python "with" command and csv reader

Question

I have both csv readers and writers. I understand you have to open and close the underlying object. One way to do it it would be to first create the file object, f, use csv reader and then use f.close().

However, I can't seem to do the following:

with open(outputpath) as f_outputfile:

    outputfile = csv.writer(f_outputfile)

OTHER CODE HERE

What I want to do is open a bunch of readers at once and a bunch of writers at once and have them all close automatically. However, does that mean I have a nested "With" block?

More appropriately, is OTHER CODE HERE the part that writes to the csv file? — Josiah
– Josiah, Commented May 17, 2012 at 19:21
Yes, it does. Say I want 3 readers and 2 writers and they all need each other. Other CODE HERE writes the CSV file. — user1357015
– user1357015, Commented May 17, 2012 at 19:22
Then you should make OTHER CODE HERE part of the with block. — Josiah
– Josiah, Commented May 17, 2012 at 19:25

Eric · Accepted Answer · 2015-02-24 23:47:47Z

5

Writing:

with open(outputpath) as f_outputfile:
    outputfile = csv.writer(f_outputfile)

OTHER CODE HERE

Is essentially the same as:

f_outputfile = open(outputpath)
try:
    outputfile = csv.writer(f_outputfile)
finally:
    f_outputfile.close() 

OTHER CODE HERE

If OTHER CODE HERE relies on the file being open, it's not going to work.

edited Feb 24, 2015 at 23:47

answered May 17, 2012 at 19:27

Eric

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1 Comment

Falmarri Over a year ago

It's only essentially the same if csv.writer can't throw exceptions (which is never true in python).

John Gaines Jr. · Accepted Answer · 2012-05-17 19:42:20Z

3

You can stack multiple items in the with statement as in (looks like this is a 2.7.x and higher feature):

with open(foo) as f_foo, open(bar) as f_bar:
    # do something

7.5. The with statement

New in version 2.5.

The with statement is used to wrap the execution of a block with methods defined by a context manager (see section With Statement Context Managers). This allows common try...except...finally usage patterns to be encapsulated for convenient reuse.

with_stmt ::=  "with" with_item ("," with_item)* ":" suite
with_item ::=  expression ["as" target]

edited May 17, 2012 at 19:42

answered May 17, 2012 at 19:24

John Gaines Jr.

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3 Comments

user1357015 Over a year ago

This throws an error, not sure why then: with open(outputpath,"w") as f_outputfile, open(inputpath, "r") as f_inputpath:

John Gaines Jr. Over a year ago

Just verified it works in 2.7.3, what version are you using? Just took a peek at the 2.6 docs and there it only allows single with_item's this is probably a 3.x feature back-ported to 2.7.x

user1357015 Over a year ago

I'm using 2.6.6, that must be why. It's ok, I can do nested "withs". Just to make sure, I don't need to worry about close commands now yes? They will now close automatically?

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