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