1. Introduction

Yapps (Yet Another Python Parser System) is an easy to use parser generator that is written in Python and generates Python code. There are several parser generator systems already available for Python, including PyLR, kjParsing, PyBison, and mcf.pars, but I had different goals for my parser. Yapps is simple, is easy to use, and produces human-readable parsers. It is not fast, powerful, or particularly flexible. Yapps is designed to be used when regular expressions are not enough and other parser systems are too much: situations where you may write your own recursive descent parser.

Some unusual features of Yapps that may be of interest are:

1.

Yapps produces recursive descent parsers that are readable by humans, as opposed to table-driven parsers that are difficult to read. A Yapps parser for a simple calculator looks similar to the one that Mark Lutz wrote by hand for Programming Python.

2.

Yapps also allows for rules that accept parameters and pass arguments to be used while parsing subexpressions. Grammars that allow for arguments to be passed to subrules and for values to be passed back are often called attribute grammars. In many cases parameterized rules can be used to perform actions at “parse time” that are usually delayed until later. For example, information about variable declarations can be passed into the rules that parse a procedure body, so that undefined variables can be detected at parse time.

3.

Yapps grammars are fairly easy to write, although there are some inconveniences having to do with LL(1) parsing that have to be worked around. For example, rules have to be left factored and rules may not be left recursive.

4.

Yapps itself is written as a single Python module with no C extensions. Yapps produces parsers that are written entirely in Python, and can be self-contained. Options are provided to control how much code is imported from the Yapps module and how much is self-contained.

5.

Yapps’s scanner is context-sensitive, picking tokens based on the types of the tokens accepted by the parser. This can be helpful when implementing certain kinds of parsers, such as for a preprocessor.

There are several disadvantages of using Yapps over another parser system:

1.

Yapps parsers are LL(1), which is less powerful than LALR (used by PyLR) or SLR (used by kjParsing), so Yapps would not be a good choice for parsing complex languages.

2.

The scanner that Yapps provides can only read from strings, not files, so an entire file has to be read in before scanning can begin. It is possible to build a custom scanner, though, so in cases where stream input is needed (from the console, a network, or a large file are examples), the Yapps parser can be given a custom scanner that reads from a stream instead of a string.

3.

Yapps is not designed with efficiency in mind.

Yapps provides an easy to use parser generator that produces parsers similar to what you might write by hand. It is not meant to be a solution for all parsing problems, but instead an aid for those times you would write a parser by hand rather than using one of the more powerful parsing packages available.