Python list extend functionality using slices -

i'm teaching myself python ahead of starting new job. django job, have stick 2.7. such, i'm reading beginning python hetland , don't understand example of using slices replicate list.extend() functionality.

first, shows extend method by

a = [1, 2, 3] b = [4, 5, 6] a.extend(b) 

produces [1, 2, 3, 4, 5, 6]

next, demonstrates extend slicing via

a = [1, 2, 3] b = [4, 5, 6] a[len(a):] = b 

which produces exact same output first example.

how work? has length of 3, , terminating slice index point empty, signifying runs end of list. how b values added a?

python's slice-assignment syntax means "make slice equal value, expanding or shrinking list if necessary". understand may want try out other slice values:

a = [1, 2, 3] b = [4, 5, 6] 

first, lets replace part of a b:

a[1:2] = b print(a) # prints [1, 4, 5, 6, 3] 

instead of replacing values, can add them assigning zero-length slice:

a[1:1] = b print(a) # prints [1, 4, 5, 6, 2, 3] 

any slice "out of bounds" instead addresses empty area @ 1 end of list or other (too large positive numbers address point off end while large negative numbers address point before start):

a[200:300] = b print(a) # prints [1, 2, 3, 4, 5, 6] 

your example code uses "accurate" out of bounds slice @ end of list. don't think code you'd use deliberately extending, might useful edge case don't need handle special logic.