Python 3 dictionaries have .keys(), .values(), and .items() methods which return memory-efficient set-like objects, not lists. (See PEP 3106.)

If your dictionaries are small enough that the memory overhead of extra list creation is not significant, stick with standard Python 3 code in your Py2/3 compatible codebase:

for item in d:
    # code here

for item in d.items():
    # code here

for value in d.values():
    # code here

If your dictionaries are large, or if you want to use the Python 3 set-like behaviour on both Py3 and Python 2.7, then use the viewkeys etc. functions from :mod:`future.utils`:

from future.utils import viewkeys, viewvalues, viewitems

for (key, value) in viewitems(hugedictionary):
    # some code here

# Set intersection:
d = {i**2: i for i in range(1000)}
both = viewkeys(d) & set(range(0, 1000, 7))

# Set union:
both = viewvalues(d1) | viewvalues(d2)

For Python 2.6 compatibility, the functions iteritems etc. are also available in :mod:`future.utils`.