Math Expression Evaluation

Question

What is the best way to implement a python program that will take a string and will output its result according to operator precedence (for example: \"4+3*5\" will output 19

Answer 1

That's what the "eval" function does in Python.

result = eval(expression)

Beware though it can do a whole lot more, primarily call functions, so to be safe you should make sure it can't access the locals or globals. Also, you can access the builtin methods, including the tricky import so you need to block access to that as well:

result = eval(expression, {'__builtins__': None}, {})

But that's only if you need security, that is if you allow anybody to type in any expression.

Of course since you this way block all locla variables from use, then you don't have any variables to use, so to have that you need to pass in just those variables which should be accessed in the dictionaries.

vars = {'__builtins__': None, 'x': x}
result = eval(expression, vars, {})

or similar.