How to read a large file - line by line?

I want to iterate over each line of an entire file. One way to do this is by reading the entire file, saving it to a list, then going over the line of interest. This method uses a lot of memory, so I am looking for an alternative.

My code so far:

for each_line in fileinput.input(input_file):
    do_something(each_line)

    for each_line_again in fileinput.input(input_file):
        do_something(each_line_again)

Executing this code gives an error message: device active.

Any suggestions?

The purpose is to calculate pair-wise string similarity, meaning for each line in file, I want to calculate the Levenshtein distance with every other line.

Nov. 2022 Edit: A related question that was asked 8 months after this question has many useful answers and comments. To get a deeper understanding of python logic, do also read this related question How should I read a file line-by-line in Python?