Doc/ref1.tex

   1 \chapter{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
  17 It is dangerous to add too many implementation details to a language
  18 reference document --- the implementation may change, and other
  19 implementations of the same language may work differently.  On the
  20 other hand, there is currently only one Python implementation, and
  21 its particular quirks are sometimes worth being mentioned, especially
  22 where the implementation imposes additional limitations.  Therefore,
  23 you'll find short ``implementation notes'' sprinkled throughout the
  24 text.
  25
  26 Every Python implementation comes with a number of built-in and
  27 standard modules.  These are not documented here, but in the separate
  28 {\em Python Library Reference} document.  A few built-in modules are
  29 mentioned when they interact in a significant way with the language
  30 definition.
  31
  32 \section{Notation}
  33
  34 The descriptions of lexical analysis and syntax use a modified BNF
  35 grammar notation.  This uses the following style of definition:
  36 \index{BNF}
  37 \index{grammar}
  38 \index{syntax}
  39 \index{notation}
  40
  41 \begin{verbatim}
  42 name:           lc_letter (lc_letter | "_")*
  43 lc_letter:      "a"..."z"
  44 \end{verbatim}
  45
  46 The first line says that a \verb@name@ is an \verb@lc_letter@ followed by
  47 a sequence of zero or more \verb@lc_letter@s and underscores.  An
  48 \verb@lc_letter@ in turn is any of the single characters `a' through `z'.
  49 (This rule is actually adhered to for the names defined in lexical and
  50 grammar rules in this document.)
  51
  52 Each rule begins with a name (which is the name defined by the rule)
  53 and a colon.  A vertical bar (\verb@|@) is used to separate
  54 alternatives; it is the least binding operator in this notation.  A
  55 star (\verb@*@) means zero or more repetitions of the preceding item;
  56 likewise, a plus (\verb@+@) means one or more repetitions, and a
  57 phrase enclosed in square brackets (\verb@[ ]@) means zero or one
  58 occurrences (in other words, the enclosed phrase is optional).  The
  59 \verb@*@ and \verb@+@ operators bind as tightly as possible;
  60 parentheses are used for grouping.  Literal strings are enclosed in
  61 quotes.  White space is only meaningful to separate tokens.
  62 Rules are normally contained on a single line; rules with many
  63 alternatives may be formatted alternatively with each line after the
  64 first beginning with a vertical bar.
  65
  66 In lexical definitions (as the example above), two more conventions
  67 are used: Two literal characters separated by three dots mean a choice
  68 of any single character in the given (inclusive) range of \ASCII{}
  69 characters.  A phrase between angular brackets (\verb@<...>@) gives an
  70 informal description of the symbol defined; e.g. this could be used
  71 to describe the notion of `control character' if needed.
  72 \index{lexical definitions}
  73 \index{ASCII}
  74
  75 Even though the notation used is almost the same, there is a big
  76 difference between the meaning of lexical and syntactic definitions:
  77 a lexical definition operates on the individual characters of the
  78 input source, while a syntax definition operates on the stream of
  79 tokens generated by the lexical analysis.  All uses of BNF in the next
  80 chapter (``Lexical Analysis'') are lexical definitions; uses in
  81 subsequent chapters are syntactic definitions.