ASTEVAL: Minimal Python AST Evaluator

The asteval package evaluates Python expressions and statements, providing a safer alternative to Python’s builtin eval() and a richer, easier to use alternative to ast.literal_eval(). It does this by building an embedded interpreter for a subset of the Python language using Python’s ast module. The emphasis and main area of application is the evaluation of mathematical expressions. Because of this emphasis, mathematical functions from Python’s math module are built-in to asteval, and a large number of functions from numpy will be available if numpy is installed on your system. For backward compatibility, a few functions that were moved from numpy to numpy_financial will be imported, if that package is installed.

While the primary goal is evaluation of mathematical expressions, many features and constructs of the Python language are supported by default. These features include array slicing and subscripting, if-then-else conditionals, while loops, for loops, try-except blocks, list comprehension, and user-defined functions. All objects in the asteval interpreter are truly Python objects, and all of the basic built-in data structures (strings, dictionaries, tuple, lists, sets, numpy arrays) are supported, including the built-in methods for these objects.

However, asteval is by no means an attempt to reproduce Python with its own ast module. There are important differences and missing features compared to Python. Many of these absences are intentional, and part of the effort to try to make a safer version of eval(), while some are simply due to the reduced requirements for an embedded mini-language. These differences and absences include:

  1. All variable and function symbol names are held in a single symbol table that can be accessed from the calling program. By default, this is a simple dictionary, giving a flat namespace. A more elaborate, still experimental, symbol table that allows both dictionary and attribute access can also be used.

  2. creating classes is not allowed.

  3. importing modules is not allowed, unless specifically enabled.

  4. decorators, generators, type hints, and lambda are not supported.

  5. yield, await, and async programming are not supported.

  6. Many builtin functions (eval(), getattr(), hasattr(), setattr(), and delattr()) are not allowed.

  7. Accessing many object attributes that can provide access to the python interpreter are not allowed.

The resulting “asteval language” acts very much like miniature version of Python, focused on mathematical calculations, and with noticeable limitations. It is the kind of toy programming language you might use to introduce simple scientific programming concepts, but also includes much of the standard Python features to be a reasonably complete language and not too restricted from what someone familiar with Python would expect.

Because asteval is designed for evaluating user-supplied input, safety against malicious or incompetent user input is an important concern. Asteval tries as hard as possible to prevent user-supplied input from crashing the Python interpreter or from returning exploitable parts of the Python interpreter. In this sense asteval is certainly safer than using eval(). However, asteval is an open source project written by volunteers, and we cannot guarantee that it is completely safe against malicious attacks.