List can be used for memory cells, but beware of negative indexes on the languages that support them.
With a small tweak, the issue can be resolved.

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are there any sequences that break the use of lists? negative index for lists acts as is. Streaming a character followed by a carriage move on the tape is a weak attempt. It is not obvious for me to use a list for memory to solve this problem.

bad idea check incoming in not_allowed
len(string.printable) = 100

len(printable) = 100
on small text possibly better use straightforward solution
after 100 chars in input must be faster, if dictionary in scope

intresting count how many times string constructor will be triggered

good explanation and so strange code, fully ignorirng explanation

Right!

how about 99`999 in input?