Does a File Object Automatically Close when its Reference Count Hits Zero?

Question

I was under the impression that file objects are immediately closed when their reference counts hit 0, hence the line:

foo = open('foo').read()

would get you the file's contents and immediately close the file. However, after reading the answer to http://stackoverflow.com/questions/1832528/is-close-necessary-when-using-iterator-on-a-python-file-object I get the impression that this does not happen, and that calling .close() on a file object is always necessary.

Does the line above do what I think it's doing and even if it does, is it the Pythonic thing to do?

Answer 1

If you want to be sure, I'd write the code like this:

from __future__ import with_statement

with open('foo') as f:
    foo = f.read()

That way, your file closes as expected, even with exceptions.

Answer 2

No, Python optimize deleting unused objects so It may never close your file (OK at the end of your script at exit it will cleanup). @ hughdbrown have pointed out nice solution.

Answer 3

The answer is in the link you provided.

Garbage collector will close file when it destroys file object, but:

• you don't really have control over when it happens.

  While CPython uses reference counting to deterministically release resources (so you can predict when object will be destroyed) other versions don't have to. For example both Jython or IronPython use JVM and .NET garbage collector which release (and finalize) objects only when there is need to recover memory and might not do that for some object until the end of the program. And even for CPython GC algorithm may change in the future as reference counting isn't very efficient.

• if exception is thrown when closing file on file object destruction, you can't really do anything about it because you won't know.

Answer 4

For the cpython implementation of python: yes, it is guaranteed to be closed when its reference count goes to zero.

For python as an abstract language (e.g., including Jython, IronPython, etc): no, it is not guaranteed to be closed. In particular, an implementation of Python may choose not to use reference counting, but to use some other form of GC.

References:

• http://docs.python.org/c-api/typeobj.html#tp%5Fdealloc
• http://eli.thegreenplace.net/2009/06/12/safely-using-destructors-in-python/