Get Original Shell Terminal Width

[George-Ogden]

What's the problem this feature will solve?

Allows developers to access the terminal width during test runtime (useful for many pretty printing libraries).

Describe the solution you'd like

Ideally, I would like to be able to do:

width, _ = shutil.get_terminal_size(fallback=(80, 24)) # still use shutil
os.get_terminal_size(sys.__stderr__.fileno()) # use a specific file descriptor

But more realistically, I could access the default terminal and view its width:

from _pytest._io.terminalwriter import default_terminal
print(default_terminal.fullwidth)

Alternative Solutions

Looking at previous issues, the only relevant one seems to be #92.
This suggests that I can do:

from _pytest._io.terminalwriter import get_terminal_width
print(get_terminal_width())

However, this fails while the test is running, falling back to the default of 80 instead of the original terminal width.

There may be a way to do this already or it may be infeasible to implement, but I'd be interested to hear what you think?

[cp50]

Hi, I'm interested in working on this issue. I've read the description and I understand the problem. I'd like to investigate why shutil.get_terminal_size() is not working in this context and propose a solution. Can you give me any more details on where I should start looking in the codebase?

[George-Ogden]

Doing some more digging, it seems this is only an issue when using --capture=fd. All the other methods produce the correct terminal width in this code sample:

def test_terminal_size():
    # captured stdout
    try:
        print(os.get_terminal_size(sys.stdout.fileno()))
    except OSError:
        print("error")
    # original stdout
    try:
        print(os.get_terminal_size(sys.__stdout__.fileno()))
    except OSError:
        print("error")
   assert False # debug the output

I'm not sure exactly where this happens or how the handling is done in the codebase.

[George-Ogden]

@cp50 - an update on this is that the issue seems to come from os.dup2 - I can view the terminal width (using os.get_terminal_size(1) before that line, but not after.

However, I can still get the terminal width on the original descriptor (which is a duplicate of the original): os.get_terminal_size(self.targetfd_save).

Ideally, there would be a method somewhere that exposes the duplicated fd (mostly for os methods). With the current setup, any attempts to use the old fd (accessible via sys.__stdout__) will fail because of the os.dup2 that ties it to the temporary fd.

I hope this helps, and let me know what you think.