[Pyparsing] A PyParsing hack

[Gre7g L. <haf...@ya...>]

First off, as a new user of PyParsing, I've gotta' say
that I am hooked. This thing rocks and is better than
sliced bread in a variety of ways.

Next, it does almost everything I could ever want...
but because it's open source, I'm able to make changes
to make it suit me. How cool is that? It's like every
day is my birthday, or something.

So, what follows is not really a question. I just
wanted to share what I'm doing and to make this
available to anyone else who might need to do
something similar. I guess if there is any element of
question to this e-mail, it would be, "Do you think
this is the cleanest way to add this functionality in
(assuming that it wasn't already here and I missed it
somehow)?" and if so, "How can I contribute this back
to the project?"

On to the show...

As I mentioned yesterday, I'm using PyParsing to parse
user code into something that my hardware can run. I
want to give the user a stripped-down version of C,
but due to extreme space limitations in my hardware, I
can't use any pre-existing virtualization code. So
let's say the user entered the function:

X=Y+5*Z

I would want to create instances that would represent
the various pieces of the instruction (one for each
variable, one for the multiplication, one for the
addition, and one for the assignment) and then call
some recursive member of the assignment instance to
generate the op-codes required to load/store and
calculate.

Teaching PyParsing to parse the assignment is pretty
trivial. With minimal effort, I can get it to churn
out the hierarchial list:

[['X', '=', ['Y', '+', [5, '*', 'Z']]]]

At this point, I *COULD* traverse back through the
output and convert the various pieces to instances,
but in essence, I would be doing the parsing process
twice. What I really would like is for PyParsing to
use its slick-as-snot setParseAction function to name
the class for me so I can just sit back and enjoy.

Here's a patch you can apply to the SVN head to make
setParseAction accept classes in addition to
functions:

http://pastie.textmate.org/162921

It only really does two things:

1. It accepts the class in lieu of a function, and
2. It provides a base class that the user can derive
from to prevent a reduce-to-text operation during the
parsing process.

With this code in place, the assignment is now
returned as (formatting added manually):

[
  <Assignment 
    <Token 'X'> =
      <Math
        [<Token 'Y'>, '+',
          <Math [5, '*', <Token 'Z'>]>
        ]
      >
  >
]

And finally, here's a really simple program to do the
above:

http://pastie.textmate.org/162922

My actual code handles a lot more kinds of code
structures and is still being debugged, but I didn't
want to bog everyone down in that, hence the tiny
snippet.

Finally, I tried to run the unitTests.py file to
verify that I hadn't broken anything in the process,
but I don't see all the required files in SVN.

Thanks again, 

Gre7g

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