Curious behaviour of Python lists [duplicate]

Question

How come this code does not throw an error when run by the Python interpreter.

a = ['A', 'B', 'C']
a[20:] = ['D', 'E']
print a

Output is ['A', 'B', 'C', 'D', 'E']. I thought Python would give me an error on the second statement since a has only 3 elements. Does this feature have any natural uses while coding?

Answer 1

That's how python works. In python for slicing no boundary checks will take place. It just expands your list since it is a mutable object.

It's also interesting when you are reading out of boundaries with slicing:

f = a[20:]

f will be an empty list.