Iterating a file object vs readlines()

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Iterating a file object is lazy - Python reads one line at a time from the OS buffer and discards it after your loop body runs. Only one line lives in memory at a time. f.readlines() loads the entire file into a list before the first iteration begins. For a 2 GB file that means 2 GB of memory consumed upfront. f.read().splitlines() is even worse - it first reads the whole file as a single string, then creates a second copy as a list of lines. When processing large files, direct iteration is not just a style preference - it's the difference between a script that works and one that crashes.