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\chapter{Lexical analysis} A Python program is read by a {\em parser}. Input to the parser is a stream of {\em tokens}, generated by the {\em lexical analyzer}. This chapter describes how the lexical analyzer breaks a file into tokens. \index{lexical analysis} \index{parser} \index{token} \section{Line structure} 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). \index{line structure} \index{logical line} \index{NEWLINE token} \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. A comment always signifies the end of the logical line. Comments are ignored by the syntax. \index{comment} \index{logical line} \index{physical line} \index{hash character} \subsection{Explicit line joining} Two or more physical lines may be joined into logical lines using 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. For example: \index{physical line} \index{line joining} \index{line continuation} \index{backslash character} % \begin{verbatim} if 1900 < year < 2100 and 1 <= month <= 12 \ and 1 <= day <= 31 and 0 <= hour < 24 \ and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date return 1 \end{verbatim} A line ending in a backslash cannot carry a comment; a backslash does not continue a comment (but it does continue a string literal, see below). \subsection{Implicit line joining} Expressions in parentheses, square brackets or curly braces can be split over more than one physical line without using backslashes. For example: \begin{verbatim} month_names = ['Januari', 'Februari', 'Maart', # These are the 'April', 'Mei', 'Juni', # Dutch names 'Juli', 'Augustus', 'September', # for the months 'Oktober', 'November', 'December'] # of the year \end{verbatim} Implicitly continued lines can carry comments. The indentation of the continuation lines is not important. Blank continuation lines are allowed. \subsection{Blank lines} A logical line that contains only spaces, tabs, and possibly a comment, is ignored (i.e., no NEWLINE token is generated), except that during interactive input of statements, an entirely blank logical line terminates a multi-line statement. \index{blank line} \subsection{Indentation} 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. \index{indentation} \index{whitespace} \index{leading whitespace} \index{space} \index{tab} \index{grouping} \index{statement grouping} 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. The indentation levels of consecutive lines are used to generate INDENT and DEDENT tokens, using a stack, as follows. \index{INDENT token} \index{DEDENT token} Before the first line of the file is read, a single zero is pushed on the stack; this will never be popped off again. The numbers pushed on the stack will always be strictly increasing from bottom to top. At the beginning of each logical line, the line's indentation level is compared to the top of the stack. If it is equal, nothing happens. If it is larger, it is pushed on the stack, and one INDENT token is generated. If it is smaller, it {\em must} be one of the numbers occurring on the stack; all numbers on the stack that are larger are popped off, and for each number popped off a DEDENT token is generated. At the end of the file, a DEDENT token is generated for each number remaining on the stack that is larger than zero. Here is an example of a correctly (though confusingly) indented piece of Python code: \begin{verbatim} def perm(l): # Compute the list of all permutations of 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 dedent \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 exist: identifiers, keywords, literals, operators, and delimiters. Spaces and tabs are not tokens, but serve to delimit tokens. Where ambiguity exists, a token comprises the longest possible string that forms a legal token, when read from left to right. \section{Identifiers} Identifiers (also referred to as names) are described by the following lexical definitions: \index{identifier} \index{name} \begin{verbatim} identifier: (letter|"_") (letter|digit|"_")* letter: lowercase | uppercase lowercase: "a"..."z" uppercase: "A"..."Z" digit: "0"..."9" \end{verbatim} Identifiers are unlimited in length. Case is significant. \subsection{Keywords} The following identifiers are used as reserved words, or {\em keywords} of the language, and cannot be used as ordinary identifiers. They must be spelled exactly as written here: \index{keyword} \index{reserved word} \begin{verbatim} and elif global not try break else if or while class except import pass continue finally in print def for is raise del from lambda return \end{verbatim} % When adding keywords, pipe it through keywords.py for reformatting \section{Literals} \label{literals} Literals are notations for constant values of some built-in types. \index{literal} \index{constant} \subsection{String literals} String literals are described by the following lexical definitions: \index{string literal} \begin{verbatim} stringliteral: shortstring | longstring shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"' longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""' shortstringitem: shortstringchar | escapeseq longstringitem: longstringchar | escapeseq shortstringchar: <any ASCII character except "\" or newline or the quote> longstringchar: <any ASCII character except "\"> escapeseq: "\" <any ASCII character> \end{verbatim} \index{ASCII} In ``long strings'' (strings surrounded by sets of three quotes), unescaped newlines and quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the string. (A ``quote'' is the character used to open the string, i.e. either \verb/'/ or \verb/"/.) Escape sequences in strings are interpreted according to rules similar to those used by Standard C. The recognized escape sequences are: \index{physical line} \index{escape sequence} \index{Standard C} \index{C} \begin{center} \begin{tabular}{|l|l|} \hline \verb/\/{\em newline} & Ignored \\ \verb/\\/ & Backslash (\verb/\/) \\ \verb/\'/ & Single quote (\verb/'/) \\ \verb/\"/ & Double quote (\verb/"/) \\ \verb/\a/ & \ASCII{} Bell (BEL) \\ \verb/\b/ & \ASCII{} Backspace (BS) \\ %\verb/\E/ & \ASCII{} Escape (ESC) \\ \verb/\f/ & \ASCII{} Formfeed (FF) \\ \verb/\n/ & \ASCII{} Linefeed (LF) \\ \verb/\r/ & \ASCII{} Carriage Return (CR) \\ \verb/\t/ & \ASCII{} Horizontal Tab (TAB) \\ \verb/\v/ & \ASCII{} Vertical Tab (VT) \\ \verb/\/{\em ooo} & \ASCII{} character with octal value {\em ooo} \\ \verb/\x/{\em xx...} & \ASCII{} character with hex value {\em xx...} \\ \hline \end{tabular} \end{center} \index{ASCII} In strict compatibility with 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 in all current implementations...). All unrecognized escape sequences are left in the string unchanged, i.e., {\em the backslash is left in the string.} (This behavior 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 otherwise passed to other modules that do their own escape handling.) \index{unrecognized escape sequence} \subsection{Numeric literals} There are three types of numeric literals: plain integers, long integers, and floating point numbers. \index{number} \index{numeric literal} \index{integer literal} \index{plain integer literal} \index{long integer literal} \index{floating point literal} \index{hexadecimal literal} \index{octal literal} \index{decimal literal} Integer and long integer literals are described by the following lexical definitions: \begin{verbatim} longinteger: integer ("l"|"L") integer: decimalinteger | octinteger | hexinteger decimalinteger: nonzerodigit digit* | "0" octinteger: "0" octdigit+ hexinteger: "0" ("x"|"X") hexdigit+ nonzerodigit: "1"..."9" octdigit: "0"..."7" hexdigit: digit|"a"..."f"|"A"..."F" \end{verbatim} Although both lower case `l' and upper case `L' are allowed as suffix for long integers, it is strongly recommended to always use `L', since the letter `l' looks too much like the digit `1'. Plain integer decimal literals must be at most 2147483647 (i.e., the largest positive integer, using 32-bit arithmetic). Plain octal and hexadecimal literals may be as large as 4294967295, but values larger than 2147483647 are converted to a negative value by subtracting 4294967296. There is no limit for long integer literals apart from what can be stored in available memory. Some examples of plain and long integer literals: \begin{verbatim} 7 2147483647 0177 0x80000000 3L 79228162514264337593543950336L 0377L 0x100000000L \end{verbatim} Floating point literals are described by the following lexical definitions: \begin{verbatim} floatnumber: pointfloat | exponentfloat pointfloat: [intpart] fraction | intpart "." exponentfloat: (intpart | pointfloat) exponent intpart: digit+ fraction: "." digit+ exponent: ("e"|"E") ["+"|"-"] digit+ \end{verbatim} The allowed range of floating point literals is implementation-dependent. Some examples of floating point literals: \begin{verbatim} 3.14 10. .001 1e100 3.14e-10 \end{verbatim} Note that numeric 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: \index{operators} \begin{verbatim} + - * / % << >> & | ^ ~ < == > <= <> != >= \end{verbatim} The comparison operators \verb@<>@ and \verb@!=@ are alternate spellings of the same operator. \section{Delimiters} The following tokens serve as delimiters or otherwise have a special meaning: \index{delimiters} \begin{verbatim} ( ) [ ] { } , : . " ` ' = ; \end{verbatim} The following printing \ASCII{} characters are not used in Python. Their occurrence outside string literals and comments is an unconditional error: \index{ASCII} \begin{verbatim} @ $ ? \end{verbatim} They may be used by future versions of the language though!


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