| \chapter{Introduction} |
| |
| This reference manual describes the Python programming language. |
| It is not intended as a tutorial. |
| |
| While I am trying to be as precise as possible, I chose to use English |
| rather than formal specifications for everything except syntax and |
| lexical analysis. This should make the document more understandable |
| to the average reader, but will leave room for ambiguities. |
| Consequently, if you were coming from Mars and tried to re-implement |
| Python from this document alone, you might have to guess things and in |
| fact you would probably end up implementing quite a different language. |
| On the other hand, if you are using |
| Python and wonder what the precise rules about a particular area of |
| the language are, you should definitely be able to find them here. |
| If you would like to see a more formal definitition of the language, |
| maybe you could volunteer your time --- or invent a cloning machine |
| :-). |
| |
| It is dangerous to add too many implementation details to a language |
| reference document --- the implementation may change, and other |
| implementations of the same language may work differently. On the |
| other hand, there is currently only one Python implementation in |
| widespread use (although a second one now exists!), and |
| its particular quirks are sometimes worth being mentioned, especially |
| where the implementation imposes additional limitations. Therefore, |
| you'll find short ``implementation notes'' sprinkled throughout the |
| text. |
| |
| Every Python implementation comes with a number of built-in and |
| standard modules. These are not documented here, but in the separate |
| {\em Python Library Reference} document. A few built-in modules are |
| mentioned when they interact in a significant way with the language |
| definition. |
| |
| \section{Notation} |
| |
| The descriptions of lexical analysis and syntax use a modified BNF |
| grammar notation. This uses the following style of definition: |
| \index{BNF} |
| \index{grammar} |
| \index{syntax} |
| \index{notation} |
| |
| \begin{verbatim} |
| name: lc_letter (lc_letter | "_")* |
| lc_letter: "a"..."z" |
| \end{verbatim} |
| |
| The first line says that a \code{name} is an \code{lc_letter} followed by |
| a sequence of zero or more \code{lc_letter}s and underscores. An |
| \code{lc_letter} in turn is any of the single characters \character{a} |
| through \character{z}. (This rule is actually adhered to for the |
| names defined in lexical and grammar rules in this document.) |
| |
| Each rule begins with a name (which is the name defined by the rule) |
| and a colon. A vertical bar (\code{|}) is used to separate |
| alternatives; it is the least binding operator in this notation. A |
| star (\code{*}) means zero or more repetitions of the preceding item; |
| likewise, a plus (\code{+}) means one or more repetitions, and a |
| phrase enclosed in square brackets (\code{[ ]}) means zero or one |
| occurrences (in other words, the enclosed phrase is optional). The |
| \code{*} and \code{+} operators bind as tightly as possible; |
| parentheses are used for grouping. Literal strings are enclosed in |
| quotes. White space is only meaningful to separate tokens. |
| Rules are normally contained on a single line; rules with many |
| alternatives may be formatted alternatively with each line after the |
| first beginning with a vertical bar. |
| |
| In lexical definitions (as the example above), two more conventions |
| are used: Two literal characters separated by three dots mean a choice |
| of any single character in the given (inclusive) range of \ASCII{} |
| characters. A phrase between angular brackets (\code{<...>}) gives an |
| informal description of the symbol defined; e.g., this could be used |
| to describe the notion of `control character' if needed. |
| \index{lexical definitions} |
| \index{ASCII@\ASCII{}} |
| |
| Even though the notation used is almost the same, there is a big |
| difference between the meaning of lexical and syntactic definitions: |
| a lexical definition operates on the individual characters of the |
| input source, while a syntax definition operates on the stream of |
| tokens generated by the lexical analysis. All uses of BNF in the next |
| chapter (``Lexical Analysis'') are lexical definitions; uses in |
| subsequent chapters are syntactic definitions. |