Add --dry-run option to pip install

news/11096.feature.rst

@@ -0,0 +1,2 @@
+Add ``--dry-run`` option to ``pip install``, to let it print what it would install but
+not actually change anything in the target environment.

src/pip/_internal/commands/install.py

@@ -85,6 +85,17 @@ def add_options(self) -> None:
         self.cmd_opts.add_option(cmdoptions.pre())
 
         self.cmd_opts.add_option(cmdoptions.editable())
+        self.cmd_opts.add_option(
+            "--dry-run",
+            action="store_true",
+            dest="dry_run",
+            default=False,
+            help=(
+                "Don't actually install anything, just print what would be. "
+                "Can be used in combination with --ignore-installed "
+                "to 'resolve' the requirements."
+            ),
+        )
         self.cmd_opts.add_option(
             "-t",
             "--target",
@@ -342,6 +353,18 @@ def run(self, options: Values, args: List[str]) -> int:
                 reqs, check_supported_wheels=not options.target_dir
             )
 
+            if options.dry_run:
+                would_install_items = sorted(
+                    (r.metadata["name"], r.metadata["version"])
+                    for r in requirement_set.requirements_to_install
+                )
+                if would_install_items:
+                    write_output(
+                        "Would install %s",
+                        " ".join("-".join(item) for item in would_install_items),
+                    )
+                return SUCCESS
+
             try:
                 pip_req = requirement_set.get_requirement("pip")
             except KeyError:

src/pip/_internal/req/req_install.py

@@ -25,7 +25,9 @@
     BaseDistribution,
     get_default_environment,
     get_directory_distribution,
+    get_wheel_distribution,
 )
+from pip._internal.metadata.base import FilesystemWheel
 from pip._internal.models.direct_url import DirectUrl
 from pip._internal.models.link import Link
 from pip._internal.operations.build.metadata import generate_metadata
@@ -558,7 +560,16 @@ def metadata(self) -> Any:
         return self._metadata
 
     def get_dist(self) -> BaseDistribution:
-        return get_directory_distribution(self.metadata_directory)
+        if self.metadata_directory:
+            return get_directory_distribution(self.metadata_directory)
+        elif self.local_file_path and self.is_wheel:
+            return get_wheel_distribution(
+                FilesystemWheel(self.local_file_path), canonicalize_name(self.name)
+            )
+        raise AssertionError(
+            f"InstallRequirement {self} has no metadata directory and no wheel: "
+            f"can't make a distribution."
+        )
 
     def assert_source_matches_version(self) -> None:
         assert self.source_dir

src/pip/_internal/req/req_set.py

@@ -67,3 +67,16 @@ def get_requirement(self, name: str) -> InstallRequirement:
     @property
     def all_requirements(self) -> List[InstallRequirement]:
         return self.unnamed_requirements + list(self.requirements.values())
+
+    @property
+    def requirements_to_install(self) -> List[InstallRequirement]:
+        """Return the list of requirements that need to be installed.
+
+        TODO remove this property together with the legacy resolver, since the new
+             resolver only returns requirements that need to be installed.
+        """
+        return [
+            install_req
+            for install_req in self.all_requirements
+            if not install_req.constraint and not install_req.satisfied_by
+        ]

tests/functional/test_install.py

@@ -2238,3 +2238,12 @@ def test_install_logs_pip_version_in_debug(
     result = script.pip("install", "-v", fake_package)
     pattern = "Using pip .* from .*"
     assert_re_match(pattern, result.stdout)
+
+
+def test_install_dry_run(script: PipTestEnvironment, data: TestData) -> None:
+    """Test that pip install --dry-run logs what it would install."""
+    result = script.pip(
+        "install", "--dry-run", "--find-links", data.find_links, "simple"
+    )
+    assert "Would install simple-3.0" in result.stdout
+    assert "Successfully installed" not in result.stdout