When writing this SO answer, I came upon this behavior, which I did not expect.

In [1]: 1 in Series([1]) Out[1]: False In [2]: 1 in Series([1, 2]) Out[2]: True 

Apparently, __contains__ looks at the index.

In [3]: 1 in Series([1], index=[1]) Out[3]: True 

I can see how that makes sense. But I think it's fair to say that it is potentially confusing. Do we rely on this behavior of __contains__ for data alignment? Might it be reasonable to change it? I'm not sure that's a good idea, but I want to raise the issue.