Differenctiate ValueErrors

ValueErrors are a bit too generic, and by raising/catching them we loose
a potential useful information; For example an invalid content type for
the index, is dealt as the same way than a missing wheel.

This introduces new exceptions for various cases, ant stop catching the
now UnsupportedContentTypeError, as I believe it should be a hard error as
we are likely having either an error with the index software, or a user
configuraition error.

See also beginning of discussion in #142

micropip/package_index.py

@@ -218,6 +218,10 @@ def _contain_placeholder(url: str, placeholder: str = "package_name") -> bool:
     return placeholder in fields
 
 
+class UnsupportedContentTypeError(Exception):
+    pass
+
+
 def _select_parser(content_type: str, pkgname: str) -> Callable[[str], ProjectInfo]:
     """
     Select the function to parse the response based on the content type.
@@ -234,7 +238,13 @@ def _select_parser(content_type: str, pkgname: str) -> Callable[[str], ProjectIn
         ):
             return partial(ProjectInfo.from_simple_html_api, pkgname=pkgname)
         case _:
-            raise ValueError(f"Unsupported content type: {content_type}")
+            raise UnsupportedContentTypeError(
+                f"Unsupported content type: {content_type}"
+            )
+
+
+class NoValidIndexForPackageError(Exception):
+    pass
 
 
 async def query_package(
@@ -296,14 +306,11 @@ async def query_package(
             raise
 
         content_type = headers.get("content-type", "").lower()
-        try:
-            parser = _select_parser(content_type, name)
-        except ValueError as e:
-            raise ValueError(f"Error trying to decode url: {url}") from e
+        parser = _select_parser(content_type, name)
         return parser(metadata)
     else:
-        raise ValueError(
-            f"Can't fetch metadata for '{name}'. "
+        raise NoValidIndexForPackageError(
+            f"Can't fetch metadata for {name!r}. "
             "Please make sure you have entered a correct package name "
             "and correctly specified index_urls (if you changed them)."
         )

micropip/transaction.py

@@ -14,7 +14,7 @@
 from ._utils import best_compatible_tag_index, check_compatible
 from .constants import FAQ_URLS
 from .package import PackageMetadata
-from .package_index import ProjectInfo
+from .package_index import NoValidIndexForPackageError, ProjectInfo
 from .wheelinfo import WheelInfo
 
 logger = logging.getLogger("micropip")
@@ -153,12 +153,23 @@ def eval_marker(e: dict[str, str]) -> bool:
             else:
                 try:
                     await self._add_requirement_from_package_index(req)
-                except ValueError:
-                    logger.debug(
-                        "Transaction: package %r not found in index, will search lock file",
+                except NoValidIndexForPackageError:
+                    logger.warning(
+                        "Transaction: package %r was not found in any index, "
+                        "falling back to pyodide lock file",
+                        req,
+                    )
+                    # This is most likely an error, and it's likely we should
+                    # hard fail here, but the legacy behavior was to ignore
+                    # errors.
+                    if not self._add_requirement_from_pyodide_lock(req):
+                        raise
+                except NoCompatibleWheelError:
+                    logger.warning(
+                        "Transaction: no compatible wheels for package %r was "
+                        "found falling back to pyodide lock file",
                         req,
                     )
-
                     # If the requirement is not found in package index,
                     # we still have a chance to find it from pyodide lockfile.
                     if not self._add_requirement_from_pyodide_lock(req):
@@ -167,7 +178,7 @@ def eval_marker(e: dict[str, str]) -> bool:
                         )
 
                         raise
-        except ValueError:
+        except (NoCompatibleWheelError, NoValidIndexForPackageError):
             self.failed.append(req)
             if not self.keep_going:
                 raise
@@ -252,6 +263,10 @@ async def add_wheel(
         self.wheels.append(wheel)
 
 
+class NoCompatibleWheelError(Exception):
+    pass
+
+
 def find_wheel(metadata: ProjectInfo, req: Requirement) -> WheelInfo:
     """Parse metadata to find the latest version of pure python wheel.
     Parameters
@@ -292,7 +307,7 @@ def find_wheel(metadata: ProjectInfo, req: Requirement) -> WheelInfo:
         if best_wheel is not None:
             return wheel
 
-    raise ValueError(
+    raise NoCompatibleWheelError(
         f"Can't find a pure Python 3 wheel for '{req}'.\n"
         f"See: {FAQ_URLS['cant_find_wheel']}\n"
         "You can use `await micropip.install(..., keep_going=True)` "