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Use uppercase() which returns a capitalized string on your input DNA string.
You define the function with a parameter dna_seqs. You then reassign dna_seqs losing what was passed in. You then have a for loop that makes no sense - is filter_seq a function or is it a string?

I don't really understand it enough. Back to the drawing board.

I thought dna_seqs = ('A','C','G','T','a','c','g','t') would just give out this part:
"The sequence contains only the nucleotide letters A, C, G or T, or their lowercase equivalents, and no ambiguous nucleotides" Don't really know my thinking with the loop.