How to read a large file – line by line?

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The correct, fully Pythonic way to read a file is the following:

with open(...) as f:
    for line in f:
        # Do something with 'line'

The with statement handles opening and closing the file, including if an exception is raised in the inner block. The for line in f treats the file object f as an iterable, which automatically uses buffered I/O and memory management so you don’t have to worry about large files.

There should be one — and preferably only one — obvious way to do it.

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