other = s or "some default value"
Ok, it must be clarified how the or operator works. It is a boolean operator, so it works in a boolean context. If the values are not boolean, they are converted to boolean for the purposes of the operator.
Note that the or operator does not return only True or False. Instead, it returns the first operand if the first operand evaluates to true, and it returns the second operand if the first operand evaluates to false.
In this case, the expression x or y returns x if it is True or evaluates to true when converted to boolean. Otherwise, it returns y. For most cases, this will serve for the very same purpose of C♯’s null-coalescing operator, but keep in mind:
42 or "something" # returns 42 0 or "something" # returns "something" None or "something" # returns "something" False or "something" # returns "something" "" or "something" # returns "something"
If you use your variable s to hold something that is either a reference to the instance of a class or None (as long as your class does not define members __nonzero__() and __len__()), it is secure to use the same semantics as the null-coalescing operator.
In fact, it may even be useful to have this side-effect of Python. Since you know what values evaluates to false, you can use this to trigger the default value without using None specifically (an error object, for example).
In some languages this behavior is referred to as the Elvis operator.