add --why option to poetry show

docs/cli.md

@@ -439,6 +439,7 @@ required by
 ### Options
 
 * `--without`: The dependency groups to ignore.
+* `--why`: Include reverse dependencies where applicable.
 * `--with`: The optional dependency groups to include.
 * `--only`: The only dependency groups to include.
 * `--default`: Only include the main dependencies. (**Deprecated**)

src/poetry/console/commands/show.py

@@ -22,6 +22,17 @@
     from poetry.repositories.repository import Repository
 
 
+def reverse_deps(pkg: Package, repo: Repository) -> dict[str, str]:
+    required_by = {}
+    for locked in repo.packages:
+        dependencies = {d.name: d.pretty_constraint for d in locked.requires}
+
+        if pkg.name in dependencies:
+            required_by[locked.pretty_name] = dependencies[pkg.name]
+
+    return required_by
+
+
 class ShowCommand(GroupCommand):
 
     name = "show"
@@ -36,6 +47,11 @@ class ShowCommand(GroupCommand):
             "Do not list the development dependencies. (<warning>Deprecated</warning>)",
         ),
         option("tree", "t", "List the dependencies as a tree."),
+        option(
+            "why",
+            None,
+            "When listing the tree for a single package, start from parents.",
+        ),
         option("latest", "l", "Show the latest version."),
         option(
             "outdated",
@@ -69,6 +85,23 @@ def handle(self) -> int | None:
         if self.option("tree"):
             self.init_styles(self.io)
 
+        if self.option("why"):
+            if self.option("tree") and package is None:
+                self.line_error(
+                    "<error>Error: --why requires a package when combined with"
+                    " --tree.</error>"
+                )
+
+                return 1
+
+            if not self.option("tree") and package:
+                self.line_error(
+                    "<error>Error: --why cannot be used without --tree when displaying"
+                    " a single package.</error>"
+                )
+
+                return 1
+
         if self.option("outdated"):
             self._io.input.set_option("latest", True)
 
@@ -83,7 +116,7 @@ def handle(self) -> int | None:
         root = self.project_with_activated_groups_only()
 
         # Show tree view if requested
-        if self.option("tree") and not package:
+        if self.option("tree") and package is None:
             requires = root.all_requires
             packages = locked_repo.packages
             for p in packages:
@@ -121,17 +154,38 @@ def handle(self) -> int | None:
             if not pkg:
                 raise ValueError(f"Package {package} not found")
 
+            required_by = reverse_deps(pkg, locked_repo)
+
             if self.option("tree"):
-                self.display_package_tree(self.io, pkg, locked_repo)
+                if self.option("why"):
+                    # The default case if there's no reverse dependencies is to query
+                    # the subtree for pkg but if any rev-deps exist we'll query for each
+                    # of them in turn
+                    packages = [pkg]
+                    if required_by:
+                        packages = [
+                            p
+                            for p in locked_packages
+                            for r in required_by.keys()
+                            if p.name == r
+                        ]
+                    else:
+                        # if no rev-deps exist we'll make this clear as it can otherwise
+                        # look very odd for packages that also have no or few direct
+                        # dependencies
+                        self._io.write_line(
+                            f"Package {package} is a direct dependency."
+                        )
 
-                return 0
+                    for p in packages:
+                        self.display_package_tree(
+                            self._io, p, locked_repo, why_package=pkg
+                        )
 
-            required_by = {}
-            for locked in locked_packages:
-                dependencies = {d.name: d.pretty_constraint for d in locked.requires}
+                else:
+                    self.display_package_tree(self._io, pkg, locked_repo)
 
-                if pkg.name in dependencies:
-                    required_by[locked.pretty_name] = dependencies[pkg.name]
+                return 0
 
             rows = [
                 ["<info>name</>", f" : <c1>{pkg.pretty_name}</>"],
@@ -163,7 +217,7 @@ def handle(self) -> int | None:
         show_all = self.option("all")
         terminal = Terminal()
         width = terminal.width
-        name_length = version_length = latest_length = 0
+        name_length = version_length = latest_length = required_by_length = 0
         latest_packages = {}
         latest_statuses = {}
         installed_repo = InstalledRepository.load(self.env)
@@ -208,6 +262,13 @@ def handle(self) -> int | None:
                             )
                         ),
                     )
+
+                    if self.option("why"):
+                        required_by = reverse_deps(locked, locked_repo)
+                        required_by_length = max(
+                            required_by_length,
+                            len(" from " + ",".join(required_by.keys())),
+                        )
             else:
                 name_length = max(name_length, current_length)
                 version_length = max(
@@ -219,9 +280,20 @@ def handle(self) -> int | None:
                     ),
                 )
 
+                if self.option("why"):
+                    required_by = reverse_deps(locked, locked_repo)
+                    required_by_length = max(
+                        required_by_length, len(" from " + ",".join(required_by.keys()))
+                    )
+
         write_version = name_length + version_length + 3 <= width
         write_latest = name_length + version_length + latest_length + 3 <= width
-        write_description = name_length + version_length + latest_length + 24 <= width
+
+        why_end_column = (
+            name_length + version_length + latest_length + required_by_length
+        )
+        write_why = self.option("why") and (why_end_column + 3) <= width
+        write_description = (why_end_column + 24) <= width
 
         for locked in locked_packages:
             color = "cyan"
@@ -273,9 +345,21 @@ def handle(self) -> int | None:
                     )
                     line += f" <fg={color}>{version:{latest_length}}</>"
 
+            if write_why:
+                required_by = reverse_deps(locked, locked_repo)
+                if required_by:
+                    content = ",".join(required_by.keys())
+                    # subtract 6 for ' from '
+                    line += f" from {content:{required_by_length - 6}}"
+                else:
+                    line += " " * required_by_length
+
             if write_description:
                 description = locked.description
-                remaining = width - name_length - version_length - 4
+                remaining = (
+                    width - name_length - version_length - required_by_length - 4
+                )
+
                 if show_latest:
                     remaining -= latest_length
 
@@ -285,10 +369,15 @@ def handle(self) -> int | None:
                 line += " " + description
 
             self.line(line)
+
         return None
 
     def display_package_tree(
-        self, io: IO, package: Package, installed_repo: Repository
+        self,
+        io: IO,
+        package: Package,
+        installed_repo: Repository,
+        why_package: Package | None = None,
     ) -> None:
         io.write(f"<c1>{package.pretty_name}</c1>")
         description = ""
@@ -297,11 +386,15 @@ def display_package_tree(
 
         io.write_line(f" <b>{package.pretty_version}</b>{description}")
 
-        dependencies = package.requires
-        dependencies = sorted(
-            dependencies,
-            key=lambda x: x.name,  # type: ignore[no-any-return]
-        )
+        if why_package is not None:
+            dependencies = [p for p in package.requires if p.name == why_package.name]
+        else:
+            dependencies = package.requires
+            dependencies = sorted(
+                dependencies,
+                key=lambda x: x.name,  # type: ignore[no-any-return]
+            )
+
         tree_bar = "├"
         total = len(dependencies)
         for i, dependency in enumerate(dependencies, 1):

tests/console/commands/test_show.py

@@ -1686,6 +1686,121 @@ def test_show_tree_no_dev(tester: CommandTester, poetry: Poetry, installed: Repo
     assert tester.io.fetch_output() == expected
 
 
+def test_show_tree_why_package(
+    tester: CommandTester, poetry: Poetry, installed: Repository
+):
+    poetry.package.add_dependency(Factory.create_dependency("a", "=0.0.1"))
+
+    a = get_package("a", "0.0.1")
+    installed.add_package(a)
+    a.add_dependency(Factory.create_dependency("b", "=0.0.1"))
+
+    b = get_package("b", "0.0.1")
+    a.add_dependency(Factory.create_dependency("c", "=0.0.1"))
+    installed.add_package(b)
+
+    c = get_package("c", "0.0.1")
+    installed.add_package(c)
+
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "a",
+                    "version": "0.0.1",
+                    "dependencies": {"b": "=0.0.1"},
+                    "python-versions": "*",
+                    "optional": False,
+                },
+                {
+                    "name": "b",
+                    "version": "0.0.1",
+                    "dependencies": {"c": "=0.0.1"},
+                    "python-versions": "*",
+                    "optional": False,
+                },
+                {
+                    "name": "c",
+                    "version": "0.0.1",
+                    "python-versions": "*",
+                    "optional": False,
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "platform": "*",
+                "content-hash": "123456789",
+                "hashes": {"a": [], "b": [], "c": []},
+            },
+        }
+    )
+
+    tester.execute("--tree --why b")
+
+    expected = """\
+a 0.0.1
+└── b =0.0.1
+    └── c =0.0.1 \n"""
+
+    assert tester.io.fetch_output() == expected
+
+
+def test_show_tree_why(tester: CommandTester, poetry: Poetry, installed: Repository):
+    poetry.package.add_dependency(Factory.create_dependency("a", "=0.0.1"))
+
+    a = get_package("a", "0.0.1")
+    installed.add_package(a)
+    a.add_dependency(Factory.create_dependency("b", "=0.0.1"))
+
+    b = get_package("b", "0.0.1")
+    a.add_dependency(Factory.create_dependency("c", "=0.0.1"))
+    installed.add_package(b)
+
+    c = get_package("c", "0.0.1")
+    installed.add_package(c)
+
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "a",
+                    "version": "0.0.1",
+                    "dependencies": {"b": "=0.0.1"},
+                    "python-versions": "*",
+                    "optional": False,
+                },
+                {
+                    "name": "b",
+                    "version": "0.0.1",
+                    "dependencies": {"c": "=0.0.1"},
+                    "python-versions": "*",
+                    "optional": False,
+                },
+                {
+                    "name": "c",
+                    "version": "0.0.1",
+                    "python-versions": "*",
+                    "optional": False,
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "platform": "*",
+                "content-hash": "123456789",
+                "hashes": {"a": [], "b": [], "c": []},
+            },
+        }
+    )
+
+    tester.execute("--why")
+
+    # this has to be on a single line due to the padding whitespace, which gets stripped
+    # by pre-commit.
+    expected = """a 0.0.1        \nb 0.0.1 from a \nc 0.0.1 from b \n"""
+
+    assert tester.io.fetch_output() == expected
+
+
 def test_show_required_by_deps(
     tester: CommandTester, poetry: Poetry, installed: Repository
 ):