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Backport PR #18245 on branch v3.3.x #18253

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Merged
merged 1 commit into from
Aug 14, 2020
Merged

Backport PR #18245 on branch v3.3.x #18253

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18 changes: 17 additions & 1 deletion lib/matplotlib/pyplot.py
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Original file line number Diff line number Diff line change
Expand Up @@ -232,10 +232,26 @@ def switch_backend(newbackend):
close("all")

if newbackend is rcsetup._auto_backend_sentinel:
current_framework = cbook._get_running_interactive_framework()
mapping = {'qt5': 'qt5agg',
'qt4': 'qt4agg',
'gtk3': 'gtk3agg',
'wx': 'wxagg',
'tk': 'tkagg',
'macosx': 'macosx',
'headless': 'agg'}

best_guess = mapping.get(current_framework, None)
if best_guess is not None:
candidates = [best_guess]
else:
candidates = []
candidates += ["macosx", "qt5agg", "gtk3agg", "tkagg", "wxagg"]

# 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", "gtk3agg", "tkagg", "wxagg"]:
for candidate in candidates:
try:
switch_backend(candidate)
except ImportError:
Expand Down
34 changes: 34 additions & 0 deletions lib/matplotlib/tests/test_backends_interactive.py
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Expand Up @@ -191,3 +191,37 @@ def test_webagg():
conn.close()
proc.send_signal(signal.SIGINT)
assert proc.wait(timeout=_test_timeout) == 0


@pytest.mark.skipif(sys.platform != "linux", reason="this a linux-only test")
@pytest.mark.backend('Qt5Agg', skip_on_importerror=True)
def test_lazy_linux_headless():
test_script = """
import os
import sys

# make it look headless
del os.environ['DISPLAY']

# we should fast-track to Agg
import matplotlib.pyplot as plt
plt.get_backend() == 'agg'
assert 'PyQt5' not in sys.modules

# make sure we really have pyqt installed
import PyQt5
assert 'PyQt5' in sys.modules

# try to switch and make sure we fail with ImportError
try:
plt.switch_backend('qt5agg')
except ImportError:
...
else:
sys.exit(1)

"""
proc = subprocess.run([sys.executable, "-c", test_script])
if proc.returncode:
pytest.fail("The subprocess returned with non-zero exit status "
f"{proc.returncode}.")