How to query the number of elements stored in a db.ListProperty in google app engine

How to query the number of elements stored in a db.ListProperty in google
app engine

The entity class is defined like:
class Item(db.Model):
list = db.ListProperty(db.Key)
What's the attribute or function to return the number of elements stored
in a ListProperty, so that I can use something like
{{ item.list.... }}
to display that in a html.