Idiomatic use of stdout as alternative

Question

Is this idiomatic?

with open(output_file, 'w') if output_file else sys.stdout as outf:
    outf.write("hello")

Will the with block cause trouble to stdout (by closing it)?

Answer 1

If you try to write to stdout after that, it will:

>>> import sys
>>> output_file = None
>>> with open(output_file, 'w') if output_file else sys.stdout as outf:
...     outf.write("hello")
... 
hello5
>>> print("test")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.

The only way this could somehow safely be used would be as the last statement in a script where you know that stdout will not be used later, e.g:

if __name__ == '__main__':
     output_file = ... # parse arguments
     with open(output_file, 'w') if output_file else sys.stdout as outf:
         outf.write("hello")

But even that feels wrong. Better: separate opening and working with the file, and be explicit about it:

if __name__ == '__main__':
     output_file = ... # parse arguments
     if output_file:
         with open(output_file, 'w') as outf:
             do_stuff(outf)
     else:
             do_stuff(sys.stdout)