matplotlib · anntzer · Mar 6, 2020 · Mar 5, 2020
diff --git a/lib/matplotlib/markers.py b/lib/matplotlib/markers.py
@@ -287,9 +287,9 @@ def set_marker(self, marker):
             try:
                 Path(marker)
                 self._marker_function = self._set_vertices
-            except ValueError:
+            except ValueError as err:
                 raise ValueError('Unrecognized marker style {!r}'
-                                 .format(marker))
+                                 .format(marker)) from err
 
         self._marker = marker
         self._recache()

diff --git a/lib/matplotlib/mathtext.py b/lib/matplotlib/mathtext.py
@@ -73,10 +73,10 @@ def get_unicode_index(symbol, math=True):
         pass
     try:  # Is symbol a TeX symbol (i.e. \alpha)
         return tex2uni[symbol.strip("\\")]
-    except KeyError:
+    except KeyError as err:
         raise ValueError(
             "'{}' is not a valid Unicode character or TeX/Type1 symbol"
-            .format(symbol))
+            .format(symbol)) from err
 
 
 class MathtextBackend:
@@ -2582,7 +2582,7 @@ def parse(self, s, fonts_object, fontsize, dpi):
             raise ValueError("\n".join(["",
                                         err.line,
                                         " " * (err.column - 1) + "^",
-                                        str(err)]))
+                                        str(err)])) from err
         self._state_stack = None
         self._em_width_cache = {}
         self._expression.resetCache()
@@ -2697,8 +2697,9 @@ def symbol(self, s, loc, toks):
         c = toks[0]
         try:
             char = Char(c, self.get_state())
-        except ValueError:
-            raise ParseFatalException(s, loc, "Unknown symbol: %s" % c)
+        except ValueError as err:
+            raise ParseFatalException(s, loc,
+                                      "Unknown symbol: %s" % c) from err
 
         if c in self._spaced_symbols:
             # iterate until we find previous character, needed for cases

diff --git a/lib/matplotlib/offsetbox.py b/lib/matplotlib/offsetbox.py
@@ -1167,8 +1167,9 @@ def set_bbox_to_anchor(self, bbox, transform=None):
         else:
             try:
                 l = len(bbox)
-            except TypeError:
-                raise ValueError("Invalid argument for bbox : %s" % str(bbox))
+            except TypeError as err:
+                raise ValueError("Invalid argument for bbox : %s" %
+                                 str(bbox)) from err
 
             if l == 2:
                 bbox = [bbox[0], bbox[1], 0, 0]

diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
@@ -1844,14 +1844,15 @@ def __new__(cls, stylename, **kw):
         _name = _list[0].lower()
         try:
             _cls = cls._style_list[_name]
-        except KeyError:
-            raise ValueError("Unknown style : %s" % stylename)
+        except KeyError as err:
+            raise ValueError("Unknown style : %s" % stylename) from err
 
         try:
             _args_pair = [cs.split("=") for cs in _list[1:]]
             _args = {k: float(v) for k, v in _args_pair}
-        except ValueError:
-            raise ValueError("Incorrect style argument : %s" % stylename)
+        except ValueError as err:
+            raise ValueError("Incorrect style argument : %s" %
+                             stylename) from err
         _args.update(kw)
 
         return _cls(**_args)

diff --git a/lib/matplotlib/projections/__init__.py b/lib/matplotlib/projections/__init__.py
@@ -52,8 +52,8 @@ def get_projection_class(projection=None):
 
     try:
         return projection_registry.get_projection_class(projection)
-    except KeyError:
-        raise ValueError("Unknown projection %r" % projection)
+    except KeyError as err:
+        raise ValueError("Unknown projection %r" % projection) from err
 
 
 get_projection_names = projection_registry.get_projection_names

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -155,9 +155,9 @@ def uninstall_repl_displayhook():
         ip = get_ipython()
         try:
             ip.events.unregister('post_execute', _IP_REGISTERED)
-        except AttributeError:
+        except AttributeError as err:
             raise NotImplementedError("Can not unregister events "
-                                      "in IPython < 2.0")
+                                      "in IPython < 2.0") from err
         _IP_REGISTERED = None
 
     if _INSTALL_FIG_OBSERVER:

diff --git a/lib/matplotlib/sphinxext/plot_directive.py b/lib/matplotlib/sphinxext/plot_directive.py
@@ -444,11 +444,11 @@ def run_code(code, code_path, ns=None, function_name=None):
         except OSError as err:
             raise OSError(str(err) + '\n`plot_working_directory` option in'
                           'Sphinx configuration file must be a valid '
-                          'directory path')
+                          'directory path') from err
         except TypeError as err:
             raise TypeError(str(err) + '\n`plot_working_directory` option in '
                             'Sphinx configuration file must be a string or '
-                            'None')
+                            'None') from err
     elif code_path is not None:
         dirname = os.path.abspath(os.path.dirname(code_path))
         os.chdir(dirname)
@@ -475,8 +475,8 @@ def run_code(code, code_path, ns=None, function_name=None):
                 if function_name is not None:
                     exec(function_name + "()", ns)
 
-        except (Exception, SystemExit):
-            raise PlotError(traceback.format_exc())
+        except (Exception, SystemExit) as err:
+            raise PlotError(traceback.format_exc()) from err
         finally:
             os.chdir(pwd)
     return ns
@@ -600,8 +600,8 @@ def render_figures(code, code_path, output_dir, output_base, context,
             for fmt, dpi in formats:
                 try:
                     figman.canvas.figure.savefig(img.filename(fmt), dpi=dpi)
-                except Exception:
-                    raise PlotError(traceback.format_exc())
+                except Exception as err:
+                    raise PlotError(traceback.format_exc()) from err
                 img.formats.append(fmt)
 
         results.append((code_piece, images))

diff --git a/lib/matplotlib/streamplot.py b/lib/matplotlib/streamplot.py
@@ -372,8 +372,9 @@ class StreamMask:
     def __init__(self, density):
         try:
             self.nx, self.ny = (30 * np.broadcast_to(density, 2)).astype(int)
-        except ValueError:
-            raise ValueError("'density' must be a scalar or be of length 2")
+        except ValueError as err:
+            raise ValueError("'density' must be a scalar or be of length "
+                             "2") from err
         if self.nx < 0 or self.ny < 0:
             raise ValueError("'density' must be positive")
         self._mask = np.zeros((self.ny, self.nx))

diff --git a/lib/matplotlib/style/core.py b/lib/matplotlib/style/core.py
@@ -114,11 +114,11 @@ def use(style):
             try:
                 rc = rc_params_from_file(style, use_default_template=False)
                 _apply_style(rc)
-            except IOError:
+            except IOError as err:
                 raise IOError(
                     "{!r} not found in the style library and input is not a "
                     "valid URL or path; see `style.available` for list of "
-                    "available styles".format(style))
+                    "available styles".format(style)) from err
 
 
 @contextlib.contextmanager

diff --git a/lib/matplotlib/table.py b/lib/matplotlib/table.py
@@ -358,8 +358,9 @@ def __setitem__(self, position, cell):
         cbook._check_isinstance(CustomCell, cell=cell)
         try:
             row, col = position[0], position[1]
-        except Exception:
-            raise KeyError('Only tuples length 2 are accepted as coordinates')
+        except Exception as err:
+            raise KeyError('Only tuples length 2 are accepted as '
+                           'coordinates') from err
         cell.set_figure(self.figure)
         cell.set_transform(self.get_transform())
         cell.set_clip_on(False)

diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
@@ -135,8 +135,8 @@ def __call__(self, orig, dest):
                 stdin=subprocess.PIPE, stdout=subprocess.PIPE)
             try:
                 self._read_until(b"\nGS")
-            except _ConverterError:
-                raise OSError("Failed to start Ghostscript")
+            except _ConverterError as err:
+                raise OSError("Failed to start Ghostscript") from err
 
         def encode_and_escape(name):
             return (os.fsencode(name)
@@ -189,8 +189,9 @@ def __call__(self, orig, dest):
             self._proc.stderr = stderr
             try:
                 self._read_until(b"\n>")
-            except _ConverterError:
-                raise OSError("Failed to start Inkscape in interactive mode")
+            except _ConverterError as err:
+                raise OSError("Failed to start Inkscape in interactive "
+                              "mode") from err
 
         # Inkscape uses glib's `g_shell_parse_argv`, which has a consistent
         # behavior across platforms, so we can just use `shlex.quote`.
@@ -207,14 +208,14 @@ def __call__(self, orig, dest):
         self._proc.stdin.flush()
         try:
             self._read_until(b"\n>")
-        except _ConverterError:
+        except _ConverterError as err:
             # Inkscape's output is not localized but gtk's is, so the output
             # stream probably has a mixed encoding.  Using the filesystem
             # encoding should at least get the filenames right...
             self._stderr.seek(0)
             raise ImageComparisonFailure(
                 self._stderr.read().decode(
-                    sys.getfilesystemencoding(), "replace"))
+                    sys.getfilesystemencoding(), "replace")) from err
 
 
 def _update_converter():

diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
@@ -192,10 +192,11 @@ def copy_baseline(self, baseline, extension):
                 os.symlink(orig_expected_path, expected_fname)
             except OSError:  # On Windows, symlink *may* be unavailable.
                 shutil.copyfile(orig_expected_path, expected_fname)
-        except OSError:
+        except OSError as err:
             raise ImageComparisonFailure(
                 f"Missing baseline image {expected_fname} because the "
-                f"following file cannot be accessed: {orig_expected_path}")
+                f"following file cannot be accessed: "
+                f"{orig_expected_path}") from err
         return expected_fname
 
     def compare(self, idx, baseline, extension):

diff --git a/lib/matplotlib/text.py b/lib/matplotlib/text.py
@@ -53,15 +53,15 @@ def get_rotation(rotation):
     """
     try:
         return float(rotation) % 360
-    except (ValueError, TypeError):
+    except (ValueError, TypeError) as err:
         if cbook._str_equal(rotation, 'horizontal') or rotation is None:
             return 0.
         elif cbook._str_equal(rotation, 'vertical'):
             return 90.
         else:
             raise ValueError("rotation is {!r}; expected either 'horizontal', "
                              "'vertical', numeric value, or None"
-                             .format(rotation))
+                             .format(rotation)) from err
 
 
 def _get_textbox(text, renderer):

diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -2848,8 +2848,9 @@ def get_locator(self, d):
 
             try:
                 ld = math.log10(d)
-            except OverflowError:
-                raise RuntimeError('AutoLocator illegal data interval range')
+            except OverflowError as err:
+                raise RuntimeError('AutoLocator illegal data interval '
+                                   'range') from err
 
             fld = math.floor(ld)
             base = 10 ** fld

diff --git a/lib/matplotlib/tri/triinterpolate.py b/lib/matplotlib/tri/triinterpolate.py
@@ -191,9 +191,9 @@ def _interpolate_multikeys(self, x, y, tri_index=None,
             # Find the return index associated with the key.
             try:
                 return_index = {'z': 0, 'dzdx': 1, 'dzdy': 2}[return_key]
-            except KeyError:
+            except KeyError as err:
                 raise ValueError("return_keys items shall take values in"
-                                 " {'z', 'dzdx', 'dzdy'}")
+                                 " {'z', 'dzdx', 'dzdy'}") from err
 
             # Sets the scale factor for f & df components
             scale = [1., 1./self._unit_x, 1./self._unit_y][return_index]

diff --git a/lib/mpl_toolkits/axes_grid1/axes_rgb.py b/lib/mpl_toolkits/axes_grid1/axes_rgb.py
@@ -114,12 +114,12 @@ def __init__(self, *args, pad=0, add_all=True, **kwargs):
         """
         try:
             axes_class = kwargs.pop("axes_class", self._defaultAxesClass)
-        except AttributeError:
+        except AttributeError as err:
             raise AttributeError(
                 'A subclass of RGBAxesBase must have a _defaultAxesClass '
                 'attribute. If you are not sure which axes class to use, '
                 'consider using mpl_toolkits.axes_grid1.mpl_axes.Axes.'
-            )
+            ) from err
 
         ax = axes_class(*args, **kwargs)
 

diff --git a/tools/boilerplate.py b/tools/boilerplate.py
@@ -343,9 +343,9 @@ def build_pyplot():
     pyplot_orig = pyplot_path.read_text().splitlines(keepends=True)
     try:
         pyplot_orig = pyplot_orig[:pyplot_orig.index(PYPLOT_MAGIC_HEADER) + 1]
-    except IndexError:
+    except IndexError as err:
         raise ValueError('The pyplot.py file *must* have the exact line: %s'
-                         % PYPLOT_MAGIC_HEADER)
+                         % PYPLOT_MAGIC_HEADER) from err
 
     with pyplot_path.open('w') as pyplot:
         pyplot.writelines(pyplot_orig)

diff --git a/tools/gh_api.py b/tools/gh_api.py
@@ -24,8 +24,8 @@ class Obj(dict):
     def __getattr__(self, name):
         try:
             return self[name]
-        except KeyError:
-            raise AttributeError(name)
+        except KeyError as err:
+            raise AttributeError(name) from err
 
     def __setattr__(self, name, val):
         self[name] = val

diff --git a/tools/memleak.py b/tools/memleak.py
@@ -6,8 +6,8 @@
 
 try:
     import psutil
-except ImportError:
-    raise ImportError("This script requires psutil")
+except ImportError as err:
+    raise ImportError("This script requires psutil") from err
 
 import numpy as np