Clarify when we are subsetting the lockfile for 3rdparty dependencies

src/python/pants/backend/python/util_rules/pex.py

@@ -310,12 +310,7 @@ async def build_pex(
     pex_runtime_env: PexRuntimeEnvironment,
 ) -> BuildPexResult:
     """Returns a PEX with the given settings."""
-
-    argv = [
-        "--output-file",
-        request.output_filename,
-        *request.additional_args,
-    ]
+    argv = ["--output-file", request.output_filename, *request.additional_args]
 
     if request.repository_pex:
         argv.extend(["--pex-repository", request.repository_pex.name])
@@ -433,32 +428,13 @@ async def build_pex(
         ),
     )
 
-    description = request.description
-    if description is None:
-        if request.requirements.req_strings:
-            description = (
-                f"Building {request.output_filename} with "
-                f"{pluralize(len(request.requirements.req_strings), 'requirement')}: "
-                f"{', '.join(request.requirements.req_strings)}"
-            )
-        elif request.requirements.file_path:
-            description = (
-                f"Building {request.output_filename} from {request.requirements.file_path}"
-            )
-        elif request.requirements.file_content:
-            description = (
-                f"Building {request.output_filename} from {request.requirements.file_content.path}"
-            )
-        else:
-            description = f"Building {request.output_filename}"
-
     process = await Get(
         Process,
         PexCliProcess(
             python=python,
             argv=argv,
             additional_input_digest=merged_digest,
-            description=description,
+            description=_build_pex_description(request),
             output_files=[request.output_filename],
         ),
     )
@@ -486,6 +462,43 @@ async def build_pex(
     )
 
 
+def _build_pex_description(request: PexRequest) -> str:
+    if request.description:
+        return request.description
+    if not request.requirements:
+        return f"Building {request.output_filename}"
+
+    if request.repository_pex:
+        repo_pex = request.repository_pex.name
+        if request.requirements.req_strings:
+            return (
+                f"Extracting {pluralize(len(request.requirements.req_strings), 'requirement')} "
+                f"to build {request.output_filename} from {repo_pex}: "
+                f"{', '.join(request.requirements.req_strings)}"
+            )
+        reqs_file = (
+            request.requirements.file_content.path
+            if request.requirements.file_content
+            else request.requirements.file_path
+        )
+        assert reqs_file is not None
+        return (
+            f"Extracting all requirements in {reqs_file} from {repo_pex} to build "
+            f"{request.output_filename}"
+        )
+
+    if request.requirements.file_path:
+        desc_suffix = f"from {request.requirements.file_path}"
+    elif request.requirements.file_content:
+        desc_suffix = f"from {request.requirements.file_content.path}"
+    else:
+        desc_suffix = (
+            f"with {pluralize(len(request.requirements.req_strings), 'requirement')}: "
+            f"{', '.join(request.requirements.req_strings)}"
+        )
+    return f"Building {request.output_filename} {desc_suffix}"
+
+
 @rule
 async def create_pex(request: PexRequest) -> Pex:
     result = await Get(BuildPexResult, PexRequest, request)

src/python/pants/backend/python/util_rules/pex_test.py

@@ -28,10 +28,11 @@
     PexResolveInfo,
     VenvPex,
     VenvPexProcess,
+    _build_pex_description,
 )
 from pants.backend.python.util_rules.pex import rules as pex_rules
 from pants.backend.python.util_rules.pex_cli import PexPEX
-from pants.engine.fs import CreateDigest, Digest, Directory, FileContent
+from pants.engine.fs import EMPTY_DIGEST, CreateDigest, Digest, Directory, FileContent
 from pants.engine.process import Process, ProcessResult
 from pants.testutil.rule_runner import QueryRule, RuleRunner
 
@@ -468,3 +469,69 @@ def test_venv_pex_resolve_info(rule_runner: RuleRunner, pex_type: type[Pex | Ven
     assert dists[3].version == Version("2.23.0")
     assert Requirement.parse('PySocks!=1.5.7,>=1.5.6; extra == "socks"') in dists[3].requires_dists
     assert dists[4].project_name == "urllib3"
+
+
+def test_build_pex_description() -> None:
+    def assert_description(
+        requirements: PexRequirements,
+        *,
+        use_repo_pex: bool = False,
+        description: str | None = None,
+        expected: str,
+    ) -> None:
+        repo_pex = Pex(EMPTY_DIGEST, "repo.pex", None) if use_repo_pex else None
+        request = PexRequest(
+            output_filename="new.pex",
+            internal_only=True,
+            requirements=requirements,
+            description=description,
+            repository_pex=repo_pex,
+        )
+        assert _build_pex_description(request) == expected
+
+    assert_description(PexRequirements(), description="Custom!", expected="Custom!")
+    assert_description(
+        PexRequirements(), description="Custom!", use_repo_pex=True, expected="Custom!"
+    )
+
+    assert_description(PexRequirements(), expected="Building new.pex")
+    assert_description(PexRequirements(), use_repo_pex=True, expected="Building new.pex")
+
+    assert_description(
+        PexRequirements(["req"]), expected="Building new.pex with 1 requirement: req"
+    )
+    assert_description(
+        PexRequirements(["req"]),
+        use_repo_pex=True,
+        expected="Extracting 1 requirement to build new.pex from repo.pex: req",
+    )
+
+    assert_description(
+        PexRequirements(["req1", "req2"]),
+        expected="Building new.pex with 2 requirements: req1, req2",
+    )
+    assert_description(
+        PexRequirements(["req1", "req2"]),
+        use_repo_pex=True,
+        expected="Extracting 2 requirements to build new.pex from repo.pex: req1, req2",
+    )
+
+    assert_description(
+        PexRequirements(file_content=FileContent("lock.txt", b"")),
+        expected="Building new.pex from lock.txt",
+    )
+    assert_description(
+        PexRequirements(file_content=FileContent("lock.txt", b"")),
+        use_repo_pex=True,
+        expected="Extracting all requirements in lock.txt from repo.pex to build new.pex",
+    )
+
+    assert_description(
+        PexRequirements(file_path="lock.txt", file_path_description_of_origin="foo"),
+        expected="Building new.pex from lock.txt",
+    )
+    assert_description(
+        PexRequirements(file_path="lock.txt", file_path_description_of_origin="foo"),
+        use_repo_pex=True,
+        expected="Extracting all requirements in lock.txt from repo.pex to build new.pex",
+    )