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Traceback to help fixing double-calls to mpl.use. #7303

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Merged
merged 1 commit into from
Oct 25, 2016
Merged

Traceback to help fixing double-calls to mpl.use. #7303

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16 changes: 12 additions & 4 deletions lib/matplotlib/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1341,10 +1341,13 @@ def rc_file_defaults():
"""
rcParams.update(rcParamsOrig)

_use_error_msg = """ This call to matplotlib.use() has no effect
because the backend has already been chosen;
matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
_use_error_msg = """
This call to matplotlib.use() has no effect because the backend has already
been chosen; matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

The backend was *originally* set to {backend!r} by the following code:
{tb}
"""


Expand Down Expand Up @@ -1385,7 +1388,12 @@ def use(arg, warn=True, force=False):
if 'matplotlib.backends' in sys.modules:
# Warn only if called with a different name
if (rcParams['backend'] != name) and warn:
warnings.warn(_use_error_msg, stacklevel=2)
import matplotlib.backends
warnings.warn(
_use_error_msg.format(
backend=rcParams['backend'],
tb=matplotlib.backends._backend_loading_tb),
stacklevel=2)

# Unless we've been told to force it, just return
if not force:
Expand Down
5 changes: 5 additions & 0 deletions lib/matplotlib/backends/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5,10 +5,15 @@

import matplotlib
import inspect
import traceback
import warnings


backend = matplotlib.get_backend()
_backend_loading_tb = "".join(
line for line in traceback.format_stack()
# Filter out line noise from importlib line.
if not line.startswith(' File "<frozen importlib._bootstrap'))


def pylab_setup(name=None):
Expand Down