LSP for dict

Liskov substitution principle tells us that if S is a subtype of T, then all occurrences of T may be replaced with S without breaking any code. That means that S should satisfy all the guarantees that T introduces.

Whenever you work with dict you usually assume that as long as x in d returns False, d[x] raises KeyError. But if your dict is a defaultdict, that it's simply not true. Does defaultdict violate the LSP then?

Strictly speaking, it doesn't. The dict documentation explicitly says that d[x] may return something even though x is not present in d (if the __missing__ method defined). So you still potentially get something from the dictionary even if the key is not in it. That means that you shouldn't ever do x in d to check the existence of element as long as you want to support all dict subclasses.

It may seem that defaultdict violates the LSP, but it “breaks” the guarantees that were never there.