Suppress chaining of cache lookup failure in color conversion.

anntzer · anntzer · commit 1aa5a83c16c1 · 2019-02-19T11:59:48.000+01:00
Currently, something like
`scatter(range(4), range(4), c=np.arange(4).reshape((2, 2)))`
results in the following traceback:

```
Traceback (most recent call last):
  File "matplotlib/lib/matplotlib/colors.py", line 173, in to_rgba
    rgba = _colors_full_map.cache[c, alpha]
TypeError: unhashable type: 'numpy.ndarray'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "matplotlib/lib/matplotlib/axes/_axes.py", line 4289, in _parse_scatter_color_args
    colors = mcolors.to_rgba_array(c)
  File "matplotlib/lib/matplotlib/colors.py", line 284, in to_rgba_array
    result[i] = to_rgba(cc, alpha)
  File "matplotlib/lib/matplotlib/colors.py", line 175, in to_rgba
    rgba = _to_rgba_no_colorcycle(c, alpha)
  File "matplotlib/lib/matplotlib/colors.py", line 240, in _to_rgba_no_colorcycle
    raise ValueError("RGBA sequence should have length 3 or 4")
ValueError: RGBA sequence should have length 3 or 4

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "&lt;string&gt;", line 4, in &lt;module&gt;
  File "matplotlib/lib/matplotlib/pyplot.py", line 2826, in scatter
    None else {}), **kwargs)
  File "matplotlib/lib/matplotlib/__init__.py", line 1512, in inner
    return func(ax, *map(sanitize_sequence, args), **kwargs)
  File "matplotlib/lib/matplotlib/axes/_axes.py", line 4458, in scatter
    get_next_color_func=self._get_patches_for_fill.get_next_color)
  File "matplotlib/lib/matplotlib/axes/_axes.py", line 4302, in _parse_scatter_color_args
    .format(nc=n_elem, xs=xsize, ys=ysize)
ValueError: 'c' argument has 2 elements, which is not acceptable for use with 'x' with size 4, 'y' with size 4.
```

At least the topmost failure, regarding cache lookup, is irrelevant to
the end user, so suppress it, which this commit does.

The middle traceback is mildly interesing, in case the user intended to
pass in RGBA but mis-shaped it; we may want to fold the message into the
scatter() error as well -- but that's not the object of this commit.
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -172,6 +172,8 @@ def to_rgba(c, alpha=None):
     try:
         rgba = _colors_full_map.cache[c, alpha]
     except (KeyError, TypeError):  # Not in cache, or unhashable.
+        rgba = None
+    if rgba is None:  # Suppress exception chaining of cache lookup failure.
         rgba = _to_rgba_no_colorcycle(c, alpha)
         try:
             _colors_full_map.cache[c, alpha] = rgba
