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Raise NotImplementedError in MovieWriter._args instead of returning it #32067

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Raise NotImplementedError in MovieWriter._args instead of returning it #32067

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2 changes: 1 addition & 1 deletion lib/matplotlib/animation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -356,7 +356,7 @@ def grab_frame(self, **savefig_kwargs):

def _args(self):
"""Assemble list of encoder-specific command-line arguments."""
return NotImplementedError("args needs to be implemented by subclass.")
raise NotImplementedError("args needs to be implemented by subclass.")

@classmethod
def bin_path(cls):
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11 changes: 11 additions & 0 deletions lib/matplotlib/tests/test_animation.py
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Expand Up @@ -135,6 +135,17 @@ def _run(self):
assert writer.dpi == fig.dpi


def test_movie_writer_args_not_implemented():
# MovieWriter._args is a placeholder that subclasses must override; it
# must *raise* NotImplementedError rather than merely returning it, so a
# subclass that forgets to override it fails loudly instead of passing an
# exception instance on as if it were the command-line arguments.
writer = animation.MovieWriter.__new__(animation.MovieWriter)

@QuLogic QuLogic Jul 23, 2026

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I think it might be clearer here if you did as the comment says and subclass MovieWriter without implementing _args:

Suggested change
writer = animation.MovieWriter.__new__(animation.MovieWriter)
class IncorrectMovieWriter(animation.MovieWriter):
pass
writer = IncorrectMovieWriter()

with pytest.raises(NotImplementedError,
match="args needs to be implemented by subclass"):
writer._args()


@animation.writers.register('null')
class RegisteredNullMovieWriter(NullMovieWriter):

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