Option 1:

func set_default_value(section:String,key:String,value:Variant)->Variant:  
	if !self.has_section_key(section,key):  
		self.set_value(section,key,value)  
	return self.get_value(section,key)  

Option 2:

func set_default_value(section:String,key:String,value:Variant)->Variant:  
	value=self.get_value(section,key,value)  
	self.set_value(section,key,value)  
	return value