how to replace (update) text in a file line by line

Question

I am trying to replace text in a text file by reading each line, testing it, then writing if it needs to be updated. I DO NOT want to save as a new file, as my script alrea

Answer 1

1. Open the file for read and copy all of the lines into memory. Close the file.
2. Apply your transformations on the lines in memory.
3. Open the file for write and write out all the lines of text in memory.

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with open(filename, "r") as f:
    lines = (line.rstrip() for line in f)
    altered_lines = [some_func(line) if regex.match(line) else line for line in lines]
with open(filename, "w") as f:
    f.write('\n'.join(altered_lines) + '\n')