3 ms·

> It's not better than a generator, but I'm surprised nobody has mentioned the very terse and still mostly readable > header, *records = [row.strip().split(

> It's not better than a generator, but I'm surprised nobody has mentioned the very terse and still mostly readable > header, *records = [row.strip().split(',') for row in open(filename).readlines()] Better would be: header, *records = [row.strip().split(',') for row in open(filename)] No need to read the lines all into memory first. Edit: Also if you want to be explicit with the file closing, you could do something like: with open(filename) as infile: header, *records = [row.strip().split(',') for row in infile] That is if we wanted to protect against future changes to semantics for garbage collection/reference counting. I always do this, but I kind of doubt it will ever really matter in any code I write.

philsnow 3y ago
> No need to read the lines all into memory first. It looks like that code does read the whole file: (with a foo.csv that is 350955 bytes long:) % python -V Python 3.11.4 % python >>> f = open("foo.csv") >>> f.tell() 0 >>> header, *records = [row.strip().split(',') for row in f] >>> f.tell() 350955 I thought that using a list comprehension to bind header and records was eagerly consuming the file, so I changed it to a generator comprehension with >>> f.close() >>> f.open("foo.csv") >>> header, *records = (row.strip().split(',') for row in f) >>> f.tell() 350955 nope, I guess the destructuring bind does it? >>> f.close() >>> f.open("foo.csv") >>> headers, records = f.readline().strip().split(','), (row.strip().split(',') for row in f) >>> f.tell() 125 not as neat, though. Is there a golf-ier way to do it?*
- me-vs-cat 3y ago
  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)

philsnow 3y ago

> No need to read the lines all into memory first. It looks like that code does read the whole file: (with a foo.csv that is 350955 bytes long:) % python -V Python 3.11.4 % python >>> f = open("foo.csv") >>> f.tell() 0 >>> header, *records = [row.strip().split(',') for row in f] >>> f.tell() 350955 I thought that using a list comprehension to bind header and records was eagerly consuming the file, so I changed it to a generator comprehension with >>> f.close() >>> f.open("foo.csv") >>> header, *records = (row.strip().split(',') for row in f) >>> f.tell() 350955 nope, I guess the destructuring bind does it? >>> f.close() >>> f.open("foo.csv") >>> headers, records = f.readline().strip().split(','), (row.strip().split(',') for row in f) >>> f.tell() 125 not as neat, though. Is there a golf-ier way to do it?*

me-vs-cat 3y ago
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)