Field calculator calculate mean values of multiple rows when some values are Nulls

Hello!

I need to calculate average value of 7 fields (Index2006, Index2007, Index2008, Index2009, Index2010, Index2011, Index2012) to field IlvesAve (see illustration).
Attachment 29714
The problem is that I have Null values in many rows. 0 values are real counting data, while Null values indicate the missing data. Null values can't be taken into account otherwise the results are incorrect. I can calculate sum value of indexes:

Code:

def stack(item1,item2,item3,item4,item5,item6,item7):
itemList = [item1,item2,item3,item4,item5,item6,item7]
myList = [item for item in itemList if (item != None)]
return count(myList)

Expression:
stack ( !Index2006! , !Index2007! , !Index2008! , !Index2009! , !Index2010! , !Index2011! , !Index2012! )
Attachment 29715

But I do not know how to calculate mean or count. I cannot divide sum value with a constant because there are always different number of fields with non Null values that need to be taken into account. Can anyone help me?

Best regards
Anne

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