Use explicit exception chaining when converting exception types

This changes Python's exception chain reporting to make it clear that the
new exception was not an unrelated failure while handling the original
one, but rather the direct cause of it.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

openslide/__init__.py

@@ -293,8 +293,8 @@ def set_cache(self, cache: OpenSlideCache) -> None:
         cache: an OpenSlideCache object."""
         try:
             llcache = cache._openslide_cache
-        except AttributeError:
-            raise TypeError('Not a cache object')
+        except AttributeError as exc:
+            raise TypeError('Not a cache object') from exc
         lowlevel.set_cache(self._osr, llcache)
 
 

openslide/lowlevel.py

@@ -84,18 +84,18 @@ def try_load(names: list[str]) -> CDLL:
     if platform.system() == 'Windows':
         try:
             return try_load(['libopenslide-1.dll', 'libopenslide-0.dll'])
-        except FileNotFoundError:
+        except FileNotFoundError as exc:
             raise ModuleNotFoundError(
                 "Couldn't locate OpenSlide DLL. "
                 "Try `pip install openslide-bin`, "
                 "or if you're using an OpenSlide binary package, "
                 "ensure you've called os.add_dll_directory(). "
                 "https://openslide.org/api/python/#installing"
-            )
+            ) from exc
     elif platform.system() == 'Darwin':
         try:
             return try_load(['libopenslide.1.dylib', 'libopenslide.0.dylib'])
-        except OSError:
+        except OSError as exc:
             # MacPorts doesn't add itself to the dyld search path, but
             # does add itself to the find_library() search path
             # (DEFAULT_LIBRARY_FALLBACK in ctypes.macholib.dyld).
@@ -107,17 +107,17 @@ def try_load(names: list[str]) -> CDLL:
                     "Couldn't locate OpenSlide dylib. "
                     "Try `pip install openslide-bin`. "
                     "https://openslide.org/api/python/#installing"
-                )
+                ) from exc
             return cdll.LoadLibrary(lib)
     else:
         try:
             return try_load(['libopenslide.so.1', 'libopenslide.so.0'])
-        except OSError:
+        except OSError as exc:
             raise ModuleNotFoundError(
                 "Couldn't locate OpenSlide shared library. "
                 "Try `pip install openslide-bin`. "
                 "https://openslide.org/api/python/#installing"
-            )
+            ) from exc
 
 
 _lib = _load_library()