Reading and running a mathematical expression in Python

Question

Using Python, how would I go about reading in (be from a string, file or url) a mathematical expression (1 + 1 is a good start) and executing it?

Aside from grabbing a string, file or url I have no idea of where to start with this.

Answer 1

Because python supports some algebraic forms, you could do:

eval("1 + 1")

But this allows the input to execute about anything defined in your env:

eval("__import__('sys').exit(1)")

Also, if you want to support something python doesn't support, the approach fails:

x³ + y² + c
----------- = 0
     z

Instead of doing this, you can implement a tokenizer and a parser with ply. Evaluating a thing like '1 + 1' ought not take more than ten lines or so.

You could also implement the tokenizer and the parser by hand. Read about LL and LR parsers. Before attempting this it's also better to learn using parser generators first.

Answer 2

If you are receiving an expression as a string you'll need to parse it into its operators and operands and then process the resulting tree. This isn't a python problem per se. but a general issue of how to deal with mathematical expressions delivered as strings.

A quick google reveals a bunch of lexical parsers for python.