DIY Interpreter

interpreter.py

import sys
def error(linenumber, clause):
	print("Error at line \#" + str(linenumber) + ": " + clause)
	sys.exit()
with open("code.diy") as f:
	data = [line.lstrip("\t") for line in f.read().splitlines()]
labels = {}
for linenum, line in enumerate(data):
	if line.endswith(":"):
		labels[line.rstrip(":")] = linenum + 1
if "start" not in labels:
	error(-1, "No start label specified")
linenum = labels["start"]
instr = [""]
class Stack:
	def __init__(self):
		self.stack = []
	def push(self, value):
		self.stack.append(value)
	def pop(self):
		return self.stack.pop()
	def length(self):
		return len(self.stack)
	def last(self):
		return self.stack[-1]
stack = Stack()
calls = []
while True:
	inst = data[linenum].lower()
	instr = inst.split(" ")
	if instr[-1].endswith(":"):
		error(linenum, "Section ended without exit instruction or jump to a label")
	elif instr[0] == "push":
		if instr[1] == "stack_len":
			stack.push(stack.length())
		else:
			stack.push(int(instr[1]))
	elif instr[0] == "goto":
		if instr[1] in labels:
			linenum = labels[instr[1]]
			continue
		else:
			error(linenum, "No label of name " + instr[1] + " exists")
	elif instr[0] == "goto-if-zero":
		if instr[1] in labels:
			if stack.last() == 0:
				linenum = labels[instr[1]]
				continue
		else:
			error(linenum, "No label of name " + instr[1] + " exists")
	elif instr[0] == "pop":
		stack.pop()
	elif instr[0] == "print-ascii":
		print(chr(stack.last()), end="")
	elif instr[0] == "print-number":
		print(stack.last(), end="")
	elif instr[0] == "exit":
		sys.exit()
	elif instr[0] == "add":
		stack.push(stack.pop() + int(instr[1]))
	elif instr[0] == "call":
		if instr[1] in labels:
			calls.append(linenum + 1)
			linenum = labels[instr[1]]
			continue
		else:
			error(linenum, "No label of name " + instr[1] + " exists")
	elif instr[0] == "back":
		if len(calls) == 0:
			error(linenum, "back instruction called but no calls have been made previously")
		else:
			linenum = calls.pop()
			continue
	else:
		error(linenum, "Unrecognized instruction " + instr[0])
	linenum += 1