[DB-SIG] "lists of tuples" vs. "tuples of tuples"

M.-A. Lemburg mal at egenix.com
Tue Feb 3 22:12:17 CET 2015


On 03.02.2015 21:30, Michael Bayer wrote:
>> [By the way, both Rows and Row objects are made immutable, so that an unwary programmer does not receive an unwelcome surprise by attempting to alter data without using an SQL statement.]
> 
> I would actually favor the result list to be an immutable one as well, and the ideal structure would be an immutable list (which is of course entirely straightforward in Python).    It’s unusual that Marc suggests the collection is explicitly mutable, and that this is a common use case.    

I was probably unclear in what I wrote. I wasn't emphasizing on the
mutability of the list elements, but rather the subsequent use of
the result set list in the applications, which often slice and dice
the list in various ways needed by the use case.

A typical use case for this is fetching result sets in chunks
and then recombining them in a larger list for further processing.

> Overall though, I don’t care much that the collection is mutable or not though in this area as well, a clearer specification would only serve the purpose to eliminate this decision / discussion needing to happen at the endpoints of every DBAPI driver.    A spec that provides clear guidance rather than loose suggestions would save a lot of effort.  Such a spec is of course more difficult to produce, but IMHO would be worth it.

The DB-API does provide guidance, but doesn't mandate specific
implementations.

What we can do is add a note to recommend that implementations
return results sets as list of tuples, unless special result set and
row objects are more appropriate or needed.

BTW: I still don't quite understand the motivation to have this
pinned down in the spec. If an application needs a list, the DB-API
mandates the sequence protocol, so running:

rs = list(cursor.fetchall())

will always give you a list as result set.

Note that tuples are faster than lists in Python, so a performance
aware module author might want to use this fact to squeeze out a
few more cycles.

People who have purely numeric needs, can also use numpy arrays as
containers (even to represent the complete result set),
since they also implement the sequence protocol.

-- 
Marc-Andre Lemburg
eGenix.com

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