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

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?