math syntax checker written in python

Question

All I need is to check, using python, if a string is a valid math expression or not.

For simplicity let\'s say I just need + - * / operators (+ -

Answer 1

Why not just evaluate it and catch the syntax error?

from math import *

def validateSyntax(expression):
  functions = {'__builtins__': None}
  variables = {'__builtins__': None}

  functions = {'acos': acos,
               'asin': asin,
               'atan': atan,
               'atan2': atan2,
               'ceil': ceil,
               'cos': cos,
               'cosh': cosh,
               'degrees': degrees,
               'exp': exp,
               'fabs':fabs,
               'floor': floor,
               'fmod': fmod,
               'frexp': frexp,
               'hypot': hypot,
               'ldexp': ldexp,
               'log': log,
               'log10': log10,
               'modf': modf,
               'pow': pow,
               'radians': radians,
               'sin': sin,
               'sinh': sinh,
               'sqrt': sqrt,
               'tan': tan,
               'tanh': tanh}

  variables = {'e': e, 'pi': pi}

  try:
    eval(expression, variables, functions)
  except (SyntaxError, NameError, ZeroDivisionError):
    return False
  else:
    return True

Here are some samples:

> print validSyntax('a+b-1') # a, b are undefined, so a NameError arises.
> False

> print validSyntax('1 + 2')
> True

> print validSyntax('1 - 2')
> True

> print validSyntax('1 / 2')
> True

> print validSyntax('1 * 2')
> True

> print validSyntax('1 +/ 2')
> False

> print validSyntax('1 + (2')
> False

> print validSyntax('import os')
> False

> print validSyntax('print "asd"')
> False

> print validSyntax('import os; os.delete("~\test.txt")')
> False # And the file was not removed

It's restricted to only mathematical operations, so it should work a bit better than a crude eval.