In need of insight regarding Python file reading mechanisms.

[Larz60+]

open files like so, instead of embedded:

with open("ROTOR_EMR4_BY_R15.cdb", "r") as old_database,
    open("old_output.txt", "r+") as old_copy:

Then, assuming that the "(3i9,6e21.13e3)" is alone by itself on a line:

        for x in old_content:
            line = str(x)
            line = line.strip()
 
            if line == "(3i9,6e21.13e3)":
                ...

You're probably including the line termination in your search, so won't match . That is the reason for the strip