[DB-SIG] PEP 249 clarification

[M.-A. Lemburg]

Stefan C. Kremer wrote:
> Hi,
> 
> The "fetchall()" command returns a list.  Lists are mutable objects in python.  Could a valid implementation return the same list object (with different contents) for multiple calls to fetchall() (with potentially different preceding execute queries)?

That would be a very unusual behavior in Python. Mutable objects
are normally not shared between function/method calls. If they
are shared, this needs to be documented. It is possible
(and common) to share immutable objects, though.

The DB-API uses the latter to implement am optimization in
the .execute() method: if you pass in the same command string
object in subsequent calls, the implementation can choose to
reuse the previously prepared access plan.

Some other examples (these are all True):

dict(some_dict) is not some_dict
list(some_list) is not some_list
tuple(some_tuple) is some_tuple
str(some_str) is some_str

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