Using `theme_matplotlib()` Raises `AttributeError`

[GrapesGoober]

Problem

Hi all, I'm using Plotnine for a first time. Currently migrating my visualizations from matplotlib and I've played around with plotnine's heatmap.

I found a possible bug when running theme_matplotlib(). The plot works fine without theme_matplotlib(), but has a problem when I add theme_matplotlib() to the ggplot

Replicate

• Using dependencies

  matplotlib==3.11.0
  plotnine==0.15.8
  

• Run python via script mode (a *.py file, not *.ipynb)

  from plotnine import (
      aes,
      geom_tile,
      ggplot,
      theme_matplotlib,
  )
  from plotnine.data import faithfuld
  
  plot = (
      ggplot(faithfuld, aes("waiting", "eruptions", fill="density"))
      + geom_tile()
      + theme_matplotlib()
  )
  plot.show()

• Expected outcome: a heatmap
• Instead I got error

  (.venv) PS C:\Users\Grapes\Desktop\Workbench\plotnine-scripts> & c:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Scripts\python.exe c:/Users/Grapes/Desktop/Workbench/plotnine-scripts/run_theme_matplotlib.py
  Traceback (most recent call last):
    File "c:\Users\Grapes\Desktop\Workbench\plotnine-scripts\run_theme_matplotlib.py", line 16, in <module>
      plot.show()
      ~~~~~~~~~^^
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\plotnine\ggplot.py", line 193, in show
      self.draw(show=True)
      ~~~~~~~~~^^^^^^^^^^^
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\plotnine\ggplot.py", line 304, in draw
      with plot_context(self, show=show):
           ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\plotnine\_utils\context.py", line 74, in __enter__
      self.rc_context.__enter__()
      ~~~~~~~~~~~~~~~~~~~~~~~~~^^
    File "C:\Program Files\Python313\Lib\contextlib.py", line 141, in __enter__
      return next(self.gen)
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\matplotlib\__init__.py", line 1210, in rc_context
      rcParams.update(rc)
      ~~~~~~~~~~~~~~~^^^^
    File "<frozen _collections_abc>", line 987, in update
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\matplotlib\__init__.py", line 779, in __setitem__
      cval = valid_key(val)
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\matplotlib\rcsetup.py", line 310, in validate_backend
      if s is _auto_backend_sentinel or backend_registry.is_valid_backend(s):
                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
    File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\matplotlib\backends\registry.py", line 224, in is_valid_backend
      if not backend.startswith("module://"):
             ^^^^^^^^^^^^^^^^^^
  AttributeError: 'object' object has no attribute 'startswith'  
  

What I found

• I dug around the plotnine source code locally a bit and found BackendRegistry.is_valid_backend method with the if-check

  if not backend.startswith("module://"):
      backend = backend.lower()

• Here, the argument backend is expected to be string, but instead it is received an object, not str. This raised the exception. I added a bandaid fix on my local installation to trigger its intended error message instead.

  if not isinstance(backend, str):
      return False
  
  if not backend.startswith("module://"):
      backend = backend.lower()

• Running the heatmap python script, I get this message instead.

  File "C:\Users\Grapes\Desktop\Workbench\plotnine-scripts\.venv\Lib\site-packages\matplotlib\__init__.py", line 781, in __setitem__
      raise ValueError(f"Key {key}: {ve}") from None
  ValueError: Key backend: '<object object at 0x000001E0483EF970>' is not a valid value for backend; supported values are ['gtk3agg', 'gtk3cairo', 'gtk4agg', 'gtk4cairo', 'macosx', 'nbagg', 'notebook', 'qtagg', 'qtcairo', 'qt5agg', 'qt5cairo', 'tkagg', 'tkcairo', 'webagg', 'wx', 'wxagg', 'wxcairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']
  

• I think this is the intended error message. After this message, I try changing my matplotlib backend to use something else, say tkagg:

  import matplotlib
  from plotnine import (
      aes,
      geom_tile,
      ggplot,
      theme_matplotlib,
  )
  from plotnine.data import faithfuld
  
  matplotlib.use("tkagg")
  
  plot = (
      ggplot(faithfuld, aes("waiting", "eruptions", fill="density"))
      + geom_tile()
      + theme_matplotlib()
  )
  plot.show()

• This can now plot the heatmap as expected.
• I hypothesize the root cause being the method BackendRegistry.is_valid_backend receives an object instead of a string.

[has2k1]

This was fixed in the current dev branch and it is in the alpha release of the next version (v0.16.0).

uv pip install --pre plotnine
pip install --pre plotnine
uv pip install "plotnine>=0.16.0a13"

In the next couple of days will backport the fix v0.15.* to and release it as v0.15.9.

[GrapesGoober]

Ok thanks. This is resolved for me. Installing the alpha works without explicitly setting the matplotlib backend. I'll look forward for new releases then.