Python: No csv.close()?

Python: No csv.close()?

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 doesnt manage any resources that would need to be cleaned up when youre done using it, so theres no need to close it; itd 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. Heres 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 youre not familiar with the with statement, its roughly equivalent to enclosing its contents in a try...finally block that closes the file in the finally part.

Hopefully this doesnt 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.

You do not close CSV readers directly; instead you should close whatever file-like object is being used. For example, in your case, youd say:

f = open(/home/rv/ncbi-blast-2.2.23+/db/output.blast)
z = csv.reader(f, delimiter=t)
...
f.close()

If you are using a recent version of Python, you can use the with statement, e.g.

with open(/home/rv/ncbi-blast-2.2.23+/db/output.blast) as f:
    z = csv.reader(f, delimiter=t)
    ...

This has the advantage that f will be closed even if you throw an exception or otherwise return inside the with-block, whereas such a case would lead to the file remaining open in the previous example. In other words, its basically equivalent to a try/finally block, e.g.

f = open(/home/rv/ncbi-blast-2.2.23+/db/output.blast)
try:
    z = csv.reader(f, delimiter=t)
    ...
finally:
    f.close()

Python: No csv.close()?

You dont close the result of the reader() method, you close the result of the open() method. So, use two statements: foo=open(...); bar=csv.reader(foo). Then you can call foo.close().

There are no bonus points awarded for doing in one line that which can be more readable and functional in two.

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