about close a file in Python

Question

I know it is a good habit of using close to close a file if not used any more in Python. I have tried to open a large number of open files, and not close them (in the same Pytho

Answer 1

To add to James Mills' answer, if you need to understand the specific reason you don't see any errors:

Python defines that when a file gets garbage collected, it will be automatically closed. But Python leaves it up to the implementation how it wants to handle garbage collection. CPython, the implementation you're probably using, does it with reference counting: as soon as the last reference to an object goes away, it's immediately collected. So, all of these will appear to work in CPython:

def spam():
    for i in range(100000):
        open('spam.txt') # collected at end of statement

def eggs():
    for i in range(100000):
        f = open('eggs.txt') # collected next loop, when f is reassigned

def beans():
    def toast():
        f = open('toast.txt') # collected when toast exits
    for i in range(100000):
        toast()

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But many other implementations (including the other big three, PyPy, Jython, and IronPython) use smarter garbage collectors that detect garbage on the fly, without having to keep track of all the references. This makes them more efficient, better at threading, etc., but it means that it's not deterministic when an object gets collected. So the same code will not work. Or, worse, it will work in your 60 tests, and then fail as soon as you're doing a demo for your investors.

It would be a shame if you needed PyPy's speed or IronPython's .NET integration, but couldn't have it without rewriting all your code. Or if someone else wanted to use your code, but needed it to work in Jython, and had to look elsewhere.

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Meanwhile, even in CPython, the interpreter doesn't collect all of its garbage at shutdown. (It's getting better, but even in 3.4 it's not perfect.) So in some cases, you're relying on the OS to close your files for you. The OS will usually flush them when it does so—but maybe not if you, e.g., had them open in a daemon thread, or you exited with os._exit, or segfaulted. (And of course definitely not if you exited by someone tripping over the power cord.)

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Finally, even CPython (since 3.3, I think) has code specifically to generate warnings if you let your files be garbage collected instead of closing them. Those warnings are off by default, but people regularly propose turning them on, and one day it may happen.