Szathmáry László honlapja @ DEIK | Nim2 / From Nim, call Python's eval() function

This is a powerful stuff. For Python's eval() —in the form of a string— you can pass any expression and the Python interpreter will evaluate it and return the value of the expression:

print(eval("1 + 1")) # 2

print(eval("2 ** 3 - 1")) # 7

Note: eval() can be a dangerous function! Don't eval a string that comes from an untrusted source! If you eval a malicious string, very bad things can happen!

Problem

I couldn't call eval() directly from a Nim program. I got an error message that globals and locals must be passed to eval(). It may be an issue of nimpy, I found a ticket about it here. However, there's a workaround.

Solution

There's a solution for the previous problem: create a Python module with a function that calls eval() for you.

$ tree .
.
├── helper.py
└── main.nim

helper.py :

def myeval(code):
return eval(code)

main.nim :

import std/os

import pkg/nimpy

proc main() =
let sys = pyImport("sys")
discard sys.path.insert(0, getAppDir()) # directory of the running binary

let
helper = pyImport("helper")
result = helper.myeval("2 ** 3 - 1").to(int)

echo result # 7
echo result.typeOf # int

main()