On: Diving into the world of hash tables

[ubernostrum]

It's easier if you look at the bytecode produced for the expression, which is:

    1           0 LOAD_NAME                0 (foo)
                2 LOAD_ATTR                1 (bar)
                4 LOAD_ATTR                2 (baz)
                6 STORE_NAME               3 (a)
                8 LOAD_CONST               0 (None)
               10 RETURN_VALUE

The CPython implementation is a stack-based virtual machine. The first instruction here, LOAD_NAME, pushes co_names['foo'] (basically, whatever 'foo' is bound to in local scope) to the top of the stack. That's at least one hash table lookup, since the scope is a hash table mapping names to values. Then LOAD_ATTR replaces the top-of-stack value with the value of its 'bar' attribute. That's another hash table lookup, since attributes are a hash table mapping names to values. Then another LOAD_ATTR to get to 'baz', that's another hash table lookup. Then STORE_NAME binds the value at the top of the stack to the name given as an argument ('a'). That's an insert or update of a hash table.

So, the expression 'a = foo.bar.baz' involves at least three hash table lookups and one insert or update.