Suppress exception chaining on rc validator failure.

anntzer · anntzer · commit afb7dce5e241 · 2019-12-08T16:24:17.000+01:00
This changes the traceback from

  Traceback (most recent call last):
    File ".../matplotlib/__init__.py", line 768, in __setitem__
      cval = self.validate[key](val)
    File ".../matplotlib/rcsetup.py", line 67, in __call__
      raise ValueError('Unrecognized %s string %r: valid strings are %s'
  ValueError: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']

  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/__init__.py", line 770, in __setitem__
      raise ValueError("Key %s: %s" % (key, str(ve)))
  ValueError: Key lines.solid_capstyle: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']

to

  Traceback (most recent call last):
    File "&lt;string&gt;", line 4, in &lt;module&gt;
    File ".../matplotlib/__init__.py", line 770, in __setitem__
      raise ValueError(f"Key {key}: {ve}") from None
  ValueError: Key lines.solid_capstyle: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -767,7 +767,7 @@ def __setitem__(self, key, val):
             try:
                 cval = self.validate[key](val)
             except ValueError as ve:
-                raise ValueError("Key %s: %s" % (key, str(ve)))
+                raise ValueError(f"Key {key}: {ve}") from None
             dict.__setitem__(self, key, cval)
         except KeyError:
             raise KeyError(
