Generate n-bit gray codes

gray_code_gen.py

#!/usr/bin/env python3
'''
python3 gray_code_gen.py 5
'''
def gray_code(n):
  #Generate n-bit gray-codes (starting from 0000..(n times))
  def __gray_codes_rec1():
    if len(CODE_SEQ) == MAX_SEQ_LEN:
        return CODE_SEQ
    current = CODE_SEQ[-1]
    i = len(current)-1
    while i >= 0:
      if current[i] == '1':
        temp = current[:i]+ '0' + current[i+1:]
        if temp not in FOUND:
          CODE_SEQ.append(temp)
          FOUND.add(temp)
          break
      if current[i] == '0':
        temp = current[:i] + '1' + current[i+1:]
        if temp not in FOUND:
          CODE_SEQ.append(temp)
          FOUND.add(temp)
          break
      i -= 1
    return __gray_codes_rec1()

  def __gray_codes_rec2(n):
    #This one exploits the property of gray-codes
    if n == 1:
      return ['0', '1']
    fwd = __gray_codes_rec2(n-1)
    gray_codes_current_level = list()
    i = 0
    while i < len(fwd):
      gray_codes_current_level.append('0' + fwd[i])
      i += 1
    i = len(fwd) - 1
    while i >= 0:
      gray_codes_current_level.append('1' + fwd[i])
      i -= 1
    return gray_codes_current_level

  if n > 0:
    if n > 9:
      return __gray_codes_rec2(n)
    else:
      START, MAX_SEQ_LEN = '0'*n, 2**n
      CODE_SEQ, FOUND = [START], {START}
      return __gray_codes_rec1()

if __name__=='__main__':
  from sys import argv
  print(gray_code(int(argv[1])))