Python: No csv.close()?

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The reader is really just a parser. When you ask it for a line of data, it delegates the reading action to the underlying file object and just converts the result into a set of fields. The reader itself doesn’t manage any resources that would need to be cleaned up when you’re done using it, so there’s no need to close it; it’d be a meaningless operation.

You should make sure to close the underlying file object, though, because that does manage a resource (an open file descriptor) that needs to be cleaned up. Here’s the way to do that:

with open('/home/rv/ncbi-blast-2.2.23+/db/output.blast') as f:
    z = csv.reader(f, delimiter="\t")
    # do whatever you need to with z

If you’re not familiar with the with statement, it’s roughly equivalent to enclosing its contents in a try...finally block that closes the file in the finally part.

Hopefully this doesn’t matter (and if it does, you really need to update to a newer version of Python), but the with statement was introduced in Python 2.5, and in that version you would have needed a __future__ import to enable it. If you were working with an even older version of Python, you would have had to do the closing yourself using try...finally.

Thanks to Jared for pointing out compatibility issues with the with statement.

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