我在Python中收到索引错误，说它超出范围，但是我不确定为什么

Question

I've created code that's supposed to turn a machine language into an assembly language, but I keep getting an error when I try to run it from the command prompt. The error I'm receiving is:

Traceback <most recent call last>:
   File "Assembler.py", line 102, in <module>
     parser.advance()
   File "Assembler.py", line 15, in advance
     self.command = self.asm_file[self.index]
IndexError: list index out of range

I'm not really sure why it's out of range though. All I'm putting into the command prompt is:

 python Assembler.py MyFile.asm

Can someone look at my code below and help me figure out why it's giving me this?

class Parser:
def __init__(self, filename):
    self.asm_file = [line for line in open(filename)]
    self.index = 0

def hasMoreCommands(self):
    return self.index < len(self.asm_file)

def advance(self):
    self.index += 1

    if self.index == len(self.asm_file):
        return

    self.command = self.asm_file[self.index]
    self.command = self.removeCommentsAndSpaces(self.command)

    if not self.command:
        self.advance()

There's just more code definitions in between these blocks, and the line 102 referenced in the error message is line 10 of the following block.

if __name__ == '__main__':
import sys

if len(sys.argv) == 1:
    print 'need filename'
    sys.exit(-1)

table = SymbolTable()
parser = Parser(sys.argv[1])
parser.advance()
line = 0

while parser.hasMoreCommands():
    if parser.commandType() == 'L_COMMAND':
        table.addEntry(parser.symbol(), line)
    else:
        line += 1

    parser.advance()

code = Code()
parser = Parser(sys.argv[1])
parser.advance()

var_stack = 16

while parser.hasMoreCommands():
    cmd_type = parser.commandType()

    if cmd_type == 'A_COMMAND':
        number = 32768

        try:
            addr = int(parser.symbol())
        except:
            if table.contains(parser.symbol()):
                addr = table.getAddress(parser.symbol())
            else:
                table.addEntry(parser.symbol(), var_stack)
                addr = var_stack
                var_stack += 1

        bin_number =  bin(number | addr)[3:]
        assembly = '0' + bin_number
        print assembly
    elif cmd_type == 'C_COMMAND':
        assembly = '111'
        assembly += code.comp(parser.comp())
        assembly += code.dest(parser.dest())
        assembly += code.jump(parser.jump())
        print assembly

    parser.advance()

Answer 1

You trying to access at the end of the script a line further then lines exist. So try to move your increment of index at the end of the function.

def advance(self):


if self.index == len(self.asm_file):
    return

self.command = self.asm_file[self.index]
self.command = self.removeCommentsAndSpaces(self.command)

self.index += 1

if not self.command:
    self.advance()

And why not reeusing code? Second way:

def advance(self):
self.index += 1

if not self.hasMoreCommands():
    return

self.command = self.asm_file[self.index]
self.command = self.removeCommentsAndSpaces(self.command)



if not self.command:
    self.advance()

Answer 2

self.index += 1

if self.index == len(self.asm_file):
    return

self.command = self.asm_file[self.index]

As you check your index right after incrementing and before accessing the list, one would assume that you are doing fine. However what might be the problem here is that before incrementing the index value, you are already at the length of the list. So after incrementing the index is one bigger than the list length.

You could easily check that by printing out the index before accessing the list. To go the safe route, just change your check though:

if self.index >= len(self.asm_file):
    return

In fact, your issue occurs on line 10 of that last snippet, ie here:

parser = Parser(sys.argv[1])
parser.advance()

Given that you advance only once, it's very likely that the file is empty, ie the length of the list is zero. So when you advance once, you have an index of 1 , which is not equal to the length ( 0 ) but is still out of range for the list access.