csv writer to stdout throws error in with block

Question

How to force csv.writer() to accept stdout inside with block?

I this works ok:

with (open(output, "w")) if output != 'stdout' else stdout as file:

this is also fine:

file = csv.writer(open(output, "w") if output != 'stdout' else stdout)

but when I try this:

with csv.writer(open(output, "w") if output != 'stdout' else stdout) as file:

I get AttributeError: __exit__

How to make this with block work please? I am looking for solution like ruby solution but for python.

`csv.writer` doesn't have to close the handle on exit. Hence no `__exit__` method, and it makes no sense to put `csv.writer` in a `with` statement. — Jean-François Fabre, Apr 29 '18 at 20:59

jedwards · Answer 1 · 2018-04-29T21:07:52.440

3

Refactored slightly:

import csv
from sys import stdout

#output = "temp.txt"
output = "stdout"

with (open(output, "w") if output != 'stdout' else stdout) as file:
    writer = csv.writer(file)
    writer.writerow(["A","B","C"])

works on Python 3.6, at least.

But... this will close stdout after the with block completes.

For example:

with (open(output, "w") if output != 'stdout' else stdout) as file:
    writer = csv.writer(file)
    writer.writerow(["A","B","C"])

print("AFTER")  # Raises "ValueError: I/O operation on closed file."

I think the best approach is just to handle the file closing directly. You do lose the benefit of the context manager auto-closing the file on exceptions, but without doing some additional work creating a pseduo-stdout that doesn't actually close on __exit__, your best bet might be:

import csv
from sys import stdout

#output = "temp.txt"
output = "stdout"

file = (open(output, "w") if output != 'stdout' else stdout)

writer = csv.writer(file)
writer.writerow(["A","B","C"])

if output != "stdout": file.close()

print("AFTER")

edited Apr 29 '18 at 21:07

answered Apr 29 '18 at 21:00

jedwards

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of course... I am coding for 12 hours now... should get some break :D Thanks! – PapeK24 Apr 29 '18 at 21:01
2

@PapeK24 See my edit, the side effect of the with block closing stdout may not be what you want. – jedwards Apr 29 '18 at 21:02
it doesnt work in python 3.4. Cannot find `__exit__` on `stdout` – Jean-François Fabre Apr 29 '18 at 21:02
1

@Jean-FrançoisFabre I don't like it even when it works. I'm editing it. – jedwards Apr 29 '18 at 21:04
@jedwards good point on closing the stdout. I was expecting that too and got `AttributeError` instead because running previous version of python. – Jean-François Fabre Apr 29 '18 at 21:06
2

with a custom object, see my answer – Jean-François Fabre Apr 29 '18 at 21:09
@PapeK24 I edited my answer to do it "manually", and the "pseudo-stdout" that I described is perfectly encapsulated in Jean-Francois' answer -- he has more patience than I do :) – jedwards Apr 29 '18 at 21:09
I think that's a pretty neat question. Why depriving from the nice `with` construct because of lame stdout? – Jean-François Fabre Apr 29 '18 at 21:12
I agree -- I hope @PapeK24 will shift the check mark. Your answer is more pythonic and more robust. – jedwards Apr 29 '18 at 21:23
@jedwards @Jean-François Fabre Is it maybe possible to inherit `FakeStdout` from clasic one and only redefine `__exit__` ? – PapeK24 Apr 29 '18 at 21:29
inherit from a class, yes, from an instance? more difficult :) what could be possible would be to redefine `__exit__` on `stdout` itself. – Jean-François Fabre Apr 29 '18 at 21:32
@Jean-FrançoisFabre thats true... yeah not gonna redefine original one, that may cause issues at some point. Thanks for help. – PapeK24 May 02 '18 at 10:52

score 2 · Accepted Answer · answered Apr 29 '18 at 21:04

csv.writer doesn't have to close the handle on exit. Hence no __exit__ method, and it makes no sense to put csv.writer in a with statement.
With my python version, your original with open code doesn't work, I get a AttributeError: __exit__
And even if it worked (seems to work on Python 3.6 as the other answer has tested it), it closes stdout. Not what you want.

I've created a "fake" file object with __enter__ and __exit__ (does nothing) methods. this object complies with the with constraints.

import csv,sys

output="stdout"
class FakeStdout:
    def write(self,data):
        sys.stdout.write(data)
    def __exit__(self,*args,**kwargs):
        pass
    def __enter__(self):
        return self

with (open(output, "w") if output != 'stdout' else FakeStdout()) as file:
    cw = csv.writer(file)
    cw.writerow([1,2,3])

print("lll")

This code prints:

1,2,3
lll

it works with standard output and doesn't close it when exiting

csv writer to stdout throws error in with block

2 Answers2