Add a --dry-run option to pip install

Author: sbidoul

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 +353,17 @@ def run(self, options: Values, args: List[str]) -> int:
                 reqs, check_supported_wheels=not options.target_dir
             )
 
+            if options.dry_run:
+                items = [
+                    f"{item.name}-{item.metadata['version']}"
+                    for item in sorted(
+                        requirement_set.all_requirements, key=lambda x: str(x.name)
+                    )
+                ]
+                if items:
+                    write_output("Would install %s", " ".join(items))
+                return SUCCESS
+
             try:
                 pip_req = requirement_set.get_requirement("pip")
             except KeyError:

tests/functional/test_install.py

@@ -2243,3 +2243,12 @@ 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