Why does this yield a list and not a Tuple?

Question

I'm trying to understand why my snippet of code ends up with the class "list" as opposed to a Tuple.

The way I'd like it to work is to open a CSV, check for the user/password header and use those to append to a tuple so that I can retrieve accounts by doing accounts[1], accounts[2]

It's important that the list of accounts is immutable to me, so I'm trying to end up with the class tuple.

# our csv file name
filename = "resources/accounts.csv"
with open(filename) as infile:
    reader = csv.DictReader(infile)
    accounts = []
    for row in reader:
        accounts.append((row['user'],row['password']))

print(type(accounts))

@AMC I believe it is, but is it more efficient than a list if you're looking for immutable items in said list? — Marcel Doe, Jan 25 '20 at 04:39
@AMC I guess what's better practice? In what case would you use a list of tuples vs a tuples of tuples? — Marcel Doe, Jan 25 '20 at 04:47
Since you’re looking for an immutable collection, a tuple of tuples sounds perfect to me. — AMC, Jan 25 '20 at 04:51
By the way, I’m guessing that the CSV contains more columns than just the two you care about? — AMC, Jan 25 '20 at 04:53
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/206620/discussion-between-marcel-doe-and-amc). — Marcel Doe, Jan 25 '20 at 05:06
Since it's immutable you can't append to a tuple. You can only make a new tuple, which is more expensive. — hpaulj, Jan 25 '20 at 05:46

John Kugelman · Accepted Answer · 2020-01-25T05:28:10.240

3

accounts = []
for row in reader:
    accounts.append((row['user'],row['password']))

You're starting with an empty list [] and appending items to it, so you end up with a list. If you want a tuple you could convert the list to one at the end:

accounts = []
for row in reader:
    accounts.append((row['user'],row['password']))
accounts = tuple(accounts)

Better yet, use a generator expression to create a tuple from the get go. That way you avoid creating a temporary list just to throw it away.

with open(filename) as infile:
    reader = csv.DictReader(infile)
    accounts = tuple((row['user'], row['password']) for row in reader)

edited Jan 25 '20 at 05:28

answered Jan 25 '20 at 04:25

John Kugelman

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John, what's the difference between doing for row in reader and then accounts = tuple(etc) on a separate line vs doing it on the same line as you did? – Marcel Doe Jan 25 '20 at 05:04
See the link I added to my answer. Does that help? – John Kugelman Jan 25 '20 at 05:23

score 0 · Answer 2 · answered Jan 25 '20 at 04:25

0

As you already know a tuple is immutable. So you are correct in making accounts a list. If you want to store it as a tuple afterwards you can convert it like this:

accounts = tuple(accounts)

answered Jan 25 '20 at 04:25

Robert Kearns

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Why does this yield a list and not a Tuple?

2 Answers2