I guess it's the overflow considerations?

what is meant with the "worst case" ?

Clippy from the Rust tool chain has a hint for optimising the code:

2 |     dna.replace("T", "U")
  |                 ^^^ help: try using a `char` instead: "T"
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern
  = note: `#[warn(clippy::single_char_pattern)]` on by default

If the goal is to pass the test, that's fine. But this code cannot be considered best practice because it does not consider the worst case.

"xrange()" in python3 test (please, fix)

Hello. I suppose in python3 in the test there is an expression from python2 (xrange()) so it does not test properly. Fix, please