In headless mode, matplotlib.use('tkagg') only errors after importing pyplot

[shoyer]

Bug report

Bug summary

Calling matplotlib.use('tkagg') doesn't work in headless mode. That's fine and to be expected.

What is bizarre is that this only results in an error when this is called after matplotlib.pyplot is imported. This results in a surprising and challenge to debug situation if matplotlib.use('tkagg') is called multiple times (e.g., in separate files).

Code for reproduction

Here's a script that I'm running with matplotlib 3.0.2:

import os

# pretend to be in "headless" mode
os.environ.pop("DISPLAY", None)

# first module
import matplotlib
matplotlib.use('TkAgg')  # silently fails?
import matplotlib.pyplot

# in another imported module
import matplotlib
matplotlib.use('TkAgg')  # this is when matplotlib crashes!
import matplotlib.pyplot

Actual outcome

Traceback (most recent call last):
  File "/usr/local/google/home/shoyer/Downloads/use_tkagg_test.py", line 13, in <module>
    matplotlib.use('TkAgg')  # this is when matplotlib crashes!
  File "/usr/local/google/home/shoyer/miniconda3/envs/xarray-py37/lib/python3.7/site-packages/matplotlib/__init__.py", line 1391, in use
    switch_backend(name)
  File "/usr/local/google/home/shoyer/miniconda3/envs/xarray-py37/lib/python3.7/site-packages/matplotlib/pyplot.py", line 222, in switch_backend
    newbackend, required_framework, current_framework))
ImportError: Cannot load backend 'TkAgg' which requires the 'tk' interactive framework, as 'headless' is currently running

Expected outcome

It would be nice if matplotlib did any of these:

1. Gave a clear error message when matplotlib.use() was called the first time.
2. Gave a clear error message when matplotlib.pyplot is imported and an invalid backend is selected.
3. Silently allowed setting an invalid backend with matplotlib.use() even after pyplot is imported.

Matplotlib version

• Operating system:
• Matplotlib version: 3.0.2
• Matplotlib backend (print(matplotlib.get_backend())): NA
• Python version: 3.7
• Jupyter version (if applicable):
• Other libraries:

As far as I can tell, the source of the problem is that matplotlib only actually validates backends if matplotlib.pyplot is imported:
https://github.com/matplotlib/matplotlib/blob/d9030efd40fe4300e775db22ba55845399f6668f/lib/matplotlib/__init__.py#L1281=L1283

It seems like one good way to avoid this would be to use a consistent codepath, whether or not matplotlib.pyplot has already been imported, e.g., by always calling matplotlib.pyplot.switch_backend(). Or maybe matplotlib.use() could do some basic validation, e.g., to verify that the backend is a valid choice.

[tacaswell]

This is the intersection of a couple of things that are trying to be nice the user. The first call to use is effecitvely being discarded as

matplotlib/lib/matplotlib/pyplot.py

Lines 2284 to 2290 in e4839b7

	# If rcParams['backend_fallback'] is true, and an interactive backend is
	# requested, ignore rcParams['backend'] and force selection of a backend that
	# is compatible with the current running interactive framework.
	if (rcParams["backend_fallback"]
	and dict.__getitem__(rcParams, "backend") in _interactive_bk
	and _get_running_interactive_framework()):
	dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)

is re-setting the backend to "auto" which does it's best to pick a compatible backend.

If you set rcParms['backend_fallback'] to False it works as expected:

amilyn@12:55  ➤  ipython
Python 3.7.1 (default, Dec 14 2018, 19:28:38) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.2.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import matplotlib                                                                                                                                                                                                                                                                 

In [2]: matplotlib.rcParams['backend_fallback']                                                                                                                                                                                                                                           
Out[2]: True

In [3]: matplotlib.rcParams['backend_fallback'] = False                                                                                                                                                                                                                                   

In [4]: import os                                                                                                                                                                                                                                                                         

In [5]: os.environ.pop("DISPLAY", None)                                                                                                                                                                                                                                                   
Out[5]: ':0'

In [6]: matplotlib.use('tkagg')                                                                                                                                                                                                                                                           

In [7]: import matplotlib.pyplot                                                                                                                                                                                                                                                          
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-7-864e826dab68> in <module>
----> 1 import matplotlib.pyplot

~/.conda/envs/dd37/lib/python3.7/site-packages/matplotlib/pyplot.py in <module>
   2370     dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)
   2371 # Set up the backend.
-> 2372 switch_backend(rcParams["backend"])
   2373 
   2374 # Just to be safe.  Interactive mode can be turned on without

~/.conda/envs/dd37/lib/python3.7/site-packages/matplotlib/pyplot.py in switch_backend(newbackend)
    220                 "Cannot load backend {!r} which requires the {!r} interactive "
    221                 "framework, as {!r} is currently running".format(
--> 222                     newbackend, required_framework, current_framework))
    223 
    224     rcParams['backend'] = rcParamsDefault['backend'] = newbackend

ImportError: Cannot load backend 'TkAgg' which requires the 'tk' interactive framework, as 'headless' is currently running

In [8]:                                                                                                                                                                                                                                                                                   

I suspect the fix is to in matplolib.use to set bacend_fallback to False to indicate that the user really did want the backend in the rcparams.

[shoyer]

I believe this can be considered fixed by #13977.

Thanks for the quick follow up!