input: Write a function that reads a string containing a numerical expression and returns the result of evaluating the expression. If the given string is empty then return 0.
  def eval_expr(expr):
     if not expr:
         return 0
     return eval(expr)
  
  assert eval_expr("1+2")==3
  assert eval_expr("5-3")==2
  assert eval_expr("10*2")==20
  assert eval_expr("15/3")==5.0
  assert eval_expr("")==0
output: def eval_expr(expr):
     if not expr:
         return 0
     return eval(expr)