Traceback to help fixing double-calls to mpl.use. by anntzer · Pull Request #7303 · matplotlib/matplotlib

anntzer · 2016-10-18T21:38:51Z

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 "<frozen importlib._bootstrap>", line 969, in _find_and_load
File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 665, in exec_module
File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
File "/home/antony/src/extern/matplotlib/lib/matplotlib/pyplot.py", line 68, in <module>
    from matplotlib.backends import pylab_setup
File "<frozen importlib._bootstrap>", line 969, in _find_and_load
File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 665, in exec_module
File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
File "/home/antony/src/extern/matplotlib/lib/matplotlib/backends/__init__.py", line 13, in <module>
    _backend_loading_tb = "".join(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.

See #7287.

tacaswell · 2016-10-19T14:24:49Z

The doc failure looks un-related and due to merge conflicts.

Kojoley · 2016-10-19T16:17:50Z

The message looks weird now. warnings.warn prints the line of code that triggered the warning after the actual message, so we have this:

>python test_use.py
test_use.py:7: 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 'TkAgg' by the following code:
  File "test_use.py", line 4, in <module>
    import matplotlib.backends
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "...\lib\matplotlib\backends\__init__.py", line 13, in <module>
    _backend_loading_tb = "".join(traceback.format_stack())


  matplotlib.use('template')

anntzer · 2016-10-19T18:32:09Z

Just to clarify, another example:
a.py

def set_to_qt4agg():
    import matplotlib
    matplotlib.use("qt4agg")
    import matplotlib.pyplot

set_to_qt4agg()
import b; b.set_to_qt5agg()

b.py

def set_to_qt5agg():
    import matplotlib
    matplotlib.use("qt5agg")
    import matplotlib.pyplot

python a.py gives

/tmp/b.py:3: 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 'Qt4Agg' by the following code:
  File "a.py", line 6, in <module>
    set_to_qt4agg()
  File "a.py", line 4, in set_to_qt4agg
    import matplotlib.pyplot
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/home/antony/src/extern/matplotlib/lib/matplotlib/pyplot.py", line 68, in <module>
    from matplotlib.backends import pylab_setup
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/home/antony/src/extern/matplotlib/lib/matplotlib/backends/__init__.py", line 13, in <module>
    _backend_loading_tb = "".join(traceback.format_stack())


  matplotlib.use("qt5agg")

efiring · 2016-10-22T20:12:21Z

I think this message will leave most users baffled. Filtering out all the importlib stuff might help.

anntzer · 2016-10-23T04:45:29Z

Filtered out importlib line noise manually (possibly relevant python.org issues: http://bugs.python.org/issue15110, http://bugs.python.org/issue15386, http://bugs.python.org/issue15425).

Now:

$ 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()

We may also want to strip out the innermost frame but that may do more harm than good...

QuLogic · 2016-10-23T04:48:38Z

Might want to update the commit message too, but it's not super important if nothing else requires any changes.

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.

anntzer · 2016-10-23T04:54:52Z

Good catch, fixed.

NelleV · 2016-10-25T15:56:19Z

Thanks @anntzer !

mdboom added the status: needs review label Oct 18, 2016

tacaswell added this to the 2.1 (next point release) milestone Oct 19, 2016

tacaswell changed the title ~~Traceback to help fixing double-calls to mpl.use.~~ [MRG+1] Traceback to help fixing double-calls to mpl.use. Oct 19, 2016

tacaswell approved these changes Oct 19, 2016

View reviewed changes

anntzer force-pushed the setting-backends-twice-traceback branch from aab3ab3 to f119e5f Compare October 23, 2016 04:44

anntzer force-pushed the setting-backends-twice-traceback branch from f119e5f to 8ce25d1 Compare October 23, 2016 04:54

NelleV merged commit a7e5f6f into matplotlib:master Oct 25, 2016

anntzer deleted the setting-backends-twice-traceback branch October 25, 2016 16:02

QuLogic changed the title ~~[MRG+1] Traceback to help fixing double-calls to mpl.use.~~ Traceback to help fixing double-calls to mpl.use. Oct 25, 2016

This was referenced Oct 26, 2016

Document that the matplotlib backend will be set to Agg sphinx-gallery/sphinx-gallery#153

Closed

Make matplotlib.use() report where the backend was set first, in case of conflict #7287

Closed

QuLogic removed the status: needs review label Oct 28, 2016

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

Traceback to help fixing double-calls to mpl.use.#7303
NelleV merged 1 commit intomatplotlib:masterfrom
anntzer:setting-backends-twice-traceback

anntzer commented Oct 18, 2016

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tacaswell commented Oct 19, 2016

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Kojoley commented Oct 19, 2016

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anntzer commented Oct 19, 2016

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efiring commented Oct 22, 2016

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anntzer commented Oct 18, 2016

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anntzer commented Oct 19, 2016

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efiring commented Oct 22, 2016

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