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Assembler for Hack Assembly Language (Python Implementation)in Python

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My Python implementation of for Hack Assembly Language. See Assembler for Hack Assembly Language - Take 2 for Java implementation. Any comment about best practices and performance improvement?

import sys
import os
def usage():
 print "Usage: %s namefile.asm" % os.path.basename(sys.argv[0])
 
def tobin(val, nbits):
 return bin((val + (1 << nbits)) % (1 << nbits))[2:]
 
def main():
 #command-line argument
 args = sys.argv[1:]
 if "-h" in args or "--help" in args or args==[]:
 usage()
 sys.exit(2)
 fw = open(args[0].split('.')[0]+".hack",'w')
 
 #init compTable
 compT = {
 '0':'0101010', '1':'0111111', '-1':'0111010', 'D':'0001100', 'A':'0110000',
 '!D':'0001101', '!A':'0110001', '-D':'0001111', '-A':'0110011','D+1':'0011111',
 'A+1':'0110111','D-1':'0001110','D+A':'0000010','D-A':'0010011','A-D':'0000111',
 'D&A':'0000000','D|A':'0010101', 'M':'1110000', '!M':'1110001', '-M':'1110011',
 'M+1':'1110111','M-1':'1110010','D+M':'1000010','D-M':'1010011','M-D':'1000111',
 'D&M':'1000000','D|M':'1010101','A-1':'0110010'
 }
 #init destTable
 destT ={'null':'000','M':'001','D':'010','MD':'011',
 'A':'100','AM':'101','AD':'110','AMD':'111'}
 #init jumpTable
 jumpT ={'null':'000','JGT':'001','JEQ':'010','JGE':'011',
 'JLT':'100','JNE':'101','JLE':'110','JMP':'111'}
 
 #init SymbolTable
 SymbolTable ={'SP':0,'LCL':1,'ARG':2,'THIS':3,'THAT':4,'SCREEN':16384,'KBD':24576,
 'R0':0,'R1':1,'R2':2,'R3':3,'R4':4,'R5':5,'R6':6,'R7':7,
 'R8':8,'R9':9,'R10':10,'R11':11,'R12':12,'R13':13,'R14':14,'R15':15}
 
 #first pass
 instr_count = 0
 with open(args[0]) as f:
 for i, l in enumerate(f):
 line = l.strip() #strip space
 line = line.split('//')[0].strip() #strip comment
 if not line:
 continue
 if line[0]=='(':
 symbol = line[1:].split(')')[0] 
 SymbolTable[symbol] = instr_count #ignoring double symbol error
 continue
 #ignoring invalid A_COMMAND & C_COMMAND
 instr_count += 1
 
 #second pass
 instr_count = 0
 symbol_counter = 16
 with open(args[0]) as f:
 for i, l in enumerate(f):
 line = l.strip() #strip space
 line = line.split('//')[0].strip() #strip comment
 if not line:
 continue
 if line[0]=='(':
 continue
 #--------------A-COMMAND------------------
 if line[0]=='@': 
 token = line[1:]
 if not token.lstrip('-').isdigit(): 
 if token not in SymbolTable:
 SymbolTable[token] = symbol_counter
 symbol_counter +=1
 token = SymbolTable[token]
 #print "0{0:015b}".format(token) #ignoring unknown symbol
 fw.write("0{0:015b}\n".format(token))
 else:
 #--- TODO: dealing with negative number
 if token[0] == '-':
 #print '0'+tobin(int(token),15)
 fw.write('0'+tobin(int(token),15)+'\n')
 else:
 #print "0{0:015b}".format(int(token)) #ignoring unknown symbol
 fw.write("0{0:015b}\n".format(int(token))) 
 instr_count += 1
 continue
 #--------------C-COMMAND------------------
 comp = dest = jump = ""
 if '=' in line:
 token = line.split('=')
 dest = destT[token[0]]
 line = token[1]
 else:
 dest = destT['null']
 token = line.split(';')
 comp = compT[token[0]]
 if ';' in line:
 jump = jumpT[token[1]]
 else:
 jump = jumpT['null']
 #print '111'+comp+dest+jump
 fw.write('111'+comp+dest+jump+'\n')
 instr_count += 1
 fw.close()
if __name__ == "__main__":
 main()