Python's defaultdict also behaves like this, inserting the nonexistent entry into the dict:

    from collections import defaultdict
    m = defaultdict(int)
    m["nonexistent"]
    len(m) # => 1

There are examples of the opposite too, though. Ruby's Hash does not insert the entry:

    m = Hash.new(0)
    m["nonexistent"]
    m.length # => 0