Return PostgreSQL UUID array as list with psycopg2

问题:

I have an SQL statement which contains a subquery embedded in an ARRAY() like so:

SELECT foo, ARRAY(SELECT x from y) AS bar ...

The query works fine, however in the psycopg2 results cursor the array is returned as a string (as in "{1,2,3}"), not a list.

My question is, what would be the best way to convert strings like these into python lists?

回答1:

It works for me without the need for parsing:

import psycopg2

query = """
    select array(select * from (values (1), (2)) s);
"""

conn = psycopg2.connect('dbname=cpn user=cpn')
cursor = conn.cursor()
cursor.execute(query)
rs = cursor.fetchall()

for l in rs:
    print l[0]

cursor.close()
conn.close()

Result when executed:

$ python stackoverflow_select_array.py 
[1, 2]

Update

You need to register the uuid type:

import psycopg2, psycopg2.extras

query = """
    select array(
        select *
        from (values
            ('A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A11'::uuid),
            ('A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A11'::uuid)
        )s
    );
"""

psycopg2.extras.register_uuid()

conn = psycopg2.connect('dbname=cpn user=cpn')
cursor = conn.cursor()
cursor.execute(query)
rs = cursor.fetchall()

for l in rs:
    print l[0]

cursor.close()
conn.close()

Result:

$ python stackoverflow_select_array.py 
[UUID('a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11'), UUID('a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11')]

回答2:

If every result cursor ARRAY is of the format '{x,y,z}', then you can do this to strip the string of the braces and split it into a list by comma-delimiter:

>>> s = '{1,2,3}'
>>> s
'{1,2,3}'
>>> l = s.rstrip('}').lstrip('{').split(',')
>>> l
['1', '2', '3']
>>>
>>> s = '{1,2,3,a,b,c}'
>>> s
'{1,2,3,a,b,c}'
>>> l = s.rstrip('}').lstrip('{').split(',')
>>> l
['1', '2', '3', 'a', 'b', 'c']

回答3:

Another way of handling this is to explicitly tell postgres that you want text, then the default psycopg2 string parsing logic will kick in and you'll get a list:

db = psycopg2.connect('...')
curs = db.cursor()
curs.execute("""
    SELECT s.id, array_agg(s.kind::text)
        FROM (VALUES ('A', 'A0EEBC99-9C0B-AEF8-BB6D-6BB9BD380A11'::uuid),
                     ('A', 'A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A12'::uuid)) AS s (id, kind)
    GROUP BY s.id
""")
for row in curs:
    print "row: {}".format(row)

Results in:

row: (u'A', [u'a0eebc99-9c0b-aef8-bb6d-6bb9bd380a11', u'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a12'])

and the query

curs.execute("""
    SELECT array(
        SELECT s.kind::text
        FROM (VALUES ('A0EEBC99-9C0B-AEF8-BB6D-6BB9BD380A11'::uuid),
                     ('A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A12'::uuid)) AS s (kind))
""")
for row in curs:
    print "row: {}".format(row)

results in:

row: ([u'a0eebc99-9c0b-aef8-bb6d-6bb9bd380a11', u'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a12'],)

The trick is specifically attaching the ::text to the fields that you care about.