Unpack a list in Python

Question

Say I have a MySQL table I access through MySQLDB. I have a standard

SELECT statement:
sql = "SELECT * FROM EMPLOYEE \
       WHERE INCOME > '%d'" % (1000)

I then execute it with the cursor and pluck out the columns as below.

   cursor.execute(sql)
   results = cursor.fetchall()
   for row in results:
      fname = row[0]
      lname = row[1]
      age = row[2]
      sex = row[3]
      income = row[4]

Is it possible to assign all the column names in a single statement? Something like:

for row in results:
    fname, lname, age, sex, income = unpack(row)

I could always do:

fname, lname, age, sex, income = row[0], row[1], row[2], row[3], row[4]

But I have over 30 columns in my table and this is getting painful. Note that though I'm using MySQL right now, I'd like this to be as DB agnostic as possible; our benevolent overlords might decide to port everything over to another database at any point.

You should remove references to SQL in your question, since it's really irrelevant. You have an array you want to deconstruct; it doesn't matter how you got the data. — Carcigenicate, Aug 19 '16 at 14:20
why do you not simply write `for fname, lname, age, sex, income in results:`? — njzk2, Aug 19 '16 at 14:20
You might want to reconsider `SELECT *` if you're expecting a fixed number and order of columns, if the table layout changes you'll break things. — Mark Ransom, Aug 19 '16 at 14:22
you can even consider running a `PRAGMA table_info()` query on the table to get a look at the number of columns and header names — Ma0, Aug 19 '16 at 14:34

score 5 · Accepted Answer · edited May 23 '17 at 11:45

5

Just do:

fname, lname, age, sex, income = row

if len(row)==5 it should works, otherwise, if you have python 3 you can use extended iterable unpacking

fname, lname, age, sex, income, *other = row

other will be a list of all remaining elements.

If you have python 2: You can use a small function as in this answer:

def unpack_list(a, b, c, d, e, *f):
    return a, b, c, d, e, f

fname, lname, age, sex, income, other = unpack_list(*row)

If you only want the 5 first elements, as @Ev.Kounis meant, you can do:

fname, lname, age, sex, income = row[:5]

edited May 23 '17 at 11:45

Community

1
1

answered Aug 19 '16 at 14:21

jrjc

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you can also add an asterisk to any of those (preferably the last) to make sure that even if there are more, execution will go on. Slicing `row` would also be an option since it doesnt throw errors when e.g., `[1,2,3][:5]` – Ma0 Aug 19 '16 at 14:24
@Ev.Kounis I edited, but I believe it's python 3 only. – jrjc Aug 19 '16 at 14:27
`fname, lname, age, sex, income = row[:5]` works in both Python 2 and 3. – vaultah Aug 19 '16 at 14:41

score 1 · Answer 2 · answered Aug 19 '16 at 15:04

How about an entirely different approach?

You could use a DictCursor and reference things by name. E.g,

cursor = db.cursor(MySQLdb.cursors.DictCursor)
cursor.execute(sql)
results = cursor.fetchall()
for row in results:
    function_with_fname(row['fname'])
    function_with_age(row['age'])

score 0 · Answer 3 · answered Aug 19 '16 at 14:59

results = [[1,2,3,4,5],
['a', 'b', 'c', 'd', 'e'],
[True, False, True, False, True]
]

for one, two, three, four, five in results:
    print one, two, three, four, five
>>> 1 2 3 4 5
>>> a b c d e
>>> True False True False True

You could also unpack the values in your for loop itself.

Unpack a list in Python

3 Answers3