Issue in Parsing Lisp input to python

Question

I am trying to parse lisp input to python list. But when I parse the input I get list of strings but I want individual element as int.

Example: When I parse following input: "(2 (10 (5 11))) (5 6) (4)" Output: [[['2'], ['10', '5']], [['5'], ['6']], [['4']]]

I use: [[map(int, x) for x in lst] for lst in test] to convert the strings to int but this function will only parse nested list of level 2. But if I have a nested list of more than 2, how should I use it to parse?

I tried pyparsing but I didn't understand it properly.

Can you show the parsing code you used? The best solution will probably involve a change there rather than a post-processing step. — user2357112
– user2357112, Commented Sep 23, 2013 at 7:17
In pyparsing, you can attach a parse action to do the integer string-to-int conversion at parse time. Look at this example pyparsing.wikispaces.com/file/view/chemicalFormulas.py/31041705/… on the pyparsing wiki. See in the internal Version 2 of the parser, how the convertIntegers method is attached to the integer expression. This does the string-to-int conversion at parse time, rather than having to post-process later. — PaulMcG
– PaulMcG, Commented Sep 23, 2013 at 7:51

falsetru · Accepted Answer · 2013-09-23 07:07:59Z

5

def as_int(xs):
    if isinstance(xs, list):
        return map(as_int, xs)
    elif isinstance(xs, str):
        return int(xs)
    return xs

Usage:

>>> xs = [[['2'], ['10', '5']], [['5'], ['6']], [['4']]]
>>> print as_int(xs)
[[[2], [10, 5]], [[5], [6]], [[4]]]

answered Sep 23, 2013 at 7:07

falsetru

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Eldritch Cheese · Accepted Answer · 2013-09-23 21:35:27Z

3

If you want to use pyparsing, you could do so without any postprocessing as follows.

import pyparsing as pp

integer = pp.Word(pp.nums).setParseAction(lambda m:int(m[0]))
expr = pp.Forward()
expr << pp.nestedExpr(content=pp.OneOrMore(integer | expr))
manyExpr = pp.OneOrMore(expr)

print manyExpr.parseString('(2 (10 (5 11))) (5 6) (4)')
    #returns [[2, [10, [5, 11]]], [5, 6], [4]]

This first defines an integer as something consisting only of numbers, and tells pyparsing how to convert it from a string of numbers to an integer object. Next, it makes defines an expression as a list of expressions or integers inside parentheses. Finally, in order to parse your example, it looks for many consecutive expressions.

answered Sep 23, 2013 at 21:35

Eldritch Cheese

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1 Comment

PaulMcG Over a year ago

Even simpler is to recognize that nestedExpr internally handles repetition and nesting of the content - just define expr as expr = pp.nestedExpr(content=integer). (Good use of a parse action to convert the numeric strings to integers at parse time, btw. And also a nice summary description of the makeup of the overall parser.)

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