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I think the bug is in the path simplification dealing with nan points (sorry for the non-contiguous numbering, only copied the interesting lines 😉 ) :

In [4]: p = Path([(0, 0), (1, 0), (1, 1), (np.nan, np.nan)],
   ...:          [Path.MOVETO, Path.LINETO, Path.LINETO, Path.CLOSEPOLY])
In [9]: p.cleaned(simplify=True)
Out[9]:
Path(array([[ 0.,  0.],
       [ 1.,  0.],
       [ 1.,  1.],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 2, 2, 0], dtype=uint8))
In [13]: p.cleaned(simplify=True).cleaned(simplify=True)
Out[13]:
Path(array([[ 0.,  0.],
       [ 1.,  0.],
       [ 1.,  1.],
       [nan, nan],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 2, 2, 2, 0], dtype=uint8))

In [14]: p.cleaned(simplify=True).cleaned(simplify=True)
Out[14]:
Path(array([[ 0.,  0.],
       [ 1.,  0.],
       [ 1.,  1.],
       [nan, nan],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 2, 2, 2, 0], dtype=uint8))

In [15]: p.cleaned(simplify=True).cleaned(simplify=True).cleaned(simplify=True)
Out[15]:
Path(array([[ 0.,  0.],
       [ 1.,  0.],
       [ 1.,  1.],
       [nan, nan],
       [nan, nan],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 2, 2, 2, 2, 0], dtype=uint8))

at it seems to insert an extra LINETO nan everytime it is run!

It is not surprising to me that the cleaning steps are not abealian so I am not sure I am too worried that

In [17]: p.cleaned(simplify=True).cleaned(remove_nans=True)
Out[17]:
Path(array([[0., 0.],
       [1., 0.],
       [1., 1.],
       [0., 0.]]), array([1, 2, 2, 0], dtype=uint8))

In [18]: p.cleaned(remove_nans=True).cleaned(simplify=True)
Out[18]:
Path(array([[ 0.,  0.],
       [ 1.,  0.],
       [ 1.,  1.],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 2, 2, 0], dtype=uint8))

are different and passing both keywords does the second:

The implemenation of the simplification is at

I suspect that the fix is to add a few lines of special-casing nan because nan is equal to nothing and all math involving nans results in nan so the "is the vector going the same direction enough" logic fails.

I'm marking this as a good first issue (there is no API design and a pretty clear tast case), but has medium difficulty.

To take this issue on you should know both Python and C++ pretty well (including templates, classes, and understanding the vertex iterator protocol). Review on this is likely to be slow.

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