The complexity of in depends entirely on what L is. e in L will become L.__contains__(e).
See this time complexity document for the complexity of several built-in types.
Here is the summary for in:
- list – Average: O(n)
- set/dict – Average: O(1), Worst: O(n)
The O(n) worst case for sets and dicts is very uncommon, but it can happen if __hash__ is implemented poorly. This only happens if everything in your set has the same hash value.