Question

I’m using the CSV module to read a tab delimited file. Code below:

z = csv.reader(open('/home/rv/ncbi-blast-2.2.23+/db/output.blast'), delimiter='\t')

But when I add Z.close() to end of my script i get and error stating “csv.reader’ object has no attribute ‘close'”

z.close()

So how do i close “Z”?

Answer 1

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.

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