>>> [1, 2, 3] + [4, 5, 6]
 [1, 2, 3, 4, 5, 6] 
>>> 'Hello,' + 'world!' 
'Hello, world!'
>>> [1, 2, 3] + 'world!' Traceback (innermost last):   
File "<pyshell>", line 1, in ?    
[1, 2, 3] + 'world!' 
TypeError: can only concatenate list (not "string") to list