converting a string to a function parameter

Paul McGuire ptmcg at austin.rr.com
Fri Mar 13 21:21:45 CET 2009


On Mar 13, 11:46 am, Aaron Brady <castiro... at gmail.com> wrote:
> On Mar 13, 2:52 am, koranthala <koranth... at gmail.com> wrote:
>
> > Hi,
> >     Is it possible to convert a string to a function parameter?
> > Ex:
> > str = 'True, type=rect, sizes=[3, 4]'
> > and I should be able to use it as:
> > test(convert(str)) and the behaviour should be same as calling test
> > with those values :
> > i.e. test(True, type=rect, sizes=[3, 4])
>
> > I tried eval, but it did not work. And any other mechanism I think
> > turns out to be creating a full fledged python parser.
>
> > Is there any mechanism with which we can do this straight away?
>
> I heard 'pyparsing' was good.  ...Not that I've even been to its
> webpage.

Did someone say 'pyparsing'? :)  Here is a first cut (partially lifted
from a previous post):


from pyparsing import *

LPAR,RPAR,LBRACK,RBRACK,EQ,COMMA = map(Suppress,"()[]=,")

noneLiteral = Literal("None")
boolLiteral = oneOf("True False")
integer = Combine(Optional(oneOf("+ -")) + Word(nums)).setName
("integer")
real = Combine(Optional(oneOf("+ -")) + Word(nums) + "." +
               Optional(Word(nums))).setName("real")

ident = Word(alphas+"_",alphanums+"_")

listStr = Forward().setName("list")
tupleStr = Forward().setName("tuple")
listItem = real | integer | noneLiteral | boolLiteral | \
    quotedString.setParseAction(removeQuotes) | Group(listStr) |
tupleStr | ident
listStr << ( LBRACK + Optional(delimitedList(listItem)) + Optional
(COMMA) + RBRACK )
tupleStr << (LPAR + Optional(delimitedList(listItem)) + Optional
(COMMA) + RPAR)

# parse actions perform parse-time conversions
noneLiteral.setParseAction(lambda: None)
boolLiteral.setParseAction(lambda toks: toks[0]=="True")
integer    .setParseAction(lambda toks: int(toks[0]))
real       .setParseAction(lambda toks: float(toks[0]))
listStr    .setParseAction(lambda toks: toks.asList())
tupleStr   .setParseAction(lambda toks: tuple(toks.asList()))

arg = Group(ident("varname") + EQ + listItem("varvalue")) | listItem


argstring = 'True, type=rect, sizes=[3, 4,], coords = ([1,2],[3,4])'

parsedArgs = delimitedList(arg).parseString(argstring)
args = []
kwargs = {}
for a in parsedArgs:
    if isinstance(a,ParseResults):
        if isinstance(a.varvalue,ParseResults):
            val = a.varvalue.asList()
        else:
            val = a.varvalue
        kwargs[a.varname] = val
    else:
        args.append(a)

print "Args:", args
print "Kwargs:", kwargs


Prints:

Args: [True]
Kwargs: {'coords': ([1, 2], [3, 4]), 'type': 'rect', 'sizes': [3, 4]}




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