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The Python standard makes no guarantee, but in CPython, the example above is deterministic. The cycle-detecting garbage collector is not involved in this case. Just refcounting.

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If open() throws an exception, perhaps after the FD is already open (Python does non-trivial IO to a file immediately after open), or read() throws an exception, the file object is at the mercy of any try/except higher in the stack, any bugs/"features" contained in those, and any sys.excepthook installed like Sentry

The example above is deterministic only on current versions of CPython and only if it is guaranteed to succeed and if it is the only line of code in the program now and in perpetuity.

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