Simple cases of item assignment don't require much explanation: X[2] += 1 makes an increment to the third item of list X (or if X is a dictionary, to the item that has 2 as its key). Less obvious are assigments using multiple indices:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

A = [2,1,5]
B = [False,1,7]
C = list(range(20))
D = [A,6,0,B,"thing",C]
B[0] = D[3][2] 
D[5][2] = "stop"
D[4][0] = 'J'
D[3] += 8
D[4] *= 0 
D[2] += [9]

Some of these assignments are Python errors:

• Line 7 is an error because D[4] is a string; thus D[4][0] = 'J' attempts item assignment to a string, which is an error in Python.

• Line 8 is an error because D[3] is a list; adding a number to a list results in a type error.

• Line 10 is an error because D[2] is a number; adding a list to a number, or trying to concatenate a number to a list, is a type error for the "+" operator.