Do not be Selfish: Use local variables to shadow your instance variables ( Python ).

Russ Hensel — Mon, 27 Jan 2025 18:42:58 +0000

Do not be Selfish: use local variables to shadow your instance variables ( Python ).

The following applies in many cases with objects. In this I will focus on a particular case: working with SQL in QT, a view and a model. A single window might have 2 models and 2 views. Setup code ( with some details left out ) might be something like this:

code with instance variables

# the model 1
self.model_1 = QSqlTableModel( ... )
self.model_1.setTable( 'table_1')

# the view 1
self.view_1 = QTableView( )
self.view_1.setModel( self.model_1  )
self.view_1.hideColumn( 0 )
self.view_1.setSelectionBehavior( QTableView.SelectRows )
self.view_1.view.setSortingEnabled( True )

# the model 2
self.model_2 = QSqlTableModel( ... )
self.model_2.setTable( 'table_2')

# the view 2 -- with a mistake
self.view_2 = QTableView( )
self.view_2.setModel( self.model_1  )
self.view_2.hideColumn( 3 )
self.view_2.setSelectionBehavior( QTableView.SelectRows )
self.view_2.setSortingEnabled( False )

less selfish code that shadows your instance variables but does exactly the same thing:

# the model 1
self.model_1 = QSqlTableModel( ... )
self.model_1 = model

model.setTable( 'table_1')

# the view 1
view = QTableView( )
self.view_1  = view
view.setModel( self.model_1  )
view.hideColumn( 0 )
view.setSelectionBehavior( QTableView.SelectRows )
view.setSortingEnabled( True )

# the model 2
model = QSqlTableModel( ... )
self.model_2 = model
model.setTable( 'table_2')

# the view 2 -- with a mistake corrected
view = QTableView( )
self.view_2 = view
view.setModel( model  )
view.hideColumn( 3 )
view.setSelectionBehavior( QTableView.SelectRows )
view.setSortingEnabled( False )

So what do I see that is different, and why do I like it:

self.model_1 mostly becomes model and self.view_1 mostly becomes view. And so on. ( in this case the shadowing is not exact it would be with self.model…. )
I have added 4 lines of code to the self.model_1….. but I have saved many characters of text.
Because of the local variables I can copy and paste the code to other places with many fewer changes.
My experiments show that using local variables like this is actually faster than instance variables.
If you mistype a local variable that is undefined your IDE will flag it as an error, this does not happen with instance variables because they can be defined very dynamically.
Is this faster coding, with faster running, and fewer bugs?

So what do you think:

worth a try?
useless?
obvious?

DEV Community: Russ Hensel

Do not be Selfish: Use local variables to shadow your instance variables ( Python ).