ValueError: malformed string when using ast.literal_eval

Question

It is widely known that using eval() is a potential security risk so the use of ast.literal_eval(node_or_string) is promoted

However In python 2.7 it re

Answer 1

It is not too hard to use pyparsing to cobble together a simple expression evaluator.

Suppose you want to eval expression, including parens, of the type of expressions of the following:

2+3
4.0^2+5*(2+3+4)
1.23+4.56-7.890
(1+2+3+4)/5
1e6^2/1e7

This simplification of the SimpleCalc example:

import pyparsing as pp
import re

ex='''\
2+3
4.0^2+5*(2+3+4)
1.23+4.56-7.890
(1+2+3+4)/5
1e6^2/1e7'''

e = pp.CaselessLiteral('E')
dec, plus, minus, mult, div, expop=map(pp.Literal,'.+-*/^')
addop  = plus | minus
multop = mult | div
lpar, rpar=map(pp.Suppress,'()')
p_m = plus | minus

num = pp.Word(pp.nums) 
integer = pp.Combine( pp.Optional(p_m) + num )
floatnumber = pp.Combine( integer +
                       pp.Optional( dec + pp.Optional(num) ) +
                       pp.Optional( e + integer ) )

stack=[]
def pushFirst(s, l, t):
    stack.append( t[0] )

expr=pp.Forward()
atom = ((floatnumber | integer).setParseAction(pushFirst) | 
         ( lpar + expr.suppress() + rpar )
       )

factor = pp.Forward()
factor << atom + pp.ZeroOrMore( ( expop + factor ).setParseAction( pushFirst ) )

term = factor + pp.ZeroOrMore( ( multop + factor ).setParseAction( pushFirst ) )
expr << term + pp.ZeroOrMore( ( addop + term ).setParseAction( pushFirst ) )    

pattern=expr+pp.StringEnd()

opn = { "+" : ( lambda a,b: a + b ),
        "-" : ( lambda a,b: a - b ),
        "*" : ( lambda a,b: a * b ),
        "/" : ( lambda a,b: a / b ),
        "^" : ( lambda a,b: a ** b ) }

def evaluateStack(stk):
    op = stk.pop()
    if op in "+-*/^":
        op2 = evaluateStack(stk)
        op1 = evaluateStack(stk)
        return opn[op](op1, op2)
    elif re.search('^[-+]?[0-9]+$',op):
        return int(op)
    else:
        return float(op)     

for line in ex.splitlines():
    parse=pattern.parseString(line)   
    s=stack[:]
    print('"{}"->{} = {}'.format(line,s,evaluateStack(stack)))   

Prints:

"2+3"->['2', '3', '+'] = 5
"4.0^2+5*(2+3+4)"->['4.0', '2', '^', '5', '2', '3', '+', '4', '+', '*', '+'] = 61.0
"1.23+4.56-7.890"->['1.23', '4.56', '+', '7.890', '-'] = -2.1000000000000005
"(1+2+3+4)/5"->['1', '2', '+', '3', '+', '4', '+', '5', '/'] = 2.0
"1e6^2/1e7"->['1E6', '2', '^', '1E7', '/'] = 100000.0