Programming Python, Second Edition

Hand-Coded Parsers

Since Python is a general purpose programming language, it’s also reasonable to consider writing a hand-coded 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 like 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 like C or C++.

The Expression Grammar

The grammar 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 grammar ...