streamline the RequirementSetWithCandidates invocation

Author: cosmicexplorer

src/pip/_internal/commands/download.py

@@ -13,6 +13,7 @@
 from pip._internal.cli.status_codes import SUCCESS
 from pip._internal.models.link import Link, LinkHash
 from pip._internal.req.req_tracker import get_requirement_tracker
+from pip._internal.resolution.base import RequirementSetWithCandidates
 from pip._internal.utils.misc import ensure_dir, normalize_path, write_output
 from pip._internal.utils.temp_dir import TempDirectory
 
@@ -236,24 +237,32 @@ def run(self, options: Values, args: List[str]) -> int:
                 downloaded.append(req.name)
 
         download_infos = DownloadInfos()
-        assert requirement_set.candidates is not None
-        for candidate in requirement_set.candidates.mapping.values():
-            # This will occur for the python version requirement, for example.
-            if candidate.name not in requirement_set.requirements:
-                download_infos.implicit_requirements.append(
-                    candidate.as_serializable_requirement()
+        if options.print_download_urls:
+            if isinstance(requirement_set, RequirementSetWithCandidates):
+                for candidate in requirement_set.candidates.mapping.values():
+                    # This will occur for the python version requirement, for example.
+                    if candidate.name not in requirement_set.requirements:
+                        download_infos.implicit_requirements.append(
+                            candidate.as_serializable_requirement()
+                        )
+                        continue
+                    req = requirement_set.requirements[candidate.name]
+                    assert req.name is not None
+                    assert req.link is not None
+                    assert req.name not in download_infos.resolution
+                    download_infos.resolution[
+                        req.name
+                    ] = RequirementDownloadInfo.from_req_and_link(
+                        req=candidate.as_serializable_requirement(),
+                        link=req.link,
+                    )
+            else:
+                logger.warning(
+                    "--print-download-urls is being used with the legacy resolver. "
+                    "The legacy resolver does not retain detailed dependency "
+                    "information, so all the fields in the output JSON file "
+                    "will be empty."
                 )
-                continue
-            req = requirement_set.requirements[candidate.name]
-            assert req.name is not None
-            assert req.link is not None
-            assert req.name not in download_infos.resolution
-            download_infos.resolution[
-                req.name
-            ] = RequirementDownloadInfo.from_req_and_link(
-                req=candidate.as_serializable_requirement(),
-                link=req.link,
-            )
 
         if downloaded:
             write_output("Successfully downloaded %s", " ".join(downloaded))

src/pip/_internal/resolution/base.py

@@ -1,3 +1,4 @@
+import abc
 from typing import TYPE_CHECKING, Callable, List, Optional
 
 from pip._internal.req.req_install import InstallRequirement
@@ -16,18 +17,24 @@
 
 
 class RequirementSetWithCandidates(RequirementSet):
-    def __init__(self, check_supported_wheels: bool = True) -> None:
+    def __init__(
+        self,
+        candidates: "Result",
+        check_supported_wheels: bool = True,
+    ) -> None:
+        self.candidates = candidates
         super().__init__(check_supported_wheels=check_supported_wheels)
-        self.candidates: Optional[Result] = None
 
 
-class BaseResolver:
+class BaseResolver(metaclass=abc.ABCMeta):
+    @abc.abstractmethod
     def resolve(
         self, root_reqs: List[InstallRequirement], check_supported_wheels: bool
-    ) -> RequirementSetWithCandidates:
-        raise NotImplementedError()
+    ) -> RequirementSet:
+        ...
 
+    @abc.abstractmethod
     def get_installation_order(
         self, req_set: RequirementSet
     ) -> List[InstallRequirement]:
-        raise NotImplementedError()
+        ...

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -107,9 +107,8 @@ def resolve(
             raise error from e
 
         req_set = RequirementSetWithCandidates(
-            check_supported_wheels=check_supported_wheels
+            candidates=result, check_supported_wheels=check_supported_wheels
         )
-        req_set.candidates = result
         for candidate in result.mapping.values():
             ireq = candidate.get_install_requirement()
             if ireq is None: