Python code for a 16x2 LCD via I2C gives alternate results

Question

I'm trying to use some Python code to write text on a 16x2 LCD display ("compatible" with classics Hitachi HD44780) connected to a RaspberryPi model B+ via an I2C "back-pack".

This I2C board has a PCF8574 IC.

This is the library (credits to the RaspberryPi forum, in turn derived from this blog post):

import smbus
from time import *

# General i2c device class so that other devices can be added easily
class i2c_device:
 def __init__(self, addr, port):
  self.addr = addr
  self.bus = smbus.SMBus(port)

 def write(self, byte):
  self.bus.write_byte(self.addr, byte)

 def read(self):
  return self.bus.read_byte(self.addr)

 def read_nbytes_data(self, data, n): # For sequential reads > 1 byte
  return self.bus.read_i2c_block_data(self.addr, data, n)


class lcd:
 #initializes objects and lcd
 '''
 Port definitions
 addr, en,rw,rs,d4,d5,d6,d7,bl
 0x27, 2, 1, 0, 4, 5, 6, 7, 3
 '''
 def __init__(self, addr, port):
  self.lcd_device = i2c_device(addr, port)
  self.backlight=1; #default backlight on

  #set 4 bit mode
  self.lcd_device_writebl(0x20)
  self.lcd_strobe()
  sleep(0.0005)
  self.lcd_strobe()
  sleep(0.0005)
  self.lcd_strobe()
  sleep(0.0005)

  self.lcd_write(0x28)
  self.lcd_write(0x08)
  self.lcd_write(0x01)
  self.lcd_write(0x06)
  self.lcd_write(0x0C)
  self.lcd_write(0x0F)

#wrapper to self.lcd_device.write for backlight control
 def lcd_device_writebl(self,value):
  if self.backlight:
   self.lcd_device.write(value | 0x08);
  else:
   self.lcd_device.write(value)

# control backlight on=1 or off=0
 def lcd_backlight(self,on):
  self.backlight=on
  self.lcd_strobe()

 # clocks EN to latch command
 def lcd_strobe(self):
 #bit 2
  self.lcd_device_writebl((self.lcd_device.read() | 0x04))
  self.lcd_device_writebl((self.lcd_device.read() & 0xFB))

 # write a command to lcd
 def lcd_write(self, cmd):
  self.lcd_device_writebl((cmd >> 4)<<4)
  self.lcd_strobe()
  self.lcd_device_writebl((cmd & 0x0F)<<4)
  self.lcd_strobe()
  self.lcd_device_writebl(0x0)

 # write a character to lcd (or character rom)
 def lcd_write_char(self, charvalue):
   self.lcd_device_writebl((0x01 | (charvalue >> 4)<<4))
   self.lcd_strobe()
   self.lcd_device_writebl((0x01 | (charvalue & 0x0F)<<4))
   self.lcd_strobe()
   self.lcd_device_writebl(0x0)

 # put char function
 def lcd_putc(self, char):
  self.lcd_write_char(ord(char))

 # put string function
 def lcd_puts(self, string, line):
  if line == 1:
   self.lcd_write(0x80)
  if line == 2:
   self.lcd_write(0xC0)
  if line == 3:
   self.lcd_write(0x94)
  if line == 4:
   self.lcd_write(0xD4)

  for char in string:
   self.lcd_putc(char)

 # clear lcd and set to home
 def lcd_clear(self):
  self.lcd_write(0x1)
  self.lcd_write(0x2)

 # add custom characters (0 - 7)
 def lcd_load_custon_chars(self, fontdata):
  self.lcd_device.bus.write(0x40);
  for char in fontdata:
   for line in char:
    self.lcd_write_char(line)

and the test code:

import time
import pylcdlib

lcd = pylcdlib.lcd(0x27,1)

lcd.lcd_puts("Hello",1) #display "Hello" on line 1
lcd.lcd_puts("World!",2) #display "World!" on line 2

time.sleep(1)
lcd.lcd_clear()

The problem is that running multiple times this sample code, in sequence, one time it goes OK and the next time the LCD displays random characters; then again, one OK, one wrong.

Does anyone know why? I'm a beginner with Python, I searched a lot but with no results.

Thanks.

EDIT: Actually, duplicating the line

lcd = pylcdlib.lcd(0x27,1)
lcd = pylcdlib.lcd(0x27,1)

makes the script to work good every time, but it doesn't make sense (well, I'm sure it does, but I don't know why).