parsing string to a list of objects - help!

Paul McGuire ptmcg at
Mon Nov 5 10:26:25 CET 2007

On Nov 5, 3:00 am, Donn Ingle < at> wrote:
> I have glanced around at parsing and all the tech-speak confuses the heck
> out of me. I am not too smart and would appreciate any help that steers
> away from cold theory and simply hits at this problem.

Donn -

Here is a pyparsing version that I hope is fairly easy-to-read and
tech-speak free:

from pyparsing import *

frame = Literal("#")
tween = Word("-") # that is, it is a "word" composed of 1 or more -'s
copy = Literal("=")
blank = Literal("_")

animation = OneOrMore((frame + Optional(
                            (tween + FollowedBy(frame)) |
                            OneOrMore(copy | blank) ) ) )

test = "#---#--#===_#"

print animation.parseString(test)

This prints the following:

['#', '---', '#', '--', '#', '=', '=', '=', '_', '#']

>From here, the next step would be to define classes that these matched
tokens could be converted to.  Pyparsing allows you to attach a method
to individual expressions using setParseAction - if you pass a class
instead of a method, then class instances are returned.  For these
tokens, you could write:

class Prop(object):
    def __init__(self,tokens):

class Tween(object):
    def __init__(self,tokens):
        self.tween = tokens[0]
        self.tweenLength = len(self.tween)

class CopyPrevious(object):
    def __init__(self,tokens):

class Blank(object):
    def __init__(self,tokens):


And now calling animation.parseString(test) returns a sequence of
objects representing the input string's frames, tweens, etc.:

[<__main__.Prop object at 0x00B9D8F0>, <__main__.Tween object at
0x00BA5390>, <__main__.Prop object at 0x00B9D9B0>, <__main__.Tween
object at 0x00BA55F0>, <__main__.Prop object at 0x00BA5230>,
<__main__.CopyPrevious object at 0x00BA58F0>, <__main__.CopyPrevious
object at 0x00BA59D0>, <__main__.CopyPrevious object at 0x00BA5AB0>,
<__main__.Blank object at 0x00BA5B70>, <__main__.Prop object at

Now you can implement behavior in these 4 classes to implement the
tweening, copying, etc. behavior that you want, and then walk through
this sequence invoking this animation logic.

-- Paul

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