matplotlib · tacaswell · Mar 9, 2020 · Mar 7, 2020
diff --git a/doc/sphinxext/github.py b/doc/sphinxext/github.py
@@ -39,7 +39,8 @@ def make_link_node(rawtext, app, type, slug, options):
             base += '/'
     except AttributeError as err:
         raise ValueError(
-            f'github_project_url configuration value is not set ({err})')
+            f'github_project_url configuration value is not set '
+            f'({err})') from err
 
     ref = base + type + '/' + slug + '/'
     set_classes(options)
@@ -137,7 +138,8 @@ def ghcommit_role(
             base += '/'
     except AttributeError as err:
         raise ValueError(
-            f'github_project_url configuration value is not set ({err})')
+            f'github_project_url configuration value is not set '
+            f'({err})') from err
 
     ref = base + text
     node = nodes.reference(rawtext, text[:6], refuri=ref, **options)

diff --git a/examples/user_interfaces/embedding_webagg_sgskip.py b/examples/user_interfaces/embedding_webagg_sgskip.py
@@ -16,8 +16,8 @@
 
 try:
     import tornado
-except ImportError:
-    raise RuntimeError("This example requires tornado.")
+except ImportError as err:
+    raise RuntimeError("This example requires tornado.") from err
 import tornado.web
 import tornado.httpserver
 import tornado.ioloop

diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -656,10 +656,10 @@ def __setitem__(self, key, val):
             except ValueError as ve:
                 raise ValueError(f"Key {key}: {ve}") from None
             dict.__setitem__(self, key, cval)
-        except KeyError:
+        except KeyError as err:
             raise KeyError(
                 f"{key} is not a valid rc parameter (see rcParams.keys() for "
-                f"a list of valid parameters)")
+                f"a list of valid parameters)") from err
 
     def __getitem__(self, key):
         if key in _deprecated_map:
@@ -942,9 +942,9 @@ def rc(group, **kwargs):
             key = '%s.%s' % (g, name)
             try:
                 rcParams[key] = v
-            except KeyError:
+            except KeyError as err:
                 raise KeyError(('Unrecognized key "%s" for group "%s" and '
-                                'name "%s"') % (key, g, name))
+                                'name "%s"') % (key, g, name)) from err
 
 
 def rcdefaults():

diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -4210,11 +4210,11 @@ def _parse_scatter_color_args(c, edgecolors, kwargs, xsize,
         if kwcolor is not None:
             try:
                 mcolors.to_rgba_array(kwcolor)
-            except ValueError:
+            except ValueError as err:
                 raise ValueError(
                     "'color' kwarg must be an color or sequence of color "
                     "specs.  For a sequence of values to be color-mapped, use "
-                    "the 'c' argument instead.")
+                    "the 'c' argument instead.") from err
             if edgecolors is None:
                 edgecolors = kwcolor
             if facecolors is None:
@@ -4264,14 +4264,14 @@ def invalid_shape_exception(csize, xsize):
         if not c_is_mapped:
             try:  # Is 'c' acceptable as PathCollection facecolors?
                 colors = mcolors.to_rgba_array(c)
-            except (TypeError, ValueError):
+            except (TypeError, ValueError) as err:
                 if not valid_shape:
-                    raise invalid_shape_exception(c.size, xsize)
+                    raise invalid_shape_exception(c.size, xsize) from err
                 # Both the mapping *and* the RGBA conversion failed: pretty
                 # severe failure => one may appreciate a verbose feedback.
                 raise ValueError(
                     f"'c' argument must be a color, a sequence of colors, or "
-                    f"a sequence of numbers, not {c}")
+                    f"a sequence of numbers, not {c}") from err
             else:
                 if len(colors) not in (0, 1, xsize):
                     # NB: remember that a single color is also acceptable.

diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -1732,10 +1732,10 @@ def axis(self, *args, emit=True, **kwargs):
                 limits = args[0]
                 try:
                     xmin, xmax, ymin, ymax = limits
-                except (TypeError, ValueError):
+                except (TypeError, ValueError) as err:
                     raise TypeError('the first argument to axis() must be an '
                                     'interable of the form '
-                                    '[xmin, xmax, ymin, ymax]')
+                                    '[xmin, xmax, ymin, ymax]') from err
             else:
                 xmin = kwargs.pop('xmin', None)
                 xmax = kwargs.pop('xmax', None)
@@ -2885,8 +2885,9 @@ def ticklabel_format(self, *, axis='both', style='', scilimits=None,
             try:
                 m, n = scilimits
                 m + n + 1  # check that both are numbers
-            except (ValueError, TypeError):
-                raise ValueError("scilimits must be a sequence of 2 integers")
+            except (ValueError, TypeError) as err:
+                raise ValueError("scilimits must be a sequence of 2 integers"
+                                 ) from err
         STYLES = {'sci': True, 'scientific': True, 'plain': False, '': None}
         is_sci_style = cbook._check_getitem(STYLES, style=style)
         axis_map = {**{k: [v] for k, v in self._get_axis_map().items()},
@@ -2904,9 +2905,9 @@ def ticklabel_format(self, *, axis='both', style='', scilimits=None,
                     axis.major.formatter.set_useLocale(
9E88
useLocale)
                 if useMathText is not None:
                     axis.major.formatter.set_useMathText(useMathText)
-        except AttributeError:
+        except AttributeError as err:
             raise AttributeError(
-                "This method only works with the ScalarFormatter")
+                "This method only works with the ScalarFormatter") from err
 
     def locator_params(self, axis='both', tight=None, **kwargs):
         """

diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -143,15 +143,16 @@ def draw_path(self, gc, path, transform, rgbFace=None):
                 p = Path(v, c)
                 try:
                     self._renderer.draw_path(gc, p, transform, rgbFace)
-                except OverflowError:
-                    raise OverflowError("Exceeded cell block limit (set "
-                                        "'agg.path.chunksize' rcparam)")
+                except OverflowError as err:
+                    raise OverflowError(
+                        "Exceeded cell block limit (set 'agg.path.chunksize' "
+                        "rcparam)") from err
         else:
             try:
                 self._renderer.draw_path(gc, path, transform, rgbFace)
-            except OverflowError:
+            except OverflowError as err:
                 raise OverflowError("Exceeded cell block limit (set "
-                                    "'agg.path.chunksize' rcparam)")
+                                    "'agg.path.chunksize' rcparam)") from err
 
     def draw_mathtext(self, gc, x, y, s, prop, angle):
         """Draw mathtext using :mod:`matplotlib.mathtext`."""

diff --git a/lib/matplotlib/backends/backend_cairo.py b/lib/matplotlib/backends/backend_cairo.py
@@ -18,10 +18,10 @@
 except ImportError:
     try:
         import cairocffi as cairo
-    except ImportError:
+    except ImportError as err:
         raise ImportError(
             "cairo backend requires that pycairo>=1.11.0 or cairocffi"
-            "is installed")
+            "is installed") from err
 
 from .. import cbook
 from matplotlib.backend_bases import (

diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
@@ -16,8 +16,8 @@
 
 try:
     import gi
-except ImportError:
-    raise ImportError("The GTK3 backends require PyGObject")
+except ImportError as err:
+    raise ImportError("The GTK3 backends require PyGObject") from err
 
 try:
     # :raises ValueError: If module/version is already loaded, already
@@ -47,7 +47,7 @@
 except TypeError as exc:
     # Happens when running headless.  Convert to ImportError to cooperate with
     # backend switching.
-    raise ImportError(exc)
+    raise ImportError(exc) from exc
 
 
 class TimerGTK3(TimerBase):

diff --git a/lib/matplotlib/backends/backend_nbagg.py b/lib/matplotlib/backends/backend_nbagg.py
@@ -164,9 +164,10 @@ def __init__(self, manager):
         display(HTML("<div id=%r></div>" % self.uuid))
         try:
             self.comm = Comm('matplotlib', data={'id': self.uuid})
-        except AttributeError:
+        except AttributeError as err:
             raise RuntimeError('Unable to create an IPython notebook Comm '
-                               'instance. Are you in the IPython notebook?')
+                               'instance. Are you in the IPython '
+                               'notebook?') from err
         self.comm.on_msg(self.on_message)
 
         manager = self.manager

diff --git a/lib/matplotlib/backends/backend_pgf.py b/lib/matplotlib/backends/backend_pgf.py
@@ -273,13 +273,14 @@ def __init__(self):
                 [self.texcommand, "-halt-on-error"],
                 stdin=subprocess.PIPE, stdout=subprocess.PIPE,
                 encoding="utf-8", cwd=self.tmpdir)
-        except FileNotFoundError:
+        except FileNotFoundError as err:
             raise RuntimeError(
                 f"{self.texcommand} not found.  Install it or change "
                 f"rcParams['pgf.texsystem'] to an available TeX "
-                f"implementation.")
-        except OSError:
-            raise RuntimeError("Error starting process %r" % self.texcommand)
+                f"implementation.") from err
+        except OSError as err:
+            raise RuntimeError("Error starting process %r" %
+                               self.texcommand) from err
         test_input = self.latex_header + latex_end
         stdout, stderr = latex.communicate(test_input)
         if latex.returncode != 0:
@@ -342,7 +343,7 @@ def get_width_height_descent(self, text, prop):
             self._expect_prompt()
         except LatexError as e:
             raise ValueError("Error processing '{}'\nLaTeX Output:\n{}"
-                             .format(text, e.latex_output))
+                             .format(text, e.latex_output)) from e
 
         # typeout width, height and text offset of the last textbox
         self._stdin_writeln(r"\typeout{\the\wd0,\the\ht0,\the\dp0}")
@@ -351,14 +352,14 @@ def get_width_height_descent(self, text, prop):
             answer = self._expect_prompt()
         except LatexError as e:
             raise ValueError("Error processing '{}'\nLaTeX Output:\n{}"
-                             .format(text, e.latex_output))
+                             .format(text, e.latex_output)) from e
 
         # parse metrics from the answer string
         try:
             width, height, offset = answer.splitlines()[0].split(",")
-        except Exception:
+        except Exception as err:
             raise ValueError("Error processing '{}'\nLaTeX Output:\n{}"
-                             .format(text, answer))
+                             .format(text, answer)) from err
         w, h, o = float(width[:-2]), float(height[:-2]), float(offset[:-2])
 
         # the height returned from LaTeX goes from base to top.

diff --git a/lib/matplotlib/backends/backend_ps.py b/lib/matplotlib/backends/backend_ps.py
@@ -1023,9 +1023,9 @@ def _print_figure_tex(
         else:
             try:
                 title = os.fspath(outfile)
-            except TypeError:
+            except TypeError as err:
                 raise ValueError(
-                    "outfile must be a path or a file-like object")
+                    "outfile must be a path or a file-like object") from err
 
         self.figure.dpi = 72  # ignore the dpi kwarg
         width, height = self.figure.get_size_inches()

diff --git a/lib/matplotlib/backends/backend_webagg.py b/lib/matplotlib/backends/backend_webagg.py
@@ -25,8 +25,8 @@
 
 try:
     import tornado
-except ImportError:
-    raise RuntimeError("The WebAgg backend requires Tornado.")
+except ImportError as err:
+    raise RuntimeError("The WebAgg backend requires Tornado.") from err
 
 import tornado.web
 import tornado.ioloop

diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -56,10 +56,11 @@
 else:
     try:
         QT_API = _ETS[QT_API_ENV]
-    except KeyError:
+    except KeyError as err:
         raise RuntimeError(
             "The environment variable QT_API has the unrecognized value {!r};"
-            "valid values are 'pyqt5', 'pyside2', 'pyqt', and 'pyside'")
+            "valid values are 'pyqt5', 'pyside2', 'pyqt', and "
+            "'pyside'") from err
 
 
 def _setup_pyqt5():

diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -721,11 +721,11 @@ def report_memory(i=0):  # argument may go away
     def call(command, os_name):
         try:
             return subprocess.check_output(command)
-        except subprocess.CalledProcessError:
+        except subprocess.CalledProcessError as err:
             raise NotImplementedError(
                 "report_memory works on %s only if "
                 "the '%s' program is found" % (os_name, command[0])
-            )
+            ) from err
 
     pid = os.getpid()
     if sys.platform == 'sunos5':

diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
@@ -100,8 +100,9 @@ def register_cmap(name=None, cmap=None, data=None, lut=None):
     if name is None:
         try:
             name = cmap.name
-        except AttributeError:
-            raise ValueError("Arguments must include a name or a Colormap")
+        except AttributeError as err:
+            raise ValueError("Arguments must include a name or a "
+                             "Colormap") from err
     if isinstance(cmap, colors.Colormap):
         cmap_d[name] = cmap
         return

diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -581,9 +581,9 @@ def set_linestyle(self, ls):
                 except ValueError:
                     dashes = [mlines._get_dash_pattern(x) for x in ls]
 
-        except ValueError:
-            raise ValueError(
-                'Do not know how to convert {!r} to dashes'.format(ls))
+        except ValueError as err:
+            raise ValueError('Do not know how to convert {!r} to '
+                             'dashes'.format(ls)) from err
 
         # get the list of raw 'unscaled' dash patterns
         self._us_linestyles = dashes

diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
@@ -1038,9 +1038,9 @@ def _get_extension_lengths(self, frac, automin, automax, default=0.05):
                 # be encountered. This is an error.
                 if np.isnan(extendlength).any():
                     raise ValueError()
-            except (TypeError, ValueError):
+            except (TypeError, ValueError) as err:
                 # Raise an error on encountering an invalid value for frac.
-                raise ValueError('invalid value for extendfrac')
+                raise ValueError('invalid value for extendfrac') from err
         return extendlength
 
     def _uniform_y(self, N):

diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -315,11 +315,11 @@ def to_rgba_array(c, alpha=None):
         # This is deprecated and will be removed in the future.
         try:
             result = np.array([to_rgba(cc, alpha) for cc in c])
-        except ValueError:
+        except ValueError as err:
             raise ValueError(
                 "'%s' is neither a valid single color nor a color sequence "
                 "consisting of single character color specifiers such as "
-                "'rgb'. Note also that the latter is deprecated." % c)
+                "'rgb'. Note also that the latter is deprecated." % c) from err
         else:
             cbook.warn_deprecated("3.2", message="Using a string of single "
                                   "character colors as a color sequence is "
@@ -443,8 +443,8 @@ def _create_lookup_table(N, data, gamma=1.0):
 
     try:
         adata = np.array(data)
-    except Exception:
-        raise TypeError("data must be convertible to an array")
+    except Exception as err:
+        raise TypeError("data must be convertible to an array") from err
     shape = adata.shape
     if len(shape) != 2 or shape[1] != 3:
         raise ValueError("data must be nx3 format")
@@ -1894,9 +1894,9 @@ def shade_rgb(self, rgb, elevation, fraction=1., blend_mode='hsv',
         else:
             try:
                 blend = blend_mode(rgb, intensity, **kwargs)
-            except TypeError:
+            except TypeError as err:
                 raise ValueError('"blend_mode" must be callable or one of {}'
-                                 .format(lookup.keys))
+                                 .format(lookup.keys)) from err
 
         # Only apply result where hillshade intensity isn't masked
         if hasattr(intensity, 'mask'):

diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -806,9 +806,9 @@ def set_stretch(self, stretch):
             stretch = int(stretch)
             if stretch < 0 or stretch > 1000:
                 raise ValueError()
-        except ValueError:
+        except ValueError as err:
             if stretch not in stretch_dict:
-                raise ValueError("stretch is invalid")
+                raise ValueError("stretch is invalid") from err
         self._stretch = stretch
 
     def set_size(self, size):
@@ -824,10 +824,10 @@ def set_size(self, size):
         except ValueError:
             try:
                 scale = font_scalings[size]
-            except KeyError:
+            except KeyError as err:
                 raise ValueError(
                     "Size is invalid. Valid font size are "
-                    + ", ".join(map(str, font_scalings)))
+                    + ", ".join(map(str, font_scalings))) from err
             else:
                 size = scale * FontManager.get_default_size()
         if size < 1.0:

diff --git a/lib/matplotlib/fontconfig_pattern.py b/lib/matplotlib/fontconfig_pattern.py
@@ -127,7 +127,7 @@ def parse(self, pattern):
             self._parser.parseString(pattern)
         except self.ParseException as e:
             raise ValueError(
-                "Could not parse font string: '%s'\n%s" % (pattern, e))
+                "Could not parse font string: '%s'\n%s" % (pattern, e)) from e
 
         self._properties = None