Getting Terminal Size In Python

Getting terminal size can be vital for your application, especially if you are doing some serious printing or drawing. However there are some few tricks worth noting that I'd like to share in this blog post.

First of all the recommended way or the "pythonic" way of retrieving terminal size for python3 is:

#!/usr/bin/env python3
import shutil
columns, rows = shutil.get_terminal_size(fallback=(80, 24))

And it works pretty great, well for the most part that is.



The Issue

This particular function is just a high level wrapper around low level cpython function os.get_terminal_size and the only real thing it does is handle an exception and returns fallback values if that's the case.

However there's a huge pitfall with this function and it's that it doesn't work with terminal pipes!
To confirm and test that with we can try this simple script size.py:

import sys
import shutil

columns, rows = shutil.get_terminal_size(fallback=(123, 456))
sys.stdout.write('cols:{}\nrows:{}\n'.format(columns, rows))
$ python size.py
cols:89
rows:22
$ python size.py | cat
cols:123
rows:456

89 to 22 is my actual size however when piping to anything, including python itself, the size seems to fallback to the fallback values, which in most cases defeats the whole purpose of retrieving the terminal size.

Making It Work!

The solution is pretty simple - use the other function instead!
If we use os.get_terminal_size(0) function, we'll get it working with piping too!

To test that lets change our script:

#!/usr/bin/env python3
import sys
import os

columns, rows = os.get_terminal_size(0)
sys.stdout.write('cols:{}\nrows:{}\n'.format(columns, rows))

The thing to note here is the positional argument 0 in os.get_terminal_size() function, which tells which file descriptor to use:

0 - Standard Input
1 - Standard Output
2 - Standard Error

By default both os and shutil functions use 1, which stands for Standard Output. This means that if we pipe and this output detaches itself we get an OsError:

OSError: [Errno 25] Inappropriate ioctl for device

This default somewhat makes sense if the size of your terminal changes when the output is displayed, but I'm having trouble even imagining an example where that would be the case.

So if we were to run our script now, we'd get the results we are looking for:

$ python size.py
cols:89
rows:22
$ python size.py | cat
cols:89
rows:22
# Even if we pipe multiple times!
$ python size.py | grep . | cat
cols:89
rows:22

Edit

As user bearded_unix_guy pointed out on reddit, using stdin(argument 0) might not always work, in particular it wont work when we pipe to our app:

cat - | size.py

In case like above we actually want to use stdout(default argument) since it's not detached. However what about if your app is in the middle:

cat - | size.py | cat

In this case neither stdout nor stdin will work, but sterr(2) will!
So to combine all of these to cover all of the cases we can simply wrap it in a for loop with exception catching:

def get_terminal_size(fallback=(80, 24):
    for i in range(0,3):
        try:
            columns, rows = os.get_terminal_size(i)
        except OSError:
            continue
        break
    else:  # set default if the loop completes which means all failed
        columns, rows = fallback
    return columns, rows

And there you go, you can use that instead of os.get_terminal_size() and have a pipe-foolproof terminal size getter!

Conclusion

I've spent more time than I'd like to admit trying to figure this out and the stackoverflow thread for this subject only left me more confused.
As always if you have any questions, critique or notices feel free to leave a comment!