The parent poster was pointing out that this requires having two in-memory complete copies of the file: [... for row in open(filename).readlines()] The re

The parent poster was pointing out that this requires having two in-memory complete copies of the file: [... for row in open(filename).readlines()] The readlines return value is one copy, and the list comprehension is another copy. However, that first copy can be avoided with: [... for row in open(filename)] The entire file must still be read to evaluate the list comprehension. Additionally, this doesn't do what you think it does: >>> header, *records = (row.strip().split(',') for row in f) Compare to this, using a variable for clarity: >>> gen = (row.strip().split(',') for row in f) >>> header, *records = next(gen)