What open() returns and what happens if you don't close it

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open() returns a file object - TextIOWrapper for text mode, BufferedReader/BufferedWriter for binary mode. Under the hood that object holds an OS file descriptor, which is a small integer from a limited per-process table. Two concrete problems arise if you skip close(). First, descriptor exhaustion: open enough files without closing them and you hit the OS limit, after which every subsequent open() raises OSError: [Errno 24] Too many open files. Second, write buffering: Python (and the OS) buffer writes for efficiency. close() flushes those buffers. If your process crashes before close() is called, the last few KB of data may never reach disk. CPython's reference-counting GC usually closes the file when the object goes out of scope, but other Python implementations (PyPy, Jython) don't guarantee this - another reason to always use 'with'.