Doc/ref/ref1.tex

   1 \chapter{Introduction\label{introduction}}
   2
   3 This reference manual describes the Python programming language.
   4 It is not intended as a tutorial.
   5
   6 While I am trying to be as precise as possible, I chose to use English
   7 rather than formal specifications for everything except syntax and
   8 lexical analysis.  This should make the document more understandable
   9 to the average reader, but will leave room for ambiguities.
  10 Consequently, if you were coming from Mars and tried to re-implement
  11 Python from this document alone, you might have to guess things and in
  12 fact you would probably end up implementing quite a different language.
  13 On the other hand, if you are using
  14 Python and wonder what the precise rules about a particular area of
  15 the language are, you should definitely be able to find them here.
  16 If you would like to see a more formal definition of the language,
  17 maybe you could volunteer your time --- or invent a cloning machine
  18 :-).
  19
  20 It is dangerous to add too many implementation details to a language
  21 reference document --- the implementation may change, and other
  22 implementations of the same language may work differently.  On the
  23 other hand, there is currently only one Python implementation in
  24 widespread use (although a second one now exists!), and
  25 its particular quirks are sometimes worth being mentioned, especially
  26 where the implementation imposes additional limitations.  Therefore,
  27 you'll find short ``implementation notes'' sprinkled throughout the
  28 text.
  29
  30 Every Python implementation comes with a number of built-in and
  31 standard modules.  These are not documented here, but in the separate
  32 \citetitle[../lib/lib.html]{Python Library Reference} document.  A few
  33 built-in modules are mentioned when they interact in a significant way
  34 with the language definition.
  35
  36 \section{Notation\label{notation}}
  37
  38 The descriptions of lexical analysis and syntax use a modified BNF
  39 grammar notation.  This uses the following style of definition:
  40 \index{BNF}
  41 \index{grammar}
  42 \index{syntax}
  43 \index{notation}
  44
  45 \begin{verbatim}
  46 name:           lc_letter (lc_letter | "_")*
  47 lc_letter:      "a"..."z"
  48 \end{verbatim}
  49
  50 The first line says that a \code{name} is an \code{lc_letter} followed by
  51 a sequence of zero or more \code{lc_letter}s and underscores.  An
  52 \code{lc_letter} in turn is any of the single characters \character{a}
  53 through \character{z}.  (This rule is actually adhered to for the
  54 names defined in lexical and grammar rules in this document.)
  55
  56 Each rule begins with a name (which is the name defined by the rule)
  57 and a colon.  A vertical bar (\code{|}) is used to separate
  58 alternatives; it is the least binding operator in this notation.  A
  59 star (\code{*}) means zero or more repetitions of the preceding item;
  60 likewise, a plus (\code{+}) means one or more repetitions, and a
  61 phrase enclosed in square brackets (\code{[ ]}) means zero or one
  62 occurrences (in other words, the enclosed phrase is optional).  The
  63 \code{*} and \code{+} operators bind as tightly as possible;
  64 parentheses are used for grouping.  Literal strings are enclosed in
  65 quotes.  White space is only meaningful to separate tokens.
  66 Rules are normally contained on a single line; rules with many
  67 alternatives may be formatted alternatively with each line after the
  68 first beginning with a vertical bar.
  69
  70 In lexical definitions (as the example above), two more conventions
  71 are used: Two literal characters separated by three dots mean a choice
  72 of any single character in the given (inclusive) range of \ASCII{}
  73 characters.  A phrase between angular brackets (\code{<...>}) gives an
  74 informal description of the symbol defined; e.g., this could be used
  75 to describe the notion of `control character' if needed.
  76 \index{lexical definitions}
  77 \index{ASCII@\ASCII{}}
  78
  79 Even though the notation used is almost the same, there is a big
  80 difference between the meaning of lexical and syntactic definitions:
  81 a lexical definition operates on the individual characters of the
  82 input source, while a syntax definition operates on the stream of
  83 tokens generated by the lexical analysis.  All uses of BNF in the next
  84 chapter (``Lexical Analysis'') are lexical definitions; uses in
  85 subsequent chapters are syntactic definitions.