Python for loop and arrays

The for statement is used for looping over a list. This is referred to as an iterator. When it is encompassed by [..], this is referred to as a list comprehension.

List comprehensions allow you to transform one list into another. This is sometimes referred to as a mapping i.e. mapping from X -> Y where a function transforms the value of X into the returned value of Y

So, for example, in

[y + 2 for y in range(...)]

the for is iterating over all values in the list produced by the range(). Each list element has 2 added to each value of y, so the final result is a list where each element is 2 greater than the corresponding element in the source list. Thus, range(3) would produce [0, 1, 2] which then transforms into [2, 3, 4].

So [y for y in range(..)] wouldn't actually accomplish much.

I see that in the example you have provided there are two iterators, which complicates things a bit. But essentially, they are providing two reference variables: r and col, from which the final result is derived using these two variables.

List comprehensions are a very powerful tool in Python. Definitely worth knowing.

These are called list comprehensions in python. If you have a function do_something then the following two blocks are equivalent:

result = [do_something(y) for y in range(10)]

...

result = []
for y in range(10):
    result.append(do_something(y))

Where range(10) could be any iterable.

Think of them as quick ways to create lists. They work for dictionaries too as of python 2.7. This tutorial may be helpful.

The "x" is an arbitrary variable name that holds the values of the sequence object. Using it in a list comprehension or in a generator expression will return the items in the iterable object that is being stepped through.