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How To Write A Calculator in 70 Python Lines, By Writing a Recursive-Descent Parser http://blog.erezsh.com/how-to-write-a-calculator-in-70-python-lines-by-writing-a-recursive-descent-parser/

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calculator.py
Python
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'''A Calculator Implemented With A Top-Down, Recursive-Descent Parser'''
# Author: Erez Shinan, Dec 2012
import re, collections
from operator import add,sub,mul,div
Token = collections.namedtuple('Token', ['name', 'value'])
RuleMatch = collections.namedtuple('RuleMatch', ['name', 'matched'])
token_map = {'+':'ADD', '-':'ADD', '*':'MUL', '/':'MUL', '(':'LPAR', ')':'RPAR'}
rule_map = {
'add' : ['mul ADD add', 'mul'],
'mul' : ['atom MUL mul', 'atom'],
'atom': ['NUM', 'LPAR add RPAR', 'neg'],
'neg' : ['ADD atom'],
}
fix_assoc_rules = 'add', 'mul'
bin_calc_map = {'*':mul, '/':div, '+':add, '-':sub}
def calc_binary(x):
while len(x) > 1:
x[:3] = [ bin_calc_map[x[1]](x[0], x[2]) ]
return x[0]
calc_map = {
'NUM' : float,
'atom': lambda x: x[len(x)!=1],
'neg' : lambda (op,num): (num,-num)[op=='-'],
'mul' : calc_binary,
'add' : calc_binary,
}
def match(rule_name, tokens):
if tokens and rule_name == tokens[0].name: # Match a token?
return tokens[0], tokens[1:]
for expansion in rule_map.get(rule_name, ()): # Match a rule?
remaining_tokens = tokens
matched_subrules = []
for subrule in expansion.split():
matched, remaining_tokens = match(subrule, remaining_tokens)
if not matched:
break # no such luck. next expansion!
matched_subrules.append(matched)
else:
return RuleMatch(rule_name, matched_subrules), remaining_tokens
return None, None # match not found
def _recurse_tree(tree, func):
return map(func, tree.matched) if tree.name in rule_map else tree[1]
def flatten_right_associativity(tree):
new = _recurse_tree(tree, flatten_right_associativity)
if tree.name in fix_assoc_rules and len(new)==3 and new[2].name==tree.name:
new[-1:] = new[-1].matched
return RuleMatch(tree.name, new)
def evaluate(tree):
solutions = _recurse_tree(tree, evaluate)
return calc_map.get(tree.name, lambda x:x)(solutions)
def calc(expr):
split_expr = re.findall('[\d.]+|[%s]' % ''.join(token_map), expr)
tokens = [Token(token_map.get(x, 'NUM'), x) for x in split_expr]
tree = match('add', tokens)[0]
tree = flatten_right_associativity( tree )
return evaluate(tree)
if __name__ == '__main__':
while True:
print( calc(raw_input('> ')) )

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