Allow --dry-run for venv updates.

pex/cli/commands/lock.py

@@ -16,7 +16,7 @@
 from pex.cli.command import BuildTimeCommand
 from pex.commands.command import JsonMixin, OutputMixin
 from pex.common import is_exe, pluralize, safe_delete, safe_open
-from pex.compatibility import commonpath
+from pex.compatibility import commonpath, shlex_quote
 from pex.dist_metadata import Distribution, MetadataType, Requirement, RequirementParseError
 from pex.enum import Enum
 from pex.interpreter import PythonInterpreter
@@ -1334,10 +1334,6 @@ def _update(self):
 
     def _sync(self):
         # type: () -> Result
-        if self.options.dry_run and any((self.options.venv, self.passthrough_args)):
-            return Error(
-                "Cannot update a venv or execute a command in it when performing a dry run."
-            )
 
         requirement_configuration = requirement_options.configure(self.options)
         resolver_configuration = cast(
@@ -1438,6 +1434,22 @@ def _sync(self):
         if not sync_target:
             return Ok()
 
+        if self.options.dry_run:
+            would_update_venv_msg = "Would sync venv at {venv}".format(
+                venv=sync_target.venv.venv_dir
+            )
+            if sync_target.command:
+                return Ok(
+                    os.linesep.join(
+                        (
+                            would_update_venv_msg + " and run the following command in it:",
+                            "  " + " ".join(shlex_quote(arg) for arg in sync_target.command),
+                        )
+                    )
+                )
+            else:
+                return Ok(would_update_venv_msg + ".")
+
         lock_file = try_(parse_lockfile(self.options, lock_file_path=lock_file_path))
         target = LocalInterpreter.create(sync_target.venv.interpreter)
         resolve_result = try_(

tests/integration/cli/commands/test_lock_sync.py

@@ -24,7 +24,7 @@
 from pex.resolve.resolved_requirement import Pin
 from pex.typing import TYPE_CHECKING
 from pex.venv.virtualenv import Virtualenv
-from testing import IntegResults, re_exact
+from testing import IntegResults, make_env, re_exact
 from testing.cli import run_pex3
 from testing.find_links import FindLinksRepo
 
@@ -659,7 +659,6 @@ def test_sync_venv_run(
     ), "We expect the initial venv to include Pip."
     assert_cowsay5(venv)
 
-    lock = os.path.join(str(tmpdir), "lock.json")
     result = run_sync(
         *(
             repo_args
@@ -695,6 +694,63 @@ def test_sync_venv_run(
     )
 
 
+def test_sync_venv_dry_run(
+    tmpdir,  # type: Any
+    repo_args,  # type: List[str]
+    path_mappings,  # type: PathMappings
+):
+    # type: (...) -> None
+
+    lock = os.path.join(str(tmpdir), "lock.json")
+    run_pex3("lock", "create", "cowsay==5.0", "-o", lock, *repo_args).assert_success()
+    locked_resolve = assert_lock(lock, path_mappings, expected_pins=[pin("cowsay", "5.0")])
+
+    venv_dir = os.path.join(str(tmpdir), "venv")
+    run_pex3("venv", "create", "-d", venv_dir, "--lock", lock, "--pip", *repo_args).assert_success()
+    venv = Virtualenv(venv_dir)
+    assert_cowsay5(venv)
+
+    run_sync(
+        *(
+            repo_args
+            + [
+                "cowsay<6.1",
+                "--lock",
+                lock,
+                "--dry-run",
+                "--",
+                "cowsay",
+                "-t",
+                "I would have mooed!",
+            ]
+        ),
+        # Simulate an activated venv with its bin dir inserted n the PATH.
+        env=make_env(
+            PATH=os.pathsep.join(
+                [venv.bin_dir] + os.environ.get("PATH", os.defpath).split(os.pathsep)
+            )
+        )
+    ).assert_success(
+        expected_output_re=re_exact(
+            dedent(
+                """\
+                Updates for lock generated by {platform}:
+                  Would update cowsay from 5 to 6
+                Updates to lock input requirements:
+                  Would update 'cowsay==5.0' to 'cowsay<6.1'
+                Would sync venv at {venv_dir} and run the following command in it:
+                  {cowsay} -t 'I would have mooed!'
+                """
+            ).format(
+                platform=locked_resolve.platform_tag,
+                venv_dir=venv_dir,
+                cowsay=venv.bin_path("cowsay"),
+            )
+        ),
+        expected_error_re=NO_OUTPUT,
+    )
+
+
 @skip_cowsay6_for_python27
 def test_sync_venv_run_retain_pip(
     tmpdir,  # type: Any