There's a slight inconsistency. The names a code object explicitly
calls out as free variables (i.e. references to cells in outer scopes)
are only a subset of the full set of free variables (every referenced
name that isn't a local variable or an attribute).

>>> from dis import show_code
>>> def outer():
...   x, y = 1, 2
...   def inner():
...     print (x, y, a, b, c.e)
...   return inner
...
>>> f = outer()
>>> show_code(f)