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Programming Python, 3rd Edition
book

Programming Python, 3rd Edition

by Mark Lutz
August 2006
Intermediate to advanced content levelIntermediate to advanced
1600 pages
51h 46m
English
O'Reilly Media, Inc.
Content preview from Programming Python, 3rd Edition

Handcoded Parsers

Since Python is a general-purpose programming language, it’s also reasonable to consider writing a handcoded parser. For instance, recursive descent parsing is a fairly well-known technique for analyzing language-based information. Since Python is a very high-level language, writing the parser itself is usually easier than it would be in a traditional language such as C or C++.

To illustrate, this section develops a custom parser for a simple grammar: it parses and evaluates arithmetic expression strings. This example also demonstrates the utility of Python as a general-purpose programming language. Although Python is often used as a frontend or rapid development language, it’s also useful for the kinds of things we’d normally write in a systems development language such as C or C++.

The Expression Grammar

The grammar that our parser will recognize can be described as follows:

goal -> <expr> END                       [number, variable, ( ]
goal -> <assign> END                     [set]

assign -> 'set' <variable> <expr>        [set]

expr -> <factor> <expr-tail>             [number, variable, ( ]

expr-tail -> ^                           [END, ) ]
expr-tail -> '+' <factor> <expr-tail>    [+]
expr-tail -> '-' <factor> <expr-tail>    [-]

factor -> <term> <factor-tail>           [number, variable, ( ]

factor-tail -> ^                         [+, -, END, ) ]
factor-tail -> '*' <term> <factor-tail>  [*]
factor-tail -> '/' <term> <factor-tail>  [/]

term -> <number>                         [number]
term -> <variable>                       [variable]
term -> '(' <expr> ')'                   [(]

tokens: (, ), num, var, -, +, /, *, set, end

This is a fairly typical ...

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Publisher Resources

ISBN: 0596009259Supplemental ContentErrata Page