[PyQt] PyQt API 2: equivalent of Null QVariant?

[Phil Thompson]

On Tue, 21 Dec 2010 11:05:03 +0100, TP <paratribulations at free.fr> wrote:
> Hello,
> 
> I currently use the API 1, and use the isNull() method of QVariant to
> detect 
> a Null QVariant. This is necessary to detect a NULL value in a database.
> 
> It seems there is no equivalent in API2 of PyQt. None is used to model
an 
> invalid QVariant. But how to model a Null QVariant?
> 
> http://www.riverbankcomputing.co.uk/static/Docs/PyQt4/pyqt4ref.html#id13
> 
> Compare the output of the two following examples.
> 
> Thanks
> 
> Julien
> 
> ########################
> # Example 1: using API 1 for QVariant
> 
> from PyQt4.QtSql import *
> import sys
> 
> db = QSqlDatabase.addDatabase( "QSQLITE" )
> db.setDatabaseName( ":memory:" )
> 
> if not db.open():
>     sys.exit(1)
> 
> query = QSqlQuery()
> 
> query.exec_( "CREATE TABLE person ( firstname VARCHAR(20)"
>         ", lastname VARCHAR(20) );" )
> query.exec_( "INSERT INTO person VALUES( 'Danny', 'Young');" )
> query.exec_( "INSERT INTO person ( firstname ) VALUES( 'Christine' );" )
> query.exec_( "INSERT INTO person VALUES( 'Lars', 'Gordon');" )
> 
> query.exec_( "SELECT * FROM person;" )
> 
> record = query.record()
> for i in range( record.count() ):
>     print record.fieldName( i ), "\t",
> 
> print "\n----------------------------"
> 
> for i in range( record.count() ):
>     while query.next():
>         for i in range( record.count() ):
>             value = query.value(i)
>             if value.isNull():
>                 print "NULL", "\t",
>             else:
>                 print value.toString(), "\t",
>         print
> 
> ########################
> # Example 2: using API 2 for QVariant
> 
> import sys, sip
> sip.setapi('QVariant', 2)
> 
> from PyQt4.QtSql import *
> 
> db = QSqlDatabase.addDatabase( "QSQLITE" )
> db.setDatabaseName( ":memory:" )
> 
> if not db.open():
>     sys.exit(1)
> 
> query = QSqlQuery()
> 
> query.exec_( "CREATE TABLE person ( firstname VARCHAR(20)"
>         ", lastname VARCHAR(20) );" )
> query.exec_( "INSERT INTO person VALUES( 'Danny', 'Young');" )
> query.exec_( "INSERT INTO person ( firstname ) VALUES( 'Christine' );" )
> query.exec_( "INSERT INTO person VALUES( 'Lars', 'Gordon');" )
> 
> query.exec_( "SELECT * FROM person;" )
> 
> record = query.record()
> for i in range( record.count() ):
>     print record.fieldName( i ), "\t",
> 
> print "\n----------------------------"
> 
> for i in range( record.count() ):
>     while query.next():
>         for i in range( record.count() ):
>             value = query.value(i)
>             if value == None:
>                 print "NULL", "\t",
>             else:
>                 print value, "\t",
>         print

The short answer is that you can't. However...

QVariant::isNull() typically delegates to the enclosed data's isNull()
method (eg. QString::isNull()). So your example for API v1 will also work
for API v2.

It gets a bit more complicated if you are also using the QString v2 API
because a null QString and an empty QString are both converted to an empty
unicode object (although None is converted to a null QString when going in
the other direction). I chose this asymmetric behaviour because QString()
creates a null QString when often what is really meant is an empty QString.

BTW, use "value is None" rather than "value == None".

Phil