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@jagt /expr.py
Created Jan 18, 2014

What would you like to do?
simple math expr parser. pratt parser really rocks.
# simple math expression pratt parser
def lexer(s):
'''token generator'''
ix = 0
while ix < len(s):
if s[ix].isspace(): ix += 1
elif s[ix] in "+-*/()":
yield s[ix]; ix += 1
elif s[ix].isdigit():
jx = ix + 1
while jx < len(s) and s[jx].isdigit(): jx += 1
yield s[ix:jx]; ix = jx
else:
raise Exception("invalid character at %d: '%s'" % (ix, s[ix]))
while True:
yield ""
# construct a simple pratt parser
def make_op_parselet(op, precedence):
def _parse(parse_func, left):
right = parse_func(precedence)
return (left, op, right)
_parse.precedence = precedence
return _parse
def make_group_parselet():
def _parse(parse_func, consume_func):
grouped = parse_func()
consume_func(')')
return grouped
return _parse
def parse(s):
tokens = lexer(s)
group_parselet = make_group_parselet()
op_parselets = {
'+' : make_op_parselet('+', 1),
'-' : make_op_parselet('-', 1),
'*' : make_op_parselet('*', 2),
'/' : make_op_parselet('/', 2),
}
# workaround the nonlocal
parse.next_token = tokens.next()
def consume(what=None):
t = parse.next_token
parse.next_token = tokens.next()
if what is not None and t != what:
raise Exception("expect %s, get %s" % (what, t))
return t
def next_precedence():
if parse.next_token and parse.next_token in '+-*/':
return op_parselets[parse.next_token].precedence
return 0
def parse_expr(precedence=0):
t = consume()
if t.isdigit():
left = t
elif t == '(':
left = group_parselet(parse_expr, consume)
else:
raise Exception("unexpected prefix token %s" % t)
while precedence < next_precedence():
t = consume()
left = op_parselets[t](parse_expr, left)
return left
return parse_expr()
if __name__ == '__main__':
from json import dumps
print dumps(parse("1 + (2 - 3) * 456"), indent=4)
print dumps(parse("1 * (2 * 3) / 5 + (2 - 3) * 456"), indent=4)
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