Code style - for with if

Question

Which of the three do you think is a better coding style, or more readable? foo should be run on the items from both dictionaries, but mydict2 can be None

Option 1:

for a,b in mydict1.items():
    foo(a,b) 
if mydict2:
    for a,b in mydict2.items():
        foo(a,b)

Option 2:

for a,b in mydict1.items():
    foo(a,b)
for a,b in mydict2.items() if mydict2 else dict().items():
    foo(a,b)

Option 3:

for a,b in chain(mydict1.items(), mydict2.items() if mydict2 else dict().items()):
    foo(a,b)

Answer 1

Detect the side case early on, and replace it with an empty dict - this is the null object pattern:

if mydict2 is None:
     mydict2 = {}

This is identical to a common pattern used to avoid mutable default arguments. Then you can always have one (very simple) loop:

for a, b in chain(mydict.items(), mydict2.items()):

If you control the relevant code, consider changing things so that mydict2 can't be None in the first place.