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PyVirtualDisplay

Python wrapper for Xvfb, Xephyr and Xvnc

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pyvirtualdisplay is a python wrapper for Xvfb, Xephyr and Xvnc

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Features:

  • python wrapper
  • supported python versions: 3.6, 3.7, 3.8, 3.9
  • back-ends: Xvfb, Xephyr and Xvnc

Possible applications:

  • headless run
  • GUI testing
  • automatic GUI screenshot

Installation

install the program:

$ python3 -m pip install pyvirtualdisplay

optional: Pillow should be installed for smartdisplay submodule:

$ python3 -m pip install pillow

If you get this error message on Linux then your Pillow version is old.

ImportError: ImageGrab is macOS and Windows only

on Ubuntu 20.04:

$ sudo apt-get install xvfb xserver-xephyr tigervnc-standalone-server xfonts-base
$ python3 -m pip install pyvirtualdisplay pillow

Usage

Controlling the display with start() and stop() methods:

from pyvirtualdisplay import Display
disp = Display()
disp.start()
# display is active
disp.stop()
# display is stopped

After Xvfb display is activated "DISPLAY" environment variable is set for Xvfb. (e.g. os.environ["DISPLAY"] = :1) After Xvfb display is stopped "DISPLAY" environment variable is restored to its original value.

Controlling the display with context manager:

from pyvirtualdisplay import Display
with Display() as disp:
    # display is active
    pass
# display is stopped

Selecting Xvfb backend:

disp=Display()
# or
disp=Display(visible=False)
# or
disp=Display(backend="xvfb")

Selecting Xephyr backend:

disp=Display(visible=True)
# or
disp=Display(backend="xephyr")

Selecting Xvnc backend:

disp=Display(backend="xvnc")

Setting display size:

disp=Display(size=(100, 60))

Setting display color depth:

disp=Display(color_depth=24)

Headless run

The display is hidden.

# pyvirtualdisplay/examples/headless.py

"Start Xvfb server. Open xmessage window."

from easyprocess import EasyProcess

from pyvirtualdisplay import Display

with Display(visible=False, size=(100, 60)) as disp:
    with EasyProcess(["xmessage", "hello"]) as proc:
        proc.wait()

If visible=True then a nested Xephyr window opens and the GUI can be controlled.

vncserver

The same as headless example, but it can be controlled with a VNC client.

# pyvirtualdisplay/examples/vncserver.py

"Start virtual VNC server. Connect with: vncviewer localhost:5904"

from easyprocess import EasyProcess

from pyvirtualdisplay import Display

with Display(backend="xvnc", size=(100, 60), rfbport=5904) as disp:
    with EasyProcess(["xmessage", "hello"]) as proc:
        proc.wait()

Run it:

$ python3 -m pyvirtualdisplay.examples.vncserver

Check it with vncviewer:

$ vncviewer localhost:5904

GUI Test

# pyvirtualdisplay/examples/lowres.py

"Testing gnumeric on low resolution."
from easyprocess import EasyProcess

from pyvirtualdisplay import Display

# start Xephyr
with Display(visible=True, size=(320, 240)) as disp:
    # start Gnumeric
    with EasyProcess(["gnumeric"]) as proc:
        proc.wait()

Run it:

$ python3 -m pyvirtualdisplay.examples.lowres

Image:

Screenshot

# pyvirtualdisplay/examples/screenshot.py

"Create screenshot of xmessage in background using 'smartdisplay' submodule"
from easyprocess import EasyProcess

from pyvirtualdisplay.smartdisplay import SmartDisplay

# 'SmartDisplay' instead of 'Display'
# It has 'waitgrab()' method.
# It has more dependencies than Display.
with SmartDisplay() as disp:
    with EasyProcess(["xmessage", "hello"]):
        # wait until something is displayed on the virtual display (polling method)
        # and then take a fullscreen screenshot
        # and then crop it. Background is black.
        img = disp.waitgrab()
img.save("xmessage.png")

Run it:

$ python3 -m pyvirtualdisplay.examples.screenshot

Image:

Nested Xephyr

# pyvirtualdisplay/examples/nested.py

"Nested Xephyr servers"
from easyprocess import EasyProcess

from pyvirtualdisplay import Display

with Display(visible=True, size=(220, 180), bgcolor="black"):
    with Display(visible=True, size=(200, 160), bgcolor="white"):
        with Display(visible=True, size=(180, 140), bgcolor="black"):
            with Display(visible=True, size=(160, 120), bgcolor="white"):
                with Display(visible=True, size=(140, 100), bgcolor="black"):
                    with Display(visible=True, size=(120, 80), bgcolor="white"):
                        with Display(visible=True, size=(100, 60), bgcolor="black"):
                            with EasyProcess(["xmessage", "hello"]) as proc:
                                proc.wait()

Run it:

$ python3 -m pyvirtualdisplay.examples.nested

Image:

xauth

Some programs require a functional Xauthority file. PyVirtualDisplay can generate one and set the appropriate environment variables if you pass use_xauth=True to the Display constructor. Note however that this feature needs xauth installed, otherwise a pyvirtualdisplay.xauth.NotFoundError is raised.

Concurrency

If more X servers are started at the same time then there is race for free display numbers.

"Recent X servers as of version 1.13 (Xvfb, too) support the -displayfd command line option: It will make the X server choose the display itself" https://stackoverflow.com/questions/2520704/find-a-free-x11-display-number/

Version 1.13 was released in 2012: https://www.x.org/releases/individual/xserver/

First help text is checked (e.g. Xvfb -help) to find if -displayfd flag is available. If -displayfd flag is available then it is used to choose the display number. If not then a free display number is generated and there are 10 retries by default which should be enough for starting 10 X servers at the same time.

displayfd usage is disabled on macOS because it doesn't work with XQuartz-2.7.11, always 0 is returned.

Thread safety

All previous examples are not thread-safe, because pyvirtualdisplay replaces $DISPLAY environment variable in global os.environ in start() and sets back to original value in stop(). To make it thread-safe you have to manage the $DISPLAY variable. Set manage_global_env to False in constructor.

# pyvirtualdisplay/examples/threadsafe.py

"Start Xvfb server. Open xmessage window. Thread safe."

from easyprocess import EasyProcess

from pyvirtualdisplay import Display

# manage_global_env=False is thread safe
with Display(manage_global_env=False) as disp:
    # disp.new_display_var should be used for new processes
    print("disp.new_display_var=" + disp.new_display_var)

    # disp.env() copies global os.environ and adds disp.new_display_var
    print("disp.env()['DISPLAY']=" + disp.env()["DISPLAY"])

    # set $DISPLAY for subprocesses
    with EasyProcess(["xmessage", "-timeout", "1", "hello"], env=disp.env()) as proc:
        proc.wait()

Run it:

$ python3 -m pyvirtualdisplay.examples.threadsafe
disp.new_display_var=:2
disp.env()['DISPLAY']=:2

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