371 lines
12 KiB
TeX
371 lines
12 KiB
TeX
\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}
|
|
access del from lambda return
|
|
and elif global not try
|
|
break else if or while
|
|
class except import pass
|
|
continue finally in print
|
|
def for is raise
|
|
\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
|
|
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 $2^{31} - 1$ (i.e., the
|
|
largest positive integer, assuming 32-bit arithmetic). Plain octal and
|
|
hexadecimal literals may be as large as $2^{32} - 1$, but values
|
|
larger than $2^{31} - 1$ are converted to a negative value by
|
|
subtracting $2^{32}$. There is no limit for long integer literals.
|
|
|
|
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!
|