what is the time complexity to check if a dictionary has a key?

Question

According to https://wiki.python.org/moin/TimeComplexity given a dictionary D the operation D[k] is constant.
What is the complexity of k in D ? Is this still constant?

Answer 1

Membership testing has the exact same cost as retrieving an item, so O(1).

That's only logical, because in order to return the value of a given key, you first need to determine if it is in the dictionary. If retrieving a key takes constant time, then determining if it is in the dictionary in the first place can only ever take constant time, too.