dictionary join

LOOKUP = {"G": "C", "C": "G", "T": "A", "A": "U"}


def to_rna(dna_strand):
    return ''.join(LOOKUP[chr] for chr in dna_strand)

This approach starts by defining a dictionary to map the DNA values to RNA values.

Python doesn't enforce having real constant values, but the LOOKUP dictionary is defined with all uppercase letters, which is the naming convention for a Python constant. It indicates that the value is not intended to be changed.

In the to_rna() function, the join() method is called on an empty string, and is passed the list created from a list comprehension.

The list comprehension iterates each character in the input, looks up the DNA character in the look-up dictionary, and outputs its matching RNA character as an element in the list.

The join() method collects the list of RNA characters back into a string. Since an empty string is the separator for the join(), there are no spaces between the RNA characters in the string.