eval in Python

This article discusses a built-in function in Python, eval.

It is an interesting hack/utility in Python which lets a Python program run Python code within itself.

The eval() method parses the expression passed to it and runs python expression(code) within the program.

The syntax of eval is:

eval(expression, globals=None, locals=None)
  • expression: this string is parsed and evaluated as a Python expression
  • globals (optional): a dictionary to specify the available global methods and variables.
  • locals (optional): another dictionary to specify the available local methods and variables.

Let us explore it with the help of a simple Python program:

from math import *

def secret_function():
    return "Secret key is 1234"

def function_creator():

    # expression to be evaluated
    expr = raw_input("Enter the function(in terms of x):")

    # variable used in expression
    x = int(raw_input("Enter the value of x:"))

    # evaluating expression
    y = eval(expr)

    # printing evaluated result
    print("y = {}".format(y))

if __name__ == "__main__":
    function_creator()

function_creator is a function which evaluates the mathematical functions created by user.

Consider an output:

Enter the function(in terms of x):x*(x+1)*(x+2)
Enter the value of x:3
y = 60

Let us analyze the code a bit:

  • The above function takes any expression in variable x as input.
  • Then user has to enter a value of x.
  • Finally, we evaluate the python expression using eval() built-in function by passing the expr as argument.

Vulnerability issues with eval

Our current version of function_creator has a few vulnerabilities.

The user can easily expose hidden values in the program, or call a dangerous function as eval will execute anything passed to it.

For example, if you input like this:

Enter the function(in terms of x):secret_function()
Enter the value of x:0

You will get output:

y = Secret key is 1234

Also, consider the situation when you have imported os module in your python program. The os module provides portable way to use operating system functionalities like: read or write a file. A single command can delete all files in your system!

Of course in most cases (like desktop programs) the user can’t do any more than they could do by writing their own python script, but in some applications (like web apps, kiosk computers), this could be a risk!

The solution is to restrict eval to only the functions and variables we want to make available.

Making eval safe

eval function comes with the facility of explicitly passing a list of functions or variables that it can access. We need to pass it as argument in the form of a dictionary.

Consider the example below:

from math import *

def secret_function():
    return "Secret key is 1234"

def function_creator():

    # expression to be evaluated
    expr = raw_input("Enter the function(in terms of x):")

    # variable used in expression
    x = int(raw_input("Enter the value of x:"))

    # passing variable x in safe dictionary
    safe_dict['x'] = x

    # evaluating expression
    y = eval(expr, {"__builtins__":None}, safe_dict)

    # printing evaluated result
    print("y = {}".format(y))

if __name__ == "__main__":

    # list of safe methods
    safe_list = ['acos', 'asin', 'atan', 'atan2', 'ceil', 'cos',
                 'cosh', 'degrees', 'e', 'exp', 'fabs', 'floor',
                 'fmod', 'frexp', 'hypot', 'ldexp', 'log', 'log10',
                 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt',
                 'tan', 'tanh']

    # creating a dictionary of safe methods
    safe_dict = dict([(k, locals().get(k, None)) for k in safe_list])

    function_creator()

Now if we try to run above program like:

Enter the function(in terms of x):secret_function()
Enter the value of x:0

We get output:

NameError: name 'secret_function' is not defined

Let us analyze above code step by step:

  • First of all, we create a list of methods we want to allow as safe_list.
  • Next, we create a dictionary of safe methods. In this dictionary, keys are the method names and values are their local namespaces.
    safe_dict = dict([(k, locals().get(k, None)) for k in safe_list])
    

    locals() is a built-in method which returns a dictionary which maps all the methods and variables in local scope with their namespaces.

  • safe_dict['x'] = x
    

    Here, we add local variable x to the safe_dict too. No local variable other than x will get identified by eval function.

  • eval accepts dictionaries of local as well as global variables as arguments. So, in order to ensure that none of the built-in methods is available to eval expression, we pass another dictionary along with safe_dict as well, as shown below:
    y = eval(expr, {"__builtins__":None}, safe_dict)
    

So, in this way, we have made our eval function safe from any possible hacks!

Uses of eval

eval is not much used due to security reasons as we explored above.

Still, it comes handy in some situations like:

  • You may want to use it to allow users to enter their own “scriptlets”: small expressions (or even small functions), that can be used to customize the behavior of a complex system.
  • eval is also sometimes used in applications needing to evaluate math expressions. This is much easier than writing an expression parser.

This blog is contributed by Nikhil Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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