[Pypgsql-users] Re: PgBoolean

[Gerhard <ger...@op...>]

You wrote:
> Can anyone tell me how to programatically check the value of a PgBoolean
> object? I am doing the following:
> 
> #####
> from pyPgSQL import PgSQL
> from pyPgSQL import libpq
> 
> db = PgSQL.connect("localhost::testdb")
> cursor = db.cursor()
> cursor.callproc("sp_TestBoolean", 1, 1)
> retval = cursor.fetchone()
> #####
> 
> When I print the value of retval, I get this:
> [<PgBoolean instance at 0x401c8900: Value: False>]
> 
> I would like to be able to do something like this:
> 
> if retval == true:
>    # do something
> else:
>    # do something else

It's not wise to test for truth values using == true. In Python, you test
wether something is nonzero, or zero.

These are examples of objects that are nonzero:

1, 2, 3, 4, 5, "foobar", [3,4,5], (3,4)

You can see this when you use them in an if-statement:

    >>> if "foobar": print "true"
    ...
    true

These are examples of objects whose truth value is "false":

0, 0.0, "", [], ()

Proof:

    >>> if not 0.0: print "false"
    ...
    false


Now, back to pyPgSQL :-) You simply test for the truth value of PgBooleans
using the if-statement:

    if retval:
       # do something
    else:
       # do something else


As far as "== true" is concerned, this does not exist in Python. *But*
there is are "True" and "False" constants in Python 2.2.2 (these are really
ints in 2.2.2). In Python 2.3 (still in development), we have a real
boolean type.

But please don't do things like:

    if foo == True:
        # ... 

just because that's possible with 2.2.2 and later. Such code hurts my eyes
;-) It's also more verbose than it needs to be. And error-prone, because it
requires that the class or type of foo support comparison to "True".
Checking for the nonzero-ness is much more rubost, and works with all
objects, as you've seen above (ints, longs, floats, strings, lists, etc.).

HTH,
-- 
Gerhard Häring
OPUS GmbH München
Tel.: +49 89 - 889 49 7 - 32
http://www.opus-gmbh.net/