Proper use of "with" together with stdout in Python

Question

I would like the following basic construct:

if fname:
    fhandle=open(fname,"w")
else:
    fhandle=sys.stdout

...code using fhandle for output....

Normally, I would open the file using "with" like this:

with open(name,"w") as fhandle:
      .... code using handle for output....

Is there a way to blend these two constructs, so that I can pass something to the open function in the with construct that will make fhandle point to sys.stdout? Or, if this is a dumb idea, what's the pythonic way to do this?

So you want to either open a file or a stdout, depending on some condition? — shybovycha, Mar 30 '15 at 15:04
Yes, the file provided as a string; and stdout is already open by default. — Jeremy Teitelbaum, Mar 30 '15 at 15:10
The `with` statement will cause `stdout` to be closed when the block exits. Is that desirable? — Martijn Pieters, Mar 30 '15 at 15:10

score 1 · Answer 1 · edited May 23 '17 at 11:56

1

I think, all you need is ternary operator:

with open('moo.txt', 'w') if YOU_CONDITION else sys.stdout as f:
    f.write('hello')

>> hello

edited May 23 '17 at 11:56

Community

1
1

answered Mar 30 '15 at 15:07

shybovycha

9,794
6
47
77

martineau · Accepted Answer · 2015-03-30T15:12:51.887

0

You could do something like this:

import sys
from contextlib import contextmanager

@contextmanager
def func(fname):
    fhandle = open(fname,"w") if fname else sys.stdout
    yield fhandle
    if fname:
        fhandle.close()

And use it like this:

with func(fname) as fhandle:
    ... your code ...

edited Mar 30 '15 at 15:12

answered Mar 30 '15 at 15:09

martineau

99,260
22
139
249

@MartijnPieters, the issue is: what happens on `stdout.__exit__`? is it in general a good idea? – bgusach Mar 30 '15 at 15:12
@ikaros45: it is just a file object, so it'll be closed. If you don't plan to write to it for the rest of the program runtime, then that's fine. – Martijn Pieters Mar 30 '15 at 15:13
@MartijnPieters, that is quite an assumption... I did think of this solution, but returning either a file, or a context manager that does not do anything on `__exit__`. – bgusach Mar 30 '15 at 15:15
@ikaros45: well, if you are using this at the top level where exiting the `with` statement means exiting the script, and errors go to `sys.stderr` instead, it is fine to close `sys.stdout`. Hence my request for clarification on the question. – Martijn Pieters Mar 30 '15 at 15:18
Yes, under these circumstances, that is perfectly fine. I wouldn't do it that way however, I prefer to forget about and specific situations. As the code grows, you're lost if you have to bear in mind specifics. – bgusach Mar 30 '15 at 15:20
FWIW, my revised answer doesn't close the file if it was set to `sys.stdout`. – martineau Mar 30 '15 at 15:23
I'm going to accept this answer because it's what I thought I wanted when I asked; but since I really don't want to close stdout, in the end I'm taking a completely different approach. Thanks all. – Jeremy Teitelbaum Mar 30 '15 at 15:27
@Jeremy: Thanks, but note, it won't close `stdout` on you. – martineau Mar 30 '15 at 15:30
@JeremyTeitelbaum out of interest, in what direction are you taking it? – NDevox Mar 30 '15 at 15:57

NDevox · Answer 3 · 2015-03-30T15:14:22.040

0

Put the code to handle output in a function.

def handle_output(fhandle):
    # do stuff

if fname:
    with open(fname,"w") as fhandle:
        handle_output(fhandle)
else:
    fhandle=sys.stdout
    handle_output(fhandle)

Feedback is always welcome on downvotes!

edited Mar 30 '15 at 15:14

answered Mar 30 '15 at 15:09

NDevox

3,751
4
18
32

I find it bizarre anyone downvoted you. I upvoted because this is the pattern I use, and it seems best to me. – ArtOfWarfare Mar 30 '15 at 15:15
@ArtOfWarfare thanks. I agree, I find the idea of closing stdout bizarre. I understand that it may not necessarily be harmful, but it's not necessary in the first place. – NDevox Mar 30 '15 at 15:17
I did not downvote, but I find this code clunky. I would create a function that returns an object with the context manager protocol (either a file handle, or a wrapper around stdout). Polymorphic and nice. – bgusach Mar 30 '15 at 15:17
@Ikaros45, I haven't used context managers (haven't had the need to) to differentiate in this way. What's the advantage over the above? The function should give the flexibility to use anything with a write method, is simple to make and is easy to understand. – NDevox Mar 30 '15 at 15:25
1

It is a subtle thing, but in the scope of such a function, I am thinking about writing some business info somewhere, and if possible, I would like not to think about where or how it happens (ie, it should be abstracted away). Moreover, passing arguments to be mutated is something I like to avoid (although a file is a perfectly fine usage) – bgusach Mar 30 '15 at 15:36
Of all the answers here, yours is the only one to gain any upvotes, so you can take solace in that... – ArtOfWarfare Mar 30 '15 at 15:56
Hah, thanks. I feel appreciated. – NDevox Mar 30 '15 at 15:57

Proper use of "with" together with stdout in Python

3 Answers3