import pyplot issue -

[terranova42]

root@MyPC:/usr/local/lib/python3.7/dist-packages# ls -lh|grep mat
drwxr-sr-x 13 root staff 4,0K mai 19 20:01 matplotlib
drwxr-sr-x 2 root staff 4,0K mai 19 20:01 matplotlib-3.1.0+832.gc4382b0b6-py3.7.egg-info
drwxr-sr-x 6 root staff 4,0K mai 19 20:04 matplotlib-3.1.0+832.gc4382b0b6-py3.7-linux-x86_64.egg
-rw-r--r-- 1 root staff 569 mai 19 20:01 matplotlib-3.1.0+832.gc4382b0b6-py3.7-nspkg.pth
root@MyPC:/usr/local/lib/python3.7/dist-packages# python3
Python 3.7.1 (default, Oct 22 2018, 11:21:55)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

from matplotlib import pyplot
Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python3.7/dist-packages/matplotlib/pyplot.py", line 2310, in
switch_backend(rcParams["backend"])
File "/usr/local/lib/python3.7/dist-packages/matplotlib/init.py", line 810, in getitem
plt.switch_backend(rcsetup._auto_backend_sentinel)
File "/usr/local/lib/python3.7/dist-packages/matplotlib/pyplot.py", line 212, in switch_backend
newbackend[9:] if newbackend.startswith("module://")
AttributeError: 'object' object has no attribute 'startswith'

[timhoffm]

Seems like no backend could be set and we fell through all candidates in switch_backend.

This has two aspects:

1. @terranova42 please try

   import matplotlib
   matplotlib.use('qt5agg')
   

   or some of the other backends listed in https://matplotlib.org/api/matplotlib_configuration_api.html?highlight=matplotlib%20use#matplotlib.use to see if you can manually set a backend.
2. @ devs: We should catch that case for candiate in ... else [hande no backend could be loaded] - pyplot l.201ff.

[anntzer]

switch_backend("agg") (the last case) should always succeed, unless something is seriously wrong with the install. So I guess the patch could be

diff --git i/lib/matplotlib/pyplot.py w/lib/matplotlib/pyplot.py
index 4f266917e..dda130995 100644
--- i/lib/matplotlib/pyplot.py
+++ w/lib/matplotlib/pyplot.py
@@ -198,8 +198,8 @@ def switch_backend(newbackend):
         # Don't try to fallback on the cairo-based backends as they each have
         # an additional dependency (pycairo) over the agg-based backend, and
         # are of worse quality.
-        for candidate in ["macosx", "qt5agg", "qt4agg", "gtk3agg", "tkagg",
-                          "wxagg", "agg"]:
+        for candidate in [
+                "macosx", "qt5agg", "qt4agg", "gtk3agg", "tkagg", "wxagg"]:
             try:
                 switch_backend(candidate)
             except ImportError:
@@ -207,6 +207,8 @@ def switch_backend(newbackend):
             else:
                 rcParamsOrig['backend'] = candidate
                 return
+        else:
+            switch_backend("agg")  # This should always succeed.
+            rcParamsOrig["backend"] = "agg"
+            return
 
     backend_name = (
         newbackend[9:] if newbackend.startswith("module://")

so that if switch_backend("agg") fails, one gets a proper exception out.

[terranova42]

Hello,
the problem seemed to come from a broken dependency. I read this documentation:
https://gist.github.com/CMCDragonkai/4e9464d9f32f5893d837f3de2c43daa4
but at first I could not import default backend 'agg' or any other.
Fixed my install by upgrading pillow:

root@MyPC:/usr/lib/python3/dist-packages# python3
Python 3.7.1 (default, Oct 22 2018, 11:21:55)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

from PIL import Image
Traceback (most recent call last):
File "", line 1, in
File "/usr/lib/python3/dist-packages/PIL/Image.py", line 60, in
from . import _imaging as core
ImportError: cannot import name '_imaging' from 'PIL' (/usr/lib/python3/dist-packages/PIL/init.py)
exit()
root@MyPC:/usr/lib/python3/dist-packages# python3 -m pip install --upgrade pillow
Collecting pillow
Downloading https://files.pythonhosted.org/packages/c1/e6/ce127fa0ac17775bc7887c432ffe945c49ae141f01b477b7cd5e63b16bb5/
|████████████████████████████████| 2.0MB 16.3MB/s
Installing collected packages: pillow
Found existing installation: Pillow 5.1.0
Uninstalling Pillow-5.1.0:
Successfully uninstalled Pillow-5.1.0
Successfully installed pillow-6.0.0
root@MyPC:/usr/lib/python3/dist-packages# python3
Python 3.7.1 (default, Oct 22 2018, 11:21:55)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
from PIL import Image
exit()

pillow upgrade seems to be the root cause (?).
then I had to upgrade numpy from 1.13.3 to 1.16.3
then installed scipy 1.3
then I was able to import any backend.

import matplotlib.rcsetup as rcsetup
print(rcsetup.all_backends)
['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo', 'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']
mpl.use('agg')
import matplotlib.pyplot as plt
plt.plot(range(20), range(20))
[<matplotlib.lines.Line2D object at 0x7ffa22c27240>]
plt.show()
main:1: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
mpl.use('TkAgg')
plt.show()
plt.plot(range(20), range(20))
[<matplotlib.lines.Line2D object at 0x7ffa1f69f7f0>]
plt.show()
print(rcsetup.interactive_bk)
['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo', 'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo']