metacircular python calculator

calc.py

#!/usr/bin/env python3

""" calc.py
To use,

$ ./calc.py "1+2+3+4"
$ 10

Use `chmod +x calc.py` or run the program with `python3 calc.py <program-string>`.
"""

import ast
import sys

def interp(program):
  """
  Interpret and evaluate a python program.
  """
  def interp_statement (statement):
      match statement:
        case ast.Expr (value):
          return interp_expression(value)

  def interp_expression (expr):
    match expr:
      case ast.Constant (value, kind):
        return value
      case ast.BinOp (left, op, right):
        valuel = interp_expression(left)
        valuer = interp_expression(right)
        match op:
          case ast.Add (): return valuel+valuer
          case ast.Sub (): return valuel-valuer
          case ast.Mult (): return valuel*valuer
          case ast.Div (): return valuel//valuer

  match program:
    case ast.Module (body, type_ignores):
      value = None
      for exp in body:
        value = interp_statement(exp)
      return value
    case ast.Expression (body):
      return interp_expression(body)

if __name__ == "__main__":
  value = interp(ast.parse(sys.argv[1]))
  print(value)