[Biopython] is_aa() returning true on nonstandard amino acids

[Ahmad Abdelzaher]

I was really happy that I found out about is_aa, however, I just found out
that it returns false positives on a lot of modified residues
like S-NITROSO-CYSTEINE (SCN) and a few others.

Is it supposed to do that? Is there another method for standard amino acids
only?

Also sorry to hijack my own thread, but I was trying to sample some random
structures with certain constraints using the PDB's advanced search. I
specifically chose no ligands or modified residues, yet  I still get those
in my structures, any idea why?

Finally, noob question, is_aa doesn't work unless I do "from
Bio.PDB.Polypeptide import is_aa" or "from Bio.PDB.Polypeptide import *",
but doesn't work if I do "from Bio.PDB import Polypeptide", doesn't the
class come with all its methods?

Regards.
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