safe_eval.r2py

"""
Author: Armon Dadgar
Description:
  Provides a method to safely evaluate a string.
"""


# Provides a method to safely evaluate a string
def safe_eval(code, context=None):
  """
  <Purpose>
    Allows code to be safely evaluated.

  <Arguments>
    code:
        The code to be evaluated.

    context:
        The context to evaluate the code in. If not specified, an empty context is used.

  <Exceptions>
    As with the code to be evaluated

  <Returns>
    The result of evaluating the code.

  <Notes>
    To perform the evaluation, a new entry '_result' is added into the
    context prior to execution and removed afterward. This will override any
    existing value. The code is evaluated by prepending '_result = ', so
    make sure the code still is sane after this. See safe_exec.

    As an example:
      val = safe_eval('123')
      print val # 123

      val = safe_eval('10 * 20 / 5')
      print val # 40

      val = safe_eval('print "Hi!"') # Exception, _result = print "Hi!" is not valid.
  """

  # Prepend _result to the code, so that we can get the result back
  code = "_result = " + code

  # Get a context
  if context == None:
    context = {}

  # Try to evaluate this
  virt_space = VirtualNamespace(code)
  context = virt_space.evaluate(context)

  # Get the value
  value = context["_result"]

  # Clear the special _result entry
  del context["_result"]

  # Return the result
  return value