Close open files windows 104/10/2023 ![]() ![]() Python does it for you, in a somewhat un-Pythonic way, magically, silently, and behind the scenes.īut what if you don’t explicitly close the file? What if you’re a bit lazy, and neither use a “with” block nor invoke f.close()? When is the file closed? When should the file be closed? Trying to read from f after we have exited from the “with” block will result in the same ValueError exception that we saw above. Thus, by using “with”, you avoid the need to explicitly close files. However, once Python exits from the “with” block, the file is automatically closed. Within the block of code opened by “with”, our file is open, and can be read from freely. That is, it assigns f to be the new file instance, pointing to the contents of /etc/passwd. The “with” statement invokes what Python calls a “context manager” on f. The preferred, best-practice way of opening files is with the “with” statement, as in the following: with open('/etc/passwd') as f: Moreover, the odds are good that you probably don’t want or need to do so, either. ValueError Traceback (most recent call last)īut here’s the thing: When I’m programming in Python, it’s pretty rare for me to explicitly invoke the “close” method on a file. Once we have done that, the object continues to exist - but we can no longer read from it, and the object’s printed representation will also indicate that the file has been closed: > f = open('/etc/passwd') ![]() The simplest answer is that we can explicitly close our file by invoking f.close(). It thus doesn’t surprise me when, soon after I introduce them to files in Python, they ask how we’re expected to close them. ![]() Most of the students in my Python courses come from other programming languages, in which they are expected to close a file when they’re done using it. In other words, f knows how to behave inside of a loop - or any other iteration context, such as a list comprehension. Note that the above code is possible because our file object “f” is an iterator. One of the first things that Python programmers learn is that you can easily read through the contents of an open file by iterating over it: f = open('/etc/passwd')
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