List comprehensions in Python let you create a list declaratively, much like the way you would describe the set in English. For example,
[x**2 for x in range(10)]
creates a list of the squares of the numbers 0 through 9.
If you wanted the sum of these numbers, you could of course say
sum( [x**2 for x in range(10)] )
but in this case the brackets are unnecessary. The code is easier to read and more efficient if you omit the brackets.
sum( x**2 for x in range(10) )
With the brackets, Python constructs the list first then sums the elements. Without the brackets, you have a generator expression. Python will sum the values as they’re generated, not saving all the values in memory. This makes no difference for a short list as above, but with a large list comprehension the generator expression could be more efficient.