Interpreter program parsing with Python like for Scheme (Lisp) ?
How is it possible to parse a program with Python like I do with Scheme ?
With Scheme, I have written some simple BNF's like for Boolean expressions, where AND expression looks like ('and' ('lit' #t) ('lit' #f)), then when I read this small program with Scheme, I get (for free) a list of things that I can process using list-ref and car/cdr.
Using Python or C how would similar parsing be done?
For Python there are several parser/parser generator libraries. One is pyparsing which has some examples on the website. Amongst them a simple boolean expression parser/evaluator:
#
# simpleBool.py
#
# Example of defining a boolean logic parser using
# the operatorGrammar helper method in pyparsing.
#
# In this example, parse actions associated with each
# operator expression will "compile" the expression
# into BoolXXX class instances, which can then
# later be evaluated for their boolean value.
#
# Copyright 2006, by Paul McGuire
# Updated 2013-Sep-14 - improved Python 2/3 cross-compatibility
#
from pyparsing import infixNotation, opAssoc, Keyword, Word, alphas
# define classes to be built at parse time, as each matching
# expression type is parsed
class BoolOperand(object):
def __init__(self,t):
self.label = t[0]
self.value = eval(t[0])
def __bool__(self):
return self.value
def __str__(self):
return self.label
__repr__ = __str__
__nonzero__ = __bool__
class BoolBinOp(object):
def __init__(self,t):
self.args = t[0][0::2]
def __str__(self):
sep = " %s " % self.reprsymbol
return "(" + sep.join(map(str,self.args)) + ")"
def __bool__(self):
return self.evalop(bool(a) for a in self.args)
__nonzero__ = __bool__
__repr__ = __str__
class BoolAnd(BoolBinOp):
reprsymbol = '&'
evalop = all
class BoolOr(BoolBinOp):
reprsymbol = '|'
evalop = any
class BoolNot(object):
def __init__(self,t):
self.arg = t[0][1]
def __bool__(self):
v = bool(self.arg)
return not v
def __str__(self):
return "~" + str(self.arg)
__repr__ = __str__
__nonzero__ = __bool__
TRUE = Keyword("True")
FALSE = Keyword("False")
boolOperand = TRUE | FALSE | Word(alphas,max=1)
boolOperand.setParseAction(BoolOperand)
# define expression, based on expression operand and
# list of operations in precedence order
boolExpr = infixNotation( boolOperand,
[
("not", 1, opAssoc.RIGHT, BoolNot),
("and", 2, opAssoc.LEFT, BoolAnd),
("or", 2, opAssoc.LEFT, BoolOr),
])
if __name__ == "__main__":
p = True
q = False
r = True
tests = [("p", True),
("q", False),
("p and q", False),
("p and not q", True),
("not not p", True),
("not(p and q)", True),
("q or not p and r", False),
("q or not p or not r", False),
("q or not (p and r)", False),
("p or q or r", True),
("p or q or r and False", True),
("(p or q or r) and False", False),
]
print("p =", p)
print("q =", q)
print("r =", r)
print()
for t,expected in tests:
res = boolExpr.parseString(t)[0]
success = "PASS" if bool(res) == expected else "FAIL"
print (t,'\n', res, '=', bool(res),'\n', success, '\n')
