Question

I’m using this code to evaluate mathematical expressions from strings. And it works:

#!/usr/bin/env python
from __future__ import division
from math import *

expression = raw_input()

# Math functions
safe_list = ['math', 'factorial', 'acos', 'asin', 'atan', 'atan2', 'ceil', 'cos', 'cosh', 'degrees', 'e', 'exp', 'fabs', 'floor', 'fmod', 'hypot', 'log', 'log10', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh'] 

# Create safe directory
safe_dict = dict([(key, locals().get(key, None)) for key in safe_list]) 
safe_dict['abs'] = abs

result = eval(expression, {"__builtins__": None}, safe_dict)
print result

I wrapped it in a function like this:

#!/usr/bin/env python
from __future__ import division
from math import *

def calculate(expression):
    # Math functions
    safe_list = ['math', 'factorial', 'acos', 'asin', 'atan', 'atan2', 'ceil', 'cos', 'cosh', 'degrees', 'e', 'exp', 'fabs', 'floor', 'fmod', 'hypot', 'log', 'log10', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh'] 

    # Create safe directory
    safe_dict = dict([(key, locals().get(key, None)) for key in safe_list]) 
    safe_dict['abs'] = abs

    result = eval(expression, {"__builtins__": None}, safe_dict)
    if isinstance(result, float) and result.is_integer():
        result = int(result)
    print result

expr = raw_input()
calculate(expr)

And it still works for the basic operations, but none of the functions defined in safe_dict are working. 5**5 works with both programs, but sin(pi) worked with the first sample of code and it’s not working with the second one. The traceback is

Traceback (most recent call last):
  File "stack.py", line 20, in <module>
    calculate(expression)
  File "stack.py", line 14, in calculate
    result = eval(expression, {"__builtins__": None}, safe_dict)
  File "<string>", line 1, in <module>
TypeError: 'NoneType' object is not callable

Answer 1

The reason it fails is that the functions you import from the math module are not local variables inside the function; they are global. So when you read locals() and insert into the dict, it inserts None for every single one. You would see this if you removed the get(key, None) and just accessed locals()[key] directly.

A better way is to use getattr on the math module. Do import math (not from math import *) and then do safe_dict = dict((k, getattr(math, k)) for k in safe_list).