Concerning “tab completion” in SageMathCell

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I don't believe that the 2nd would be supported in the cell, because it has no way of knowing what your object is until evaluated, and hence no idea of what you can do to that object. That is the downside of a "one-off" cell; if you need this functionality a lot (as I do) you will want to download Sage or use it in an online instance other than the cell server. It's a great tool, it just can't read your mind :)

Alternately, if you already know what sort of object you want, some of them will work. Try x.[tab], for instance or graphs.[tab]; you just have to have something predefined. But often that will be enough, if you know what sort of object you are looking at and what it is called.