets or curly braces can be
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split over more than one physical line without using backslashes.
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For example:
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\begin{verbatim}
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month_names = ['Januari', 'Februari', 'Maart', # These are the
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'April', 'Mei', 'Juni', # Dutch names
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'Juli', 'Augustus', 'September', # for the months
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'Oktober', 'November', 'December'] # of the year
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\end{verbatim}
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Implicitly continued lines can carry comments. The indentation of the
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continuation lines is not important. Blank continuation lines are
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allowed.
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\subsection{Blank lines}
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A logical line that contains only spaces, tabs, and possibly a
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comment, is ignored (i.e., no NEWLINE token is generated), except that
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during interactive input of statements, an entirely blank logical line
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terminates a multi-line statement.
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\index{blank line}
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\subsection{Indentation}
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Leading whitespace (spaces and tabs) at the beginning of a logical
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line is used to compute the indentation level of the line, which in
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turn is used to determine the grouping of statements.
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\index{indentation}
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\index{whitespace}
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\index{leading whitespace}
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\index{space}
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\index{tab}
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\index{grouping}
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\index{statement grouping}
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First, tabs are replaced (from left to right) by one to eight spaces
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such that the total number of characters up to there is a multiple of
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eight (this is intended to be the same rule as used by \UNIX{}). The
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total number of spaces preceding the first non-blank character then
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determines the line's indentation. Indentation cannot be split over
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multiple physical lines using 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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\index{INDENT token}
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\index{DEDENT token}
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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 on
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the stack will always be strictly increasing from bottom to top. At
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the beginning of each logical line, the line's indentation level is
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compared to the top of the stack. If it is equal, nothing happens.
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If it is larger, it is pushed on the stack, and one INDENT token is
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generated. If it is smaller, it \emph{must} be one of the numbers
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occurring on the stack; all numbers on the stack that are larger are
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popped off, and for each number popped off a DEDENT token is
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generated. At the end of the file, a DEDENT token is generated for
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each number remaining on the stack that is larger than zero.
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Here is an example of a correctly (though confusingly) indented piece
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of Python code:
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\begin{verbatim}
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def perm(l):
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# Compute the list of all permutations of l
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if len(l) <= 1:
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return [l]
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r = []
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for i in range(len(l)):
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s = l[:i] + l[i+1:]
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p = perm(s)
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for x in p:
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r.append(l[i:i+1] + x)
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return r
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\end{verbatim}
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The following example shows various indentation errors:
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\begin{verbatim}
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def perm(l): # error: first line indented
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for i in range(len(l)): # error: not indented
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s = l[:i] + l[i+1:]
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p = perm(l[:i] + l[i+1:]) # error: unexpected indent
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for x in p:
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r.append(l[i:i+1] + x)
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return r # error: inconsistent dedent
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\end{verbatim}
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(Actually, the first three errors are detected by the parser; only the
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last error is found by the lexical analyzer --- the indentation of
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\code{return r} does not match a level popped off the stack.)
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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. Where
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ambiguity exists, a token comprises the longest possible string that
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forms a legal token, when read from left to right.
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\section{Identifiers}
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Identifiers (also referred to as names) are described by the following
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lexical definitions:
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\index{identifier}
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\index{name}
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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"..."z"
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uppercase: "A"..."Z"
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digit: "0"..."9"
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\end{verbatim}
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Identifiers are unlimited in length. Case is significant.
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\subsection{Keywords}
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The following identifiers are used as reserved words, or
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\emph{keywords} of the language, and cannot be used as ordinary
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identifiers. They must be spelled exactly as written here:%
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\index{keyword}%
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\index{reserved word}
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\begin{verbatim}
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and elif global not try
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break else if or while
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class except import pass
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continue finally in print
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def for is raise
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del from lambda return
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\end{verbatim}
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% When adding keywords, pipe it through keywords.py for reformatting
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\section{Literals} \label{literals}
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Literals are notations for constant values of some built-in types.
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\index{literal}
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\index{constant}
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\subsection{String literals}
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String literals are described by the following lexical definitions:
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\index{string literal}
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\begin{verbatim}
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stringliteral: shortstring | longstring
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shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"'
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longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
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shortstringitem: shortstringchar | escapeseq
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longstringitem: longstringchar | escapeseq
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shortstringchar: <any ASCII character except "\" or newline or the quote>
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longstringchar: <any ASCII character except "\">
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escapeseq: "\" <any ASCII character>
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\end{verbatim}
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\index{ASCII@\ASCII{}}
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In ``long strings'' (strings surrounded by sets of three quotes),
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unescaped newlines and quotes are allowed (and are retained), except
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that three unescaped quotes in a row terminate the string. (A
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``quote'' is the character used to open the string, i.e. either
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\code{'} or \code{"}.)
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Escape sequences in strings are interpreted according to rules similar
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to those used by Standard C. The recognized escape sequences are:
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\index{physical line}
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\index{escape sequence}
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\index{Standard C}
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\index{C}
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\begin{center}
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\begin{tabular}{|l|l|}
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\hline
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\code{\e}\emph{newline} & Ignored \\
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\code{\e\e} & Backslash (\code{\e}) \\
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\code{\e'} & Single quote (\code{'}) \\
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\code{\e"} & Double quote (\code{"}) \\
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\code{\e a} & \ASCII{} Bell (BEL) \\
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\code{\e b} & \ASCII{} Backspace (BS) \\
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%\code{\e E} & \ASCII{} Escape (ESC) \\
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\code{\e f} & \ASCII{} Formfeed (FF) \\
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\code{\e n} & \ASCII{} Linefeed (LF) \\
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\code{\e r} & \ASCII{} Carriage Return (CR) \\
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\code{\e t} & \ASCII{} Horizontal Tab (TAB) \\
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\code{\e v} & \ASCII{} Vertical Tab (VT) \\
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\code{\e}\emph{ooo} & \ASCII{} character with octal value \emph{ooo} \\
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\code{\e x}\emph{xx...} & \ASCII{} character with hex value \emph{xx...} \\
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\hline
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\end{tabular}
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\end{center}
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\index{ASCII@\ASCII{}}
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In strict compatibility with Standard \C, up to three octal digits are
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accepted, but an unlimited number of hex digits is taken to be part of
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the hex escape (and then the lower 8 bits of the resulting hex number
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are used in all current implementations...).
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All unrecognized escape sequences are left in the string unchanged,
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i.e., \emph{the backslash is left in the string.} (This behavior is
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useful when debugging: if an escape sequence is mistyped, the
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resulting output is more easily recognized as broken. It also helps a
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great deal for string literals used as regular expressions or
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otherwise passed to other modules that do their own escape handling.)
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\index{unrecognized escape sequence}
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\subsection{Numeric literals}
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There are three types of numeric literals: plain integers, long
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integers, and floating point numbers.
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\index{number}
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\index{numeric literal}
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\index{integer literal}
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\index{plain integer literal}
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\index{long integer literal}
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\index{floating point literal}
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\index{hexadecimal literal}
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\index{octal literal}
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\index{decimal literal}
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Integer and long integer literals are described by the following
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lexical definitions:
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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"..."9"
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octdigit: "0"..."7"
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hexdigit: digit|"a"..."f"|"A"..."F"
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\end{verbatim}
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Although both lower case `l' and upper case `L' are allowed as suffix
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for long integers, it is strongly recommended to always use `L', since
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the letter `l' looks too much like the digit `1'.
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Plain integer decimal literals must be at most 2147483647 (i.e., the
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largest positive integer, using 32-bit arithmetic). Plain octal and
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hexadecimal literals may be as large as 4294967295, but values larger
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than 2147483647 are converted to a negative value by subtracting
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4294967296. There is no limit for long integer literals apart from
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what can be stored in available memory.
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Some examples of plain and long integer literals:
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\begin{verbatim}
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7 2147483647 0177 0x80000000
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3L 79228162514264337593543950336L 0377L 0x100000000L
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\end{verbatim}
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Floating point literals are described by the following lexical
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definitions:
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\begin{verbatim}
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floatnumber: pointfloat | exponentfloat
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pointfloat: [intpart] fraction | intpart "."
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exponentfloat: (intpart | pointfloat) 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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The allowed range of floating point literals is
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implementation-dependent.
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Some examples of floating point literals:
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\begin{verbatim}
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3.14 10. .001 1e100 3.14e-10
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\end{verbatim}
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Note that numeric literals do not include a sign; a phrase like
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\code{-1} is actually an expression composed of the operator
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\code{-} and the literal \code{1}.
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\section{Operators}
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The following tokens are operators:
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\index{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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The comparison operators \code{<>} and \code{!=} are alternate
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spellings of the same operator.
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\section{Delimiters}
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The following tokens serve as delimiters or otherwise have a special
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meaning:
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\index{delimiters}
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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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The following printing \ASCII{} characters are not used in Python. Their
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occurrence outside string literals and comments is an unconditional
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error:
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\index{ASCII@\ASCII{}}
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\begin{verbatim}
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@ $ ?
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\end{verbatim}
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They may be used by future versions of the language though!
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