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Lots of changes to get this in sync with the Frame version. 

Added raw strings, imaginary literals, assert and exec (!) keywords, a
table about Resererved classes of identifiers, and more.
This commit is contained in:
Guido van Rossum 1998-06-15 18:00:50 +00:00
parent 0bd3795d6a
commit 60f2f0cf8e
1 changed files with 217 additions and 50 deletions
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267
Doc/ref/ref2.tex
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@ -7,25 +7,61 @@ chapter describes how the lexical analyzer breaks a file into tokens.
\index{parser}
\index{token}
Python uses the 7-bit \ASCII{} character set for program text and string
literals. 8-bit characters may be used in string literals and comments
but their interpretation is platform dependent; the proper way to
insert 8-bit characters in string literals is by using octal or
hexadecimal escape sequences.
The run-time character set depends on the I/O devices connected to the
program but is generally a superset of \ASCII{}.
\strong{Future compatibility note:} It may be tempting to assume that the
character set for 8-bit characters is ISO Latin-1 (an \ASCII{}
superset that covers most western languages that use the Latin
alphabet), but it is possible that in the future Unicode text editors
will become common. These generally use the UTF-8 encoding, which is
also an \ASCII{} superset, but with very different use for the
characters with ordinals 128-255. While there is no consensus on this
subject yet, it is unwise to assume either Latin-1 or UTF-8, even
though the current implementation appears to favor Latin-1. This
applies both to the source character set and the run-time character
set.
\section{Line structure}
A Python program is divided in a number of logical lines. The end of
A Python program is divided into a number of \emph{logical lines}.
\index{line structure}
\subsection{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}
A logical line is constructed from one or more \emph{physical lines}
by following the explicit or implicit \emph{line joining} rules.
\index{logical line}
\index{physical line}
\index{line joining}
\index{NEWLINE token}
\subsection{Physical lines}
A physical line ends in whatever the current platform's convention is
for terminating lines. On \UNIX{}, this is the \ASCII{} LF (linefeed)
character. On DOS/Windows, it is the \ASCII{} sequence CR LF (return
followed by linefeed). On Macintosh, it is the \ASCII{} CR (return)
character.
\subsection{Comments}
A comment starts with a hash character (\code{\#}) 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.
signifies the end of the logical line unless the implicit line joining
rules are invoked.
Comments are ignored by the syntax; they are not tokens.
\index{comment}
\index{logical line}
\index{physical line}
\index{hash character}
\subsection{Explicit line joining}
@ -47,9 +83,11 @@ if 1900 < year < 2100 and 1 <= month <= 12 \
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).
A line ending in a backslash cannot carry a comment. A backslash does
not continue a comment. A backslash does not continue a token except
for string literals (i.e., tokens other than string literals cannot be
split across physical lines using a backslash). A backslash is
illegal elsewhere on a line outside a string literal.
\subsection{Implicit line joining}
@ -66,13 +104,16 @@ month_names = ['Januari', 'Februari', 'Maart', # These are the
Implicitly continued lines can carry comments. The indentation of the
continuation lines is not important. Blank continuation lines are
allowed.
allowed. There is no NEWLINE token between implicit continuation
lines. Implicitly continued lines can also occur within triple-quoted
strings (see below); in that case they cannot carry comments.
\subsection{Blank lines}
A logical line that contains only spaces, tabs, and possibly a
A logical line that contains only spaces, tabs, formfeeds 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
(i.e. one containing not even whitespace or a comment)
terminates a multi-line statement.
\index{blank line}
@ -90,11 +131,23 @@ turn is used to determine the grouping of statements.
\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
such that the total number of characters up to and including the
replacement 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.
multiple physical lines using backslashes; the whitespace up to the
first backslash determines the indentation.
\strong{Cross-platform compatibility note:} because of the nature of
text editors on non-UNIX platforms, it is unwise to use a mixture of
spaces and tabs for the indentation in a single source file.
A formfeed character may be present at the start of the line; it will
be ignored for the indentation calculations above. A formfeed
characters occurring elsewhere in the leading whitespace have an
undefined effect (for instance, they may reset the space count to
zero).
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.
@ -119,7 +172,6 @@ of Python code:
\begin{verbatim}
def perm(l):
# Compute the list of all permutations of l
if len(l) <= 1:
return [l]
r = []
@ -134,7 +186,7 @@ def perm(l):
The following example shows various indentation errors:
\begin{verbatim}
def perm(l): # error: first line indented
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
@ -147,17 +199,28 @@ The following example shows various indentation errors:
last error is found by the lexical analyzer --- the indentation of
\code{return r} does not match a level popped off the stack.)
\subsection{Whitespace between tokens}
Except at the beginning of a logical line or in string literals, the
whitespace characters space, tab and formfeed can be used
interchangeably to separate tokens. Whitespace is needed between two
tokens only if their concatenation could otherwise be interpreted as a
different token (e.g., ab is one token, but a b is two tokens).
\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
exist: \emph{identifiers}, \emph{keywords}, \emph{literals},
\emph{operators}, and \emph{delimiters}.
Whitespace characters (other than line terminators, discussed earlier)
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}
\section{Identifiers and keywords}
Identifiers (also referred to as names) are described by the following
Identifiers (also referred to as \emph{names}) are described by the following
lexical definitions:
\index{identifier}
\index{name}
@ -181,15 +244,34 @@ identifiers. They must be spelled exactly as written here:%
\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
and del for is raise
assert elif from lambda return
break else global not try
class except if or while
continue exec import pass
def finally in print
\end{verbatim}
% When adding keywords, pipe it through keywords.py for reformatting
% When adding keywords, use reswords.py for reformatting
\subsection{Reserved classes of identifiers}
Certain classes of identifiers (besides keywords) have special
meanings. These are:
\begin{center}
\begin{tabular}{|l|l|}
\hline
Form & Meaning \\
\hline
\code{_*} & Not imported by \code{from \var{module} import *} \\
\code{__*__} & System-defined name \\
\code{__*} & Class-private name mangling \\
\hline
\end{tabular}
\end{center}
(XXX need section references here.)
\section{Literals} \label{literals}
@ -214,14 +296,27 @@ escapeseq: "\" <any ASCII character>
\end{verbatim}
\index{ASCII@\ASCII{}}
In ``long strings'' (strings surrounded by sets of three quotes),
In plain English: String literals can be enclosed in matching single
quotes (\code{'}) or double quotes (\code{"}). They can also be
enclosed in matching groups of three single or double quotes (these
are generally referred to as \emph{triple-quoted strings}). The
backslash (\code{\e}) character is used to escape characters that
otherwise have a special meaning, such as newline, backslash itself,
or the quote character. String literals may optionally be prefixed
with a letter `r' or `R'; such strings are called raw strings and use
different rules for backslash escape sequences.
\index{triple-quoted string}
\index{raw string}
In triple-quoted strings,
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
\code{'} or \code{"}.)
Escape sequences in strings are interpreted according to rules similar
to those used by Standard C. The recognized escape sequences are:
Unless an `r' or `R' prefix is present, 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}
@ -230,20 +325,21 @@ to those used by Standard C. The recognized escape sequences are:
\begin{center}
\begin{tabular}{|l|l|}
\hline
Escape Sequence & Meaning \\
\hline
\code{\e}\emph{newline} & Ignored \\
\code{\e\e} & Backslash (\code{\e}) \\
\code{\e'} & Single quote (\code{'}) \\
\code{\e"} & Double quote (\code{"}) \\
\code{\e a} & \ASCII{} Bell (BEL) \\
\code{\e b} & \ASCII{} Backspace (BS) \\
%\code{\e E} & \ASCII{} Escape (ESC) \\
\code{\e f} & \ASCII{} Formfeed (FF) \\
\code{\e n} & \ASCII{} Linefeed (LF) \\
\code{\e r} & \ASCII{} Carriage Return (CR) \\
\code{\e t} & \ASCII{} Horizontal Tab (TAB) \\
\code{\e v} & \ASCII{} Vertical Tab (VT) \\
\code{\e}\emph{ooo} & \ASCII{} character with octal value \emph{ooo} \\
\code{\e x}\emph{xx...} & \ASCII{} character with hex value \emph{xx...} \\
\code{\e x}\emph{hh...} & \ASCII{} character with hex value \emph{hh...} \\
\hline
\end{tabular}
\end{center}
@ -252,20 +348,55 @@ to those used by Standard C. The recognized escape sequences are:
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...).
are used in 8-bit implementations).
All unrecognized escape sequences are left in the string unchanged,
Unlike Standard \C{},
all unrecognized escape sequences are left in the string unchanged,
i.e., \emph{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.)
resulting output is more easily recognized as broken.)
\index{unrecognized escape sequence}
When an `r' or `R' prefix is present, backslashes are still used to
quote the following character, but \emph{all backslashes are left in
the string}. For example, the string literal \code{r"\e n"} consists
of two characters: a backslash and a lowercase `n'. String quotes can
be escaped with a backslash, but the backslash remains in the string;
for example, \code{r"\""} is a valid string literal consisting of two
characters: a backslash and a double quote; \code{r"\"} is not a value
string literal (even a raw string cannot end in an odd number of
backslashes). Specifically, \emph{a raw string cannot end in a single
backslash} (since the backslash would escape the following quote
character).
\subsection{String literal concatenation}
Multiple adjacent string literals (delimited by whitespace), possibly
using different quoting conventions, are allowed, and their meaning is
the same as their concatenation. Thus, \code{"hello" 'world'} is
equivalent to \code{"helloworld"}. This feature can be used to reduce
the number of backslashes needed, to split long strings conveniently
across long lines, or even to add comments to parts of strings, for
example:
\begin{verbatim}
re.compile("[A-Za-z_]" # letter or underscore
"[A-Za-z0-9_]*" # letter, digit or underscore
)
\end{verbatim}
Note that this feature is defined at the syntactical level, but
implemented at compile time. The `+' operator must be used to
concatenate string expressions at run time. Also note that literal
concatenation can use different quoting styles for each component
(even mixing raw strings and triple quoted strings).
\subsection{Numeric literals}
There are three types of numeric literals: plain integers, long
integers, and floating point numbers.
There are four types of numeric literals: plain integers, long
integers, floating point numbers, and imaginary numbers. There are no
complex literals (complex numbers can be formed by adding a real
number and an imaginary number).
\index{number}
\index{numeric literal}
\index{integer literal}
@ -275,6 +406,14 @@ integers, and floating point numbers.
\index{hexadecimal literal}
\index{octal literal}
\index{decimal literal}
\index{imaginary literal}
\index{complex literal}
Note that numeric literals do not include a sign; a phrase like
\code{-1} is actually an expression composed of the unary operator
`\code{-}' and the literal \code{1}.
\subsection{Integer and long integer literals}
Integer and long integer literals are described by the following
lexical definitions:
@ -285,7 +424,6 @@ 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"
@ -309,6 +447,8 @@ Some examples of plain and long integer literals:
3L 79228162514264337593543950336L 0377L 0x100000000L
\end{verbatim}
\subsection{Floating point literals}
Floating point literals are described by the following lexical
definitions:
@ -316,14 +456,15 @@ definitions:
floatnumber: pointfloat | exponentfloat
pointfloat: [intpart] fraction | intpart "."
exponentfloat: (intpart | pointfloat) exponent
intpart: digit+
intpart: nonzerodigit digit* | "0"
fraction: "." digit+
exponent: ("e"|"E") ["+"|"-"] digit+
\end{verbatim}
Note that the integer part of a floating point number cannot look like
an octal integer.
The allowed range of floating point literals is
implementation-dependent.
Some examples of floating point literals:
\begin{verbatim}
@ -334,30 +475,58 @@ Note that numeric literals do not include a sign; a phrase like
\code{-1} is actually an expression composed of the operator
\code{-} and the literal \code{1}.
\subsection{Imaginary literals}
Imaginary literals are described by the following lexical definitions:
\begin{verbatim}
imagnumber: (floatnumber | intpart) ("j"|"J")
\end{verbatim}
An imaginary literals yields a complex number with a real part of
0.0. Complex numbers are represented as a pair of floating point
numbers and have the same restrictions on their range. To create a
complex number with a nonzero real part, add a floating point number
to it, e.g. \code{(3+4j)}. Some examples of imaginary literals:
\begin{verbatim}
3.14j 10.j 10 j .001j 1e100j 3.14e-10j
\end{verbatim}
\section{Operators}
The following tokens are operators:
\index{operators}
\begin{verbatim}
+ - * / %
+ - * ** / %
<< >> & | ^ ~
< == > <= <> != >=
< > <= >= == != <>
\end{verbatim}
The comparison operators \code{<>} and \code{!=} are alternate
spellings of the same operator.
spellings of the same operator. \code{!=} is the preferred spelling;
\code{<>} is obsolescent.
\section{Delimiters}
The following tokens serve as delimiters or otherwise have a special
meaning:
The following tokens serve as delimiters in the grammar:
\index{delimiters}
\begin{verbatim}
( ) [ ] { }
, : . " ` '
= ;
, : . ` = ;
\end{verbatim}
The period can also occur in floating-point and imaginary literals. A
sequence of three periods has a special meaning as ellipses in slices.
The following printing ASCII characters have special meaning as part
of other tokens or are otherwise significant to the lexical analyzer:
\begin{verbatim}
' " # \
\end{verbatim}
The following printing \ASCII{} characters are not used in Python. Their
@ -368,5 +537,3 @@ error:
\begin{verbatim}
@ $ ?
\end{verbatim}
They may be used by future versions of the language though!