DEV Community

How would you rewrite this Python code?

Dustin King on May 26, 2018

I ran across this problem while writing some code for a challenge. I have some code that I want to write to a particular file if a filename is pro...

Read full post

rhymes • May 26 '18 • Edited

I think the simplest is the first one :D

Or you can wrap that if in a normal function if you plan to use it more than once:

def file_or_stdout(fname=None):
  if fname:
    return open(fname, 'w')
  else:
    return sys.stdout

I don't think you need to use a context manager in this case because there are no other operations between the opening and closing of the file.

Anyway I wouldn't do it like that.

Keep in mind that if you use sys.stdout this way you will close it which means that any attempt to write on it after your with statement will result in an error:

>>> import sys
>>> with file_or_stdout() as f:
...     f.write("Yo!\n")
...
Yo!
4
>>> sys.stdout.write("Yo 2\n")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.
>>> print("yo 3")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.

So why do you need to hide stdout this way? It's probably easier to use StringIO, write to it and then use a print to display it on stdout.

Dustin King • May 27 '18

So why do you need to hide stdout this way?

I'm just trying to keep the code clear and non-repetitive. In terms of level of effort and maintainability, if it needs to be able to optionally write to a file, breaking the inner logic out into a function as inspired by @thinkslynk is probably best.

But if it's a one-off project it might be even better to only write to stdout and pipe it to a file in the shell if needed. (Sometimes it's fun to over-engineer though.)

Tomer • May 26 '18

Actually the context manager solves this by only opening/closing the fp if it isn't to stdout so it might be the best solution

rhymes • May 26 '18

Oh yes, you're right :-)

+1 to the context manager, though it might be overkill if the function is used only once.

Stephen Dycus • May 26 '18

I think the cleanest way is this:

if filename:
    with (open(filename, 'w') as file:
        do_something(file)
else:
    do_something(sys.stdout)

Dustin King • May 26 '18

Certainly works if do_something is just a function call. I meant it as a stand-in for a larger block of code. But maybe that block should actually be a function.

Niels Böhm • May 27 '18

Yes, it should be a function. stdout/stderr/stdin are different from other streams in that they're managed outside (by the OS and Python, not by your app) whereas an open should be used with with in simple cases if possible.

You can start treating them the same when you start writing to / reading from them and stop when you stop doing that. All that can nicely be stuck away in a function. That also enables you to call the actual I/O part with a different file-like thing, say a StringIO instance when testing.

Dustin King • Jun 15 '18

I don't think I'd heard of CQS before. I'll have to think more about it. It does make the code clear in this case.

On first examination, one issue I have with the general concept is that it seems that functions that act like constructors, such as file_or_stdout() in my example, or even just output = open(...) would violate it because they both do something (e.g. open the file, build a context manager) and return a reference to the thing they've opened or created.

CQS seems similar to a rule of thumb that parameters are for input and return values are for output, which I'm violating with do_something(), because it was a stand-in for a couple lines in the original code.

Maybe the larger takeaway of this exercise is that mutability has its pitfalls.

Derek Bartron • May 27 '18

How is the first one harder to read then the rest that more lines and more complicated? If a Python coder can't read an inline expression, how is turning it into a context manager helping? I'm confused by this entire article.

Dustin King • May 27 '18

There is a lot of subjectivity involved in deciding which code is "better", and that line may be perfectly readable for you.

To me it seemed like there was too much happening in that one line, which would mean someone has to stop on it for a few seconds to figure out what it's doing. Having a more complicated with clause also seemed to distract from the code after with(...):. This isn't shown very well here, since I just used do_something(file) as a stand-in, instead of the couple lines in the linked code this was derived from.

But you may have a point in that one or two long lines might be preferable to a lot of short lines. The point of this post was that I ran across a situation in which the clearest way to write the code wasn't obvious, and I thought it would be valuable to hear peoples opinions on different ways to do it, and the tradeoffs between them.

Yoandy Rodriguez Martinez • May 26 '18 • Edited

I'll tell you as my old Python sensei used to tell me: When in doubt, import this.
I think your second solution is the one. It is both concise and does not messes with PEP 8.

Dustin King • May 26 '18

What do you mean by "import this"?

Pierre Bouillon • May 27 '18

Just type "import this" I'm your interpreter, you will understand :)

Dustin King • May 27 '18

Ah yes, the good ol' Zen of Python.

Tomer • May 26 '18

TIL building a context manager is so simple and clean.