Simple For Loop

for i in range (1, 5):		#Start value, Exit value (loop won't run with this value as it exits, so needs to be desired final value + 1)
	print i:

Will output: 1 2 3 4

Reverse Loop

for i in range (5, -1, -1):		#Start value, Exit value (loop won't run with this value as it exits, so needs to be desired final value + 1), adjust value
	print i:

Will output: 5 4 3 2 1 0

Nested For Loops

for level in xrange(255, -1, -1):
    for index in xrange(0, 24):
        led_values[index] = level
    bus.write_i2c_block_data(PCA9626_ADDRESS, (0x80 | PCA9626_REG_PWM0), led_values)
    time.sleep(0.01)