From 8ce25d1221697c3f2ccf4e94f7cd2bc0d692c3a6 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Tue, 18 Oct 2016 14:32:51 -0700
Subject: [PATCH] Traceback to help fixing double-calls to mpl.use.

Example session:

    $ python -c 'import matplotlib as mpl; mpl.use("agg"); import matplotlib.pyplot; mpl.use("qt5agg")'
    -c:1: UserWarning:
    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 'agg' by the following code:
    File "<string>", line 1, in <module>
    File "/home/antony/src/extern/matplotlib/lib/matplotlib/pyplot.py", line 68, in <module>
        from matplotlib.backends import pylab_setup
    File "/home/antony/src/extern/matplotlib/lib/matplotlib/backends/__init__.py", line 14, in <module>
        line for line in traceback.format_stack()

(In practice, you'd expect the two calls to be further apart from each
other, of course.)

Note that the *current* traceback can be accessed by standard means,
e.g. by setting the warnings filter.

Not sure how easy this is to test.
---
 lib/matplotlib/__init__.py          | 16 ++++++++++++----
 lib/matplotlib/backends/__init__.py |  5 +++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
index a1dd5adbef34..5df9bd58fdb8 100644
--- 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
index 68c3a91b1c59..c2b29d474d8d 100644
--- 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):