poetry show <package> now also shows which packages depend on it

docs/cli.md

@@ -407,10 +407,13 @@ name        : pendulum
 version     : 1.4.2
 description : Python datetimes made easy
 
-dependencies:
+dependencies
  - python-dateutil >=2.6.1
  - tzlocal >=1.4
  - pytzdata >=2017.2.2
+
+required by
+ - calendar >=1.4.0
 ```
 
 ### Options

poetry/console/commands/show.py

@@ -180,6 +180,13 @@ def handle(self) -> Optional[int]:
 
                 return 0
 
+            required_by = {}
+            for locked in locked_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]
+
             rows = [
                 ["<info>name</>", " : <c1>{}</>".format(pkg.pretty_name)],
                 ["<info>version</>", " : <b>{}</b>".format(pkg.pretty_version)],
@@ -199,6 +206,14 @@ def handle(self) -> Optional[int]:
                         )
                     )
 
+            if required_by:
+                self.line("")
+                self.line("<info>required by</info>")
+                for parent, requires_version in required_by.items():
+                    self.line(
+                        " - <c1>{}</c1> <b>{}</b>".format(parent, requires_version)
+                    )
+
             return 0
 
         show_latest = self.option("latest")

tests/console/commands/test_show.py

@@ -1419,3 +1419,78 @@ def test_show_tree_no_dev(tester, poetry, installed):
 """
 
     assert expected == tester.io.fetch_output()
+
+
+def test_show_required_by_deps(tester, poetry, installed):
+    poetry.package.add_dependency(Factory.create_dependency("cachy", "^0.2.0"))
+    poetry.package.add_dependency(Factory.create_dependency("pendulum", "2.0.0"))
+
+    cachy2 = get_package("cachy", "0.2.0")
+    cachy2.add_dependency(Factory.create_dependency("msgpack-python", ">=0.5 <0.6"))
+
+    pendulum = get_package("pendulum", "2.0.0")
+    pendulum.add_dependency(Factory.create_dependency("CachY", "^0.2.0"))
+
+    installed.add_package(cachy2)
+    installed.add_package(pendulum)
+
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "cachy",
+                    "version": "0.2.0",
+                    "description": "",
+                    "category": "main",
+                    "optional": False,
+                    "platform": "*",
+                    "python-versions": "*",
+                    "checksum": [],
+                    "dependencies": {"msgpack-python": ">=0.5 <0.6"},
+                },
+                {
+                    "name": "pendulum",
+                    "version": "2.0.0",
+                    "description": "Pendulum package",
+                    "category": "main",
+                    "optional": False,
+                    "platform": "*",
+                    "python-versions": "*",
+                    "checksum": [],
+                    "dependencies": {"cachy": ">=0.2.0 <0.3.0"},
+                },
+                {
+                    "name": "msgpack-python",
+                    "version": "0.5.1",
+                    "description": "",
+                    "category": "main",
+                    "optional": False,
+                    "platform": "*",
+                    "python-versions": "*",
+                    "checksum": [],
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "platform": "*",
+                "content-hash": "123456789",
+                "hashes": {"cachy": [], "pendulum": [], "msgpack-python": []},
+            },
+        }
+    )
+
+    tester.execute("cachy")
+
+    expected = """\
+ name         : cachy
+ version      : 0.2.0
+ description  :
+
+dependencies
+ - msgpack-python >=0.5 <0.6
+
+required by
+ - pendulum >=0.2.0 <0.3.0
+""".splitlines()
+    actual = [line.rstrip() for line in tester.io.fetch_output().splitlines()]
+    assert actual == expected