How does python close files that have been gc'

Question

I had always assumed that a file would leak if it was opened without being closed, but I just verified that if I enter the following lines of code, the file will close:

>>> f = open('somefile.txt')
>>> del f

Just out of sheer curiosity, how does this work? I notice that file doesn't include a __del__ method.

Answer 1

Python uses reference counting and deterministic destruction in addition to garbage collection. When there is no more references to an object, the object is released immediately. Releasing a file closes it.

This is different than e.g. Java where there is only nondeterministic garbage collection. This means you connot know when the object is released, so you will have to close the file manually.

Note that reference counting is not perfect. You can have objects with circular references, which is not reachable from the progam. Thats why Python has garbage collection in addition to reference counting.

Answer 2

Best guess is that because the file type is a built-in type, the interpreter itself handles closing the file on garbage collection.

Alternatively, you are only checking after the python interpreter has exited, and all "leaked" file handles are closed anyways.

Answer 3

In CPython, at least, files are closed when the file object is deallocated. See the file_dealloc function in Objects/fileobject.c in the CPython source. Dealloc methods are sort-of like __del__ for C types, except without some of the problems inherent to __del__.

Answer 4

Hence the with statement.

For Python 2.5, use

from __future__ import with_statement

(For Python 2.6 or 3.x, do nothing)

with open( "someFile", "rU" ) as aFile:
    # process the file
    pass
# At this point, the file was closed by the with statement.
# Bonus, it's also out of scope of the with statement,
# and eligible for GC.