Question

How can I allow users to execute mathematical expressions in a safe way?
Do I need to write a full parser?

Is there something like ast.literal_eval(), but for expressions?

Answer 1

The examples provided with Pyparsing include several expression parsers:

• https://github.com/pyparsing/pyparsing/blob/master/examples/fourFn.py is a conventional arithmetic infix notation parser/evaluator implementation using pyparsing. (Despite its name, this actually does 5-function arithmetic, plus several trig functions.)

• https://github.com/pyparsing/pyparsing/blob/master/examples/simpleBool.py is a boolean infix notation parser/evaluator, using a pyparsing helper method operatorPrecedence, which simplifies the definition of infix operator notations.

• https://github.com/pyparsing/pyparsing/blob/master/examples/simpleArith.py and https://github.com/pyparsing/pyparsing/blob/master/examples/eval_arith.py recast fourFn.py using operatorPrecedence. The first just parses and returns a parse tree; the second adds evaluation logic.

If you want a more pre-packaged solution, look at plusminus, a pyparsing-based extensible arithmetic parsing package.