On 2007-05-23 13:37, Carsten Haese wrote:
> On Wed, 2007-05-23 at 12:36 +0200, M.-A. Lemburg wrote:
>> Just an aside:
>>
>> Note that the named style allows binding the same parameter
>> more than once.
>
> Ditto for numeric style.
>
>> This poses a few problems for interfaces which
>> rely on the database telling the interface how to best bind
>> a parameter, since it may well be that case that a parameter
>> needs to be bound as e.g. integer in one place (e.g. as index)
>> and float in another (e.g. if used in a formula).
>>
>> For numbers, it's fairly obvious what to do (create multiple
>> bindings for the object), but it's not for objects that don't
>> easily allow retrieving the same value twice, such as
>> iterators or files.
>>
>> I don't know how interfaces that do support named style deal
>> with this problem.
>
> InformixDB supports binding by name, and in the real world this is not a
> problem at all. The parameter values must be supplied in a mapping, and
> the input binding loop simply calls __getitem__ on that mapping,
> possibly requesting the same key twice. If the same key is requested
> twice, you'd have to work hard to make that *not* return the same object
> twice. You'd have to pass in a dictionary-like object whose __getitem__
> method has deliberate side-effects, and if you do that, you deserve to
> be punished.
That's not what I was asking. The problem (or maybe it's a non-issue
in the real word) is: what happens at binding time to the data
fetched from the object you bind to a command parameter, e.g.
say the interface supports reading data from a file (instead of
just using a string):
file = open('my.dat', 'rb')
cursor.execute('insert into mytable values (:data, :data)',
{'data': file})
In theory, the interface would have to read and buffer
the data from the file in order to be able to provide
two bindings to the database.
Regards,
--
Marc-Andre Lemburg
eGenix.com
Professional Python Services directly from the Source (#1, May 23 2007)
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