How to handle both `with open(…)` and `sys.stdout` nicely?

Why LBYL when you can EAFP?

try:
    with open(target, 'w') as h:
        h.write(content)
except TypeError:
    sys.stdout.write(content)

Why rewrite it to use the with/as block uniformly when you have to make it work in a convoluted way? You’ll add more lines and reduce performance.

Stick with your current code. It’s simple and you can tell exactly what it’s doing just by glancing at it.

Another way would be with an inline if:

handle = open(target, 'w') if target else sys.stdout
handle.write(content)

if handle is not sys.stdout:
    handle.close()

But that isn’t much shorter than what you have and it looks arguably worse.

You could also make sys.stdout unclosable, but that doesn’t seem too Pythonic:

sys.stdout.close = lambda: None

with (open(target, 'w') if target else sys.stdout) as handle:
    handle.write(content)

I’d also go for a simple wrapper function, which can be pretty simple if you can ignore the mode (and consequently stdin vs. stdout), for example:

from contextlib import contextmanager
import sys

@contextmanager
def open_or_stdout(filename):
    if filename != '-':
        with open(filename, 'w') as f:
            yield f
    else:
        yield sys.stdout

If you really must insist on something more “elegant”, i.e. a one-liner:

>>> import sys
>>> target = "foo.txt"
>>> content = "foo"
>>> (lambda target, content: (lambda target, content: filter(lambda h: not h.write(content), (target,))[0].close())(open(target, 'w'), content) if target else sys.stdout.write(content))(target, content)

foo.txt appears and contains the text foo.

Okay, if we are getting into one-liner wars, here’s:

(target and open(target, 'w') or sys.stdout).write(content)

I like Jacob’s original example as long as context is only written in one place. It would be a problem if you end up re-opening the file for many writes. I think I would just make the decision once at the top of the script and let the system close the file on exit:

output = target and open(target, 'w') or sys.stdout
...
output.write('thing onen')
...
output.write('thing twon')

You could include your own exit handler if you think its more tidy

import atexit

def cleanup_output():
    global output
    if output is not sys.stdout:
        output.close()

atexit(cleanup_output)

Another possible solution: do not try to avoid the context manager exit method, just duplicate stdout.

with (os.fdopen(os.dup(sys.stdout.fileno()), 'w')
      if target == '-'
      else open(target, 'w')) as f:
      f.write("Foo")

How about opening a new fd for sys.stdout? This way you won’t have any problems closing it:

if not target:
    target = "/dev/stdout"
with open(target, 'w') as f:
    f.write(content)

if (out != sys.stdout):
    with open(out, 'wb') as f:
        f.write(data)
else:
    out.write(data)

Slight improvement in some cases.

An improvement of Wolph’s answer

import sys
import contextlib

@contextlib.contextmanager
def smart_open(filename: str, mode: str = 'r', *args, **kwargs):
    '''Open files and i/o streams transparently.'''
    if filename == '-':
        if 'r' in mode:
            stream = sys.stdin
        else:
            stream = sys.stdout
        if 'b' in mode:
            fh = stream.buffer  # type: IO
        else:
            fh = stream
        close = False
    else:
        fh = open(filename, mode, *args, **kwargs)
        close = True

    try:
        yield fh
    finally:
        if close:
            try:
                fh.close()
            except AttributeError:
                pass

This allows binary IO and pass eventual extraneous arguments to open if filename is indeed a file name.

import contextlib
import sys

with contextlib.ExitStack() as stack:
    h = stack.enter_context(open(target, 'w')) if target else sys.stdout
    h.write(content)

Just two extra lines if you’re using Python 3.3 or higher: one line for the extra import and one line for the stack.enter_context.

If it’s fine that sys.stdout is closed after with body, you can also use patterns like this:

# Use stdout when target is "-"
with open(target, "w") if target != "-" else sys.stdout as f:
    f.write("hello world")

# Use stdout when target is falsy (None, empty string, ...)
with open(target, "w") if target else sys.stdout as f:
    f.write("hello world")

or even more generally:

with target if isinstance(target, io.IOBase) else open(target, "w") as f:
    f.write("hello world")

The following solution is not a beauty, but from a time long, long ago; just before with …

handler = open(path, mode = 'a') if path else sys.stdout
try:
    print('stuff', file = handler)
    ... # other stuff or more writes/prints, etc.
except Exception as e:
    if not (path is None): handler.close()
    raise e
handler.close()

One way to solve it is with polymorphism. Pathlib.path has an open method that functions as you would expect:

from pathlib import Path

output = Path("/path/to/file.csv")

with output.open(mode="w", encoding="utf-8") as f:
    print("hello world", file=f)

we can copy this interface for printing

import sys

class Stdout:
    def __init__(self, *args):
        pass

    def open(self, mode=None, encoding=None):
        return self

    def __enter__(self):
        return sys.stdout

    def __exit__(self, exc_type, exc_value, traceback):
        pass

Now we simply replace Path with Stdout

output = Stdout("/path/to/file.csv")

with output.open(mode="w", encoding="utf-8") as f:
    print("hello world", file=f)

This isn’t necessarily better than overloading open, but it’s a convenient solution if you’re using Path objects.

With python 3 you can used wrap stdout file descriptor with IO object and avoid closing on context leave it with closefd=False:

h = open(target, 'w') if target else open(sys.stdout.fileno(), 'w', closefd=False)

with h as h:
    h.write(content)

As pointed in Conditional with statement in Python, Python 3.7 allows using contextlib.nullcontext for that:

from contextlib import nullcontext

with open(target, "w") if target else nullcontext(sys.stdout) as f:
    f.write(content)

This is a simpler and shorter version of the accepted answer

import contextlib, sys


def writer(fn): 
    @contextlib.contextmanager
    def stdout():
        yield sys.stdout
    return open(fn, 'w') if fn else stdout()

usage:

with writer('') as w:
    w.write('hellon')

with writer('file.txt') as w:
    w.write('hellon')