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    <p>On Windows, NamedTemporaryFile uses a Windows-specific extension (os.O_TEMPORARY) to ensure that the file is deleted when it is closed. This probably also works if the process is killed in any way. However there is no obvious equivalent on POSIX, most likely because on POSIX you can simply delete files that are still in use; it only deletes the name, and the file's content is only removed after it is closed (in any way). But indeed assuming that we want the file name to persist until the file is closed, like with NamedTemporaryFile, then we need "magic".</p> <p>We cannot use the same magic as for flushing buffered files. What occurs there is that the C library handles it (in Python 2): the files are FILE objects in C, and the C guarantees that they are flushed on normal program exit (but not if the process is killed). In the case of Python 3, there is custom C code to achieve the same effect. But it's specific to this use case, not anything directly reusable.</p> <p>That's why NamedTemporaryFile uses a custom <code>__del__</code>. And indeed, <code>__del__</code> are not guaranteed to be called when the interpreter exits. (We can prove it with a global cycle of references that also references a NamedTemporaryFile instance; or running PyPy instead of CPython.)</p> <p>As a side note, NamedTemporaryFile could be implemented a bit more robustly, e.g. by registering itself with <code>atexit</code> to ensure that the file name is removed then. But you can call it yourself too: if your process doesn't use an unbounded number of NamedTemporaryFiles, it's simply <code>atexit.register(my_named_temporary_file.close)</code>.</p>
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    1. CO+1, excellent answer.
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    2. COFor the Python 3 case: in 3.0, `io.BufferedWriter` is implemented in pure Python, and in Python 3.1 and later a pure Python implementation is still available as `_pyio`. How can these modules use "custom C code"?
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    3. COI don't know Python 3 in details, but I do know that the `_io` module is written in C at least in the current Python 3.x (Python 3.0 is old by now). If there used to be a pure Python version, it was or is probably using `atexit`; if it was using `__del__` then it would be suffering from the same issues that this question is about.
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