Looks like the most easy solution *to use* will be the one which is hardest to implement: write parser of tuple and hope PG will eventually fully support generics :)
Oh, I wrote this email yesterday and forgot to send it, sorry.... On Tue, Sep 5, 2017 at 4:40 PM, Dmytro Starosud <d.starosud@gmail.com> wrote: > Hello guys > > I do following interactions with DB: > > In [48]: conn = psycopg2.connect("dbname=... user=... password=...") > In [49]: cur = conn.cursor() > In [50]: cur.execute("select (1, 2)") > In [51]: cur.fetchone() > Out[51]: ('(1,2)',) > > Is it possible to get that tuple parsed into python tuple in the same way > array works? > > In [55]: cur.execute("select array[1, 2]") > In [56]: cur.fetchone() > Out[56]: ([1, 2],) > > Looks like that can be done if I register composite type for that tuple. > But I would like it to work with any tuple.
No, the problem is that the server doesn't pass extra information of the types in a ROW, e.g. it doesn't tell that the first type is a number and the second is a date. With the array it returns this information, saying e.g. that the result is an array of dates, so we can split on the elements and parse the dates one by one. What register_composite does is exactly that: it queries the server to check what type is each field, stores that information, and then uses it at parsing time to parse each element correctly.
You can definitely write a typecaster yourself and assign it to the `record` type
=# select pg_typeof((1,2)); pg_typeof ----------- record
you can check the docs about registering a new typecaster: you will have a function receiving '(1,2)' and you can parse it any way you want. What you have to solve is this ambiguity though:
=# select (1,'2'); row ------- (1,2)
your program must be able to tell whether the second element is a string or a number. The server won't tell it and psycopg will not punt. You may have this knowledge in your program, so you can actually do something this way. -- Daniele
