The chapter explains, but stepper shows.


List Comprehensions

A = [ 2**i - 1 for i in range(10) ]
B = [ w[2] for w in ("one","two","three") ]
v = ("over","to","you")
C = [ len(w) for w in v ]
x = [100,1000,500,600]
E = [ x[i+1]-x[i] for i in range(len(x)-1) ]


The last one, using range(len(x)-1) and k[i+1] is an idiom of programming. Instead of directly accessing items in a sequence, the comprehension uses index numbers. Because the term in the comprehension has k[i+1], it is important that the range of index values stop just one short of last (so that i+1 will not be an error).

Python2 Bug

Python version 2 makes visible the name of the comprehension item, even after the comprehension is finished. Python 3 remedies this situation. You might see in the example above that names w and i are each used in two comprehensions, and different values are given for these two names during the stepper run of the code. This can actually be considered a bug in the way that Python 2 behaves (the stepper uses Python 2). Please ignore this aspect of list comprehension in Python 2, though be mindful and do not use a temporary name in a comprehension expression that your program will define elsewhere to have some value.