Summary

PyMeta is an implementation of OMeta, an object-oriented pattern-matching language developed by Alessandro Warth (http://www.cs.ucla.edu/~awarth/ometa/). PyMeta provides a compact syntax based on Parsing Expression Grammars (PEGs) for common lexing, parsing and tree-transforming activities in a way that’s easy to reason about for Python programmers.

How It Works

PyMeta compiles a grammar to a Python class, with the rules as methods. The rules specify parsing expressions, which consume input and return values if they succeed in matching.

Interface

The starting point for defining a new grammar is pymeta.grammar.OMeta.makeGrammar, which takes a grammar definition and a dict of variable bindings for its embedded expressions and produces a Python class. Grammars can be subclassed as usual, and makeGrammar can be called on these classes to override rules and provide new ones. To invoke a grammar rule, call grammarObject.apply() with its name.

Example Usage

>>> from pymeta.grammar import OMeta
>>> exampleGrammar = """
ones ::= '1' '1' => 1
twos ::= '2' '2' => 2
stuff ::= (<ones> | <twos>)+
"""
>>> Example = OMeta.makeGrammar(exampleGrammar, {})
>>> g = Example("11221111")
>>> result, error = g.apply("stuff")
>>> result
[1, 2, 1, 1]