style: improve stack traces

pyproject.toml

@@ -103,6 +103,7 @@ lint.select = [
   "C4",     # flake8-comprehensions
   "D",      # pydocstyle
   "E",      # Error detected by Pycodestyle
+  "EM",     # Error messages
   "F",      # Errors detected by Pyflakes
   "I",      # isort
   "RUF100", # Report unused noqa directives

src/scverse_misc/_extensions.py

@@ -91,14 +91,14 @@ def _check_namespace_signature(ns_class: type, cls: type, canonical_instance_nam
 
     # Ensure there are at least two parameters (self and mdata)
     if len(params) < 2:
-        raise TypeError(f"Namespace initializer must accept a {cls.__name__} instance as the second parameter.")
+        msg = f"Namespace initializer must accept a {cls.__name__} instance as the second parameter."
+        raise TypeError(msg)
 
     # Get the second parameter (expected to be `canonical_instance_name`)
     [_, param, *_] = params.values()
     if param.annotation is inspect.Parameter.empty:
-        raise AttributeError(
-            f"Namespace initializer's second parameter must be annotated as the {cls.__name__!r} class, got empty annotation."
-        )
+        msg = f"Namespace initializer's second parameter must be annotated as the {cls.__name__!r} class, got empty annotation."
+        raise AttributeError(msg)
 
     name_ok = param.name == canonical_instance_name
 
@@ -107,30 +107,28 @@ def _check_namespace_signature(ns_class: type, cls: type, canonical_instance_nam
         type_hints = get_type_hints(ns_class.__init__)  # type: ignore[misc]  # https://github.com/python/mypy/issues/21236
         resolved_type = type_hints.get(param.name, param.annotation)
     except NameError as e:
-        raise NameError(
-            f"Namespace initializer's second parameter must be named {canonical_instance_name!r}, got '{param.name}'."
-        ) from e
+        msg = f"Namespace initializer's second parameter must be named {canonical_instance_name!r}, got {param.name!r}."
+        raise NameError(msg) from e
 
     type_ok = resolved_type is cls
 
     match (name_ok, type_ok):
         case (True, True):
             return  # Signature is correct.
         case (False, True):
-            raise TypeError(
-                f"Namespace initializer's second parameter must be named {canonical_instance_name!r}, got {param.name!r}."
-            )
+            msg = f"Namespace initializer's second parameter must be named {canonical_instance_name!r}, got {param.name!r}."
+            raise TypeError(msg)
         case (True, False):
             type_repr = getattr(resolved_type, "__name__", str(resolved_type))
-            raise TypeError(
-                f"Namespace initializer's second parameter must be annotated as the {cls.__name__!r} class, got {type_repr!r}."
-            )
+            msg = f"Namespace initializer's second parameter must be annotated as the {cls.__name__!r} class, got {type_repr!r}."
+            raise TypeError(msg)
         case _:
             type_repr = getattr(resolved_type, "__name__", str(resolved_type))
-            raise TypeError(
+            msg = (
                 f"Namespace initializer's second parameter must be named {canonical_instance_name!r}, got {param.name!r}. "
                 f"And must be annotated as {cls.__name__!r}, got {type_repr!r}."
             )
+            raise TypeError(msg)
 
 
 def _create_namespace[NameSpT: ExtensionNamespace](
@@ -141,7 +139,8 @@ def _create_namespace[NameSpT: ExtensionNamespace](
     def namespace(ns_class: type[NameSpT]) -> type[NameSpT]:
         _check_namespace_signature(ns_class, cls, canonical_instance_name)  # Perform the runtime signature check
         if name in reserved_namespaces:
-            raise AttributeError(f"cannot override reserved attribute {name!r}")
+            msg = f"cannot override reserved attribute {name!r}"
+            raise AttributeError(msg)
         elif hasattr(cls, name):
             warnings.warn(
                 f"Overriding existing custom namespace {name!r} (on {cls.__name__!r})", UserWarning, stacklevel=2

src/scverse_misc/datasets/_fetcher.py

@@ -121,10 +121,12 @@ def download(file: FileEntry, /, dest: Path | None = None, processor: Processor
                     return download(replace(file, url=fallback, fallback_urls=None), dest=dest, processor=processor)
                 except (OSError, ValueError) as e:
                     exceptions.append(e)
-            raise ExceptionGroup(f"Could not download {file.name}", exceptions) from None
+            msg = f"Could not download {file.name}"
+            raise ExceptionGroup(msg, exceptions) from None
 
     if entry.type not in _LOADERS:
-        raise KeyError(f"No loader registered for type {entry.type!r}. Available: {available_loaders()}")
+        msg = f"No loader registered for type {entry.type!r}. Available: {available_loaders()}"
+        raise KeyError(msg)
     return cast("Loader[T]", _LOADERS[entry.type])(entry, target, download, **kwargs)
 
 
@@ -151,5 +153,6 @@ def _load_spatialdata(entry: DatasetEntry, target: Path, download: DownloadCB, /
     download(entry.file(suffix=".zip"), dest=dest, processor=pooch.Unzip(extract_dir="."))
     zarrs = sorted(dest.glob("*.zarr"))
     if len(zarrs) != 1:
-        raise RuntimeError(f"Expected exactly one .zarr extracted under {dest}, found {len(zarrs)}: {zarrs}.")
+        msg = f"Expected exactly one .zarr extracted under {dest}, found {len(zarrs)}: {zarrs}."
+        raise RuntimeError(msg)
     return sd.read_zarr(zarrs[0], **cast("dict[str, Any]", kwargs))

src/scverse_misc/datasets/_registry.py

@@ -40,7 +40,8 @@ def resolve_url(self, base_url: str | None = None) -> str:
             return self.url
         if base_url and self.s3_key:
             return f"{base_url.rstrip('/')}/{self.s3_key}"
-        raise ValueError(f"FileEntry {self.name!r} has neither `url` nor `s3_key` (with a registry `base_url`).")
+        msg = f"FileEntry {self.name!r} has neither `url` nor `s3_key` (with a registry `base_url`)."
+        raise ValueError(msg)
 
 
 @dataclass(frozen=True, slots=True)
@@ -68,9 +69,11 @@ def file(self, *, name: str | None = None, suffix: str | None = None) -> FileEnt
             matches = [f for f in self.files if f.name.endswith(suffix)]
             crit = f"suffix={suffix!r}"
         else:
-            raise ValueError("Pass exactly one of `name` or `suffix`.")
+            msg = "Pass exactly one of `name` or `suffix`."
+            raise ValueError(msg)
         if len(matches) != 1:
-            raise ValueError(f"Expected exactly one file with {crit} in {self.name!r}, found {len(matches)}.")
+            msg = f"Expected exactly one file with {crit} in {self.name!r}, found {len(matches)}."
+            raise ValueError(msg)
         return matches[0]