Merge pull request #7303 from anntzer/setting-backends-twice-traceback

NelleV · web-flow · commit a7e5f6fa34f4 · 2016-10-25T11:56:22.000-04:00
[MRG+1] Traceback to help fixing double-calls to mpl.use.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -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}
 """
 
 
@@ -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:
diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -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):
