code: def test_unknown_format(self):
      with pytest.raises(Exception):
          p.save(format="unknown", verbose=False)
  
imports: import warnings
  from pathlib import Path
  import matplotlib.pyplot
  import pandas
  import pytest
  from plotnine import aes, facet_wrap, geom_point, geom_text, ggplot, ggsave, theme_xkcd
  from plotnine.data import mtcars
  from plotnine.exceptions import PlotnineError, PlotnineWarning
label_desc: Use image node_id 1 for calling the TestExceptions obj's underlying member method code with example usage: obj.test_unknown_format() without return types