Few nitpicks and suggested improvements:

• if not val in registers: is a bit less natural as if val not in registers:

• you can replace if str == '': with if not str:

• str, though, is not a good variable name since it shadows a built-in str type

• is_int though can probably use the EAFP approach - try converting it to int and handle possible errors:

   def is_int(value): try: int(value) return True except (ValueError, TypeError): return False 

• max_register can be simplified to:

   return max(registers.values()) 

• it is a good practice to define regular expression strings as raw strings

• I think you can unpack the captured group values into variables:

   res = re.search(r'([^ ]*) ([^ ]*) ([^ ]*)', line) self.left, self.operator, self.right = res.groups() 

• the Condition class may benefit from the operator module. Instead of having multiple if/elifs, you may define a mapping of operator strings to operator functions:

   OPERATIONS = { '==': operator.eq, '!=': operator.ne, '>': operator.gt, '>=': operator.gte, '<': operator.lt, '<=': operator.le } l, r = (value(registers, self.left), value(registers, self.right)) return OPERATIONS[self.operator](l, r) 

Few nitpicks, possible improvements and random ideas:

• if not val in registers: is a bit less natural as if val not in registers:

• you can replace if str == '': with if not str:

• str, though, is not a good variable name since it shadows a built-in str type

• is_int though can probably use the EAFP approach - try converting it to int and handle possible errors:

   def is_int(value): try: int(value) return True except (ValueError, TypeError): return False 

• max_register can be simplified to:

   return max(registers.values()) 

• it is a good practice to define regular expression strings as raw strings

• I think you can unpack the captured group values into variables:

   res = re.search(r'([^ ]*) ([^ ]*) ([^ ]*)', line) self.left, self.operator, self.right = res.groups() 

• the Condition class may benefit from the operator module. Instead of having multiple if/elifs, you may define a mapping of operator strings to operator functions:

   import operator OPERATIONS = { '==': operator.eq, '!=': operator.ne, '>': operator.gt, '>=': operator.gte, '<': operator.lt, '<=': operator.le } l, r = value(registers, self.left), value(registers, self.right) return OPERATIONS[self.operator](l, r) 

Note that I would check for validness of the operator when you extract it via the regular expression in the __init__. Also, OPERATIONS can probably a module or class-level constant.

• think of pre-compiling regular expressions with re.compile() and re-using