`if key in dict` vs. `try/except` - which is more readable idiom?

Question

I have a question about idioms and readability, and there seems to be a clash of Python philosophies for this particular case:

I want to build dictionary A from dict

Answer 1

There is also a third way that avoids both exceptions and double-lookup, which can be important if the lookup is expensive:

value = B.get("blah", None)
if value is not None: 
    A["blah"] = value

In case you expect the dictionary to contain None values, you can use some more esoteric constants like NotImplemented, Ellipsis or make a new one:

MyConst = object()
def update_key(A, B, key):
    value = B.get(key, MyConst)
    if value is not MyConst: 
        A[key] = value

Anyway, using update() is the most readable option for me:

a.update((k, b[k]) for k in ("foo", "bar", "blah") if k in b)