Is the R interface identical to R in a R.<x> assignment?

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It would be worth learning a bit of Python syntax if you are going to use Sage - in particular, identifiers belong to whatever you assign them to, which can change any time you assign them to something else.

Thanks, I didn't realize that the R.<x> assignment rebinds R to a polynomial ring. But, as you can check, the docs misleadingly build polynomial ring generator with the syntax R.<some_id>, (nearly) always with the R name in such a way I thought that the letter R was mandatory. This is not true since S.<x> is valid and create a new binding from S to a polynomial ring. In fact, what your response just explains is the non-negligible difference between Python syntax and Sage syntax (I imagine, due to the Sage preprocessor). In Python, R.<x> raises a syntax error. And worse, in Python R.attr always refers to an R attribute (not the case here with Sage) and doesn't bind or rebind R. I consider the R.<x> as an unfortunate choice (the is no attribute setting) and R<x> would have been clearer.

The rationale is that some other programs offered this syntax, so it was enabled (long, long ago) to make it easy to migrate. Most users of Magma are unlikely to be also needing R :)