Parsing in Python

I recently had to do some simple parsing in Python. Aside: the Python requirement isn’t particularly principled (I am actually not a huge fan), but I go where matplotlib is. I don’t think that I have seen a more useful plotting library or program yet. Having the full power of a programming language available to munge data into an appropriate form is a major improvement over ad-hoc data manipulation or selection primitives in tools like gnuplot. My only complaint at this point is that showing a legend for a particular plot outside of the plot is more difficult than it should be.

Anyway, manually writing a recursive-descent parser would have been simple enough, but I can pretty safely say that I’ll probably never do that again. I decided to take the opportunity to look around the Python parsing library/generator landscape, with the constraint that I did not want something too complicated since this is basically a one-off parser and intrinsically a simple grammar.

pyparsing

The first thing I found was pyparsing. It seems robust and well thought out, but under-documented. The intended documentation seems to be some book by the author of the library. That is a fine idea, but also completely unappealing to me.

pyPEG

Next up was pyPEG. I had actually heard good things about this parsing framework already, and it passed the documentation test pretty handily. The library is actually very small, which is a good thing (there is just one file that you can drop into your project trivially).

PEGs are Parsing Expression Grammars, and it looks substantially like a basic top-down parsing strategy with one nice additional feature: production rules can include the ?,+, and \* operators from the realm of regular expressions (hence Expression Grammar). These operators provide a very convenient way to express optional or repeated parts of a grammar without having to introduce unnecessary recursive rules.

This reminds me a great deal of the typical constructs provided by a parser combinator library, though obviously more restricted.

Evaluation

The library was pretty much just what I needed. I can deal with slow for this application since I only need to parse one thing. The grammar is very readable, though the AST that it gives you back is a bit cumbersome: it is composed of nested tuples and lists. It is not exactly fun to pick through.

The Good

The Not So Great

Notes

pyPEG cuts down on boilerplate by relying heavily on convention. This results in very readable grammars at the source level at the expense of flexibility. The lack of semantic actions is probably related to this choice. A simple example demonstrates all of the features that I found:

MANY = -1
MANY1 = -2

def _whitespace(): return MANY, [" ", "\t"]
def _number(): return _whitespace, re.compile(r'\d+'), _whitespace
def sample(): return MANY1, _number, '\n'

Starting from the top:

The AST is a bit annoying to work with. Non-terminals that are not suppressed with a leading underscore are tagged by the name of the production rule that generated them. What happens when you want to generate sub-parsers using higher order functions? Consider

def pairParserGen(name, regex):
    def pairParser(): return name, "=", "\"", re.compile(regex), "\""
    return pairParser

A function like this would generate production rules to parse key-value pairs with a given name as the key and a value that matches the provided regex. pyPEG uses the name of the function that represents the production rule it is applying when generating the tag element of a tuple in the AST. With this higher order rule generation, all of the pairs parsed would share the same tag in the AST: ‘pairParser’. That makes actually using the AST even more challenging. The fix is actually straightforward:

def pairParserGen(name, regex):
    def pairParser(): return name, "=", "\"", re.compile(regex), "\""
    pairParser.__name__ = name
    return pairParser

Conclusion

I don’t think I actually have a conclusion. pyPEG is pretty nice, though.