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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.
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Within the block of code opened by “with”, our file is open, and can be read from freely. However, once Python exits from the “with” block, the file is automatically closed.
You can use the with statement to open the file, which will ensure that the file is closed. See http://www.python.org/dev/peps/pep-0343/ for more details.
The close() method of a file object flushes any unwritten information and closes the file object, after which no more writing can be done. Python automatically closes a file when the reference object of a file is reassigned to another file. It is a good practice to use the close() method to close a file.
You've learned why it's important to close files in Python. Because files are limited resources managed by the operating system, making sure files are closed after use will protect against hard-to-debug issues like running out of file handles or experiencing corrupted data.
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__.
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