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See python's PEP-8 for the policy for python's standard library (identifiers must be ASCII) and PEP-3131 for the proposal that led to unicode identifiers being possible. Sage library code has more-or-less been following PEP-8, so I expect non-ascii identifiers will not be allowed there. What you do in your own files is your own concern (go wild and name your eigenvalue $\lambda$).

Non-ascii unicode characters may be difficult to type (depending on keyboard layout) and they can be extremely confusing, because unicode contains different code-points that have (nearly) identical print-shapes. I'd be strongly in favour for keeping them out of library code.