[DB-SIG] Some obscurity with paramstyle

M.-A. Lemburg mal at egenix.com
Mon Jul 18 09:33:06 CEST 2011

Christoph Zwerschke wrote:
> At the PyGreSQL mailing list we're currently wondering whether the
> 'format' and 'pyformat' paramstyles allow specifying parameters with
> types other than '%s' - e.g. can I specify my parameter as '%.2f' or
> '%(name).2f' if I want to round floats to 2 digits?
> PEP 249 has only '%s' in the example, but does not exclude other types,
> does this mean these are allowed?

Not really. The DB-API 2.0 allows for a certain number of binding
parameter formats, but you shouldn't really invent more.

For the next version, we're likely going to reduce the number
of allowed formats to just two (%s and ?, IIRC), since having
lots of different formats has resulted in much confusion.

> Also, should we define a new paramstyle for the advanced string
> formatting syntax available since Py 2.6?

No. See above.

The problem with %s or any other format that resembles a
Python string formatting character is that people (rightly)
expect the full functionality of those to be available, which
is not the case.

There's a significant difference between string formatting
characters and binding parameter characters: the latter just
let the database module know where a certain parameter
value should be added, but don't literally include the
parameters in the SQL string, nor define how the parameters
are passed to the database.

Binding parameters just define an association between the
SQL and the parameter list, nothing more.

Marc-Andre Lemburg

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