Do not error when adding existing packages

Resolves: #98

poetry/console/commands/add.py

+# -*- coding: utf-8 -*-
+from typing import Dict
+from typing import List
+
 from cleo import argument
 from cleo import option
 
@@ -86,18 +90,19 @@ class AddCommand(InstallerCommand, InitCommand):
         if section not in poetry_content:
             poetry_content[section] = {}
 
-        for name in packages:
-            for key in poetry_content[section]:
-                if key.lower() == name.lower():
-                    pair = self._parse_requirements([name])[0]
-                    if (
-                        "git" in pair
-                        or "url" in pair
-                        or pair.get("version") == "latest"
-                    ):
-                        continue
+        existing_packages = self.get_existing_packages_from_input(
+            packages, poetry_content, section
+        )
 
-                    raise ValueError("Package {} is already present".format(name))
+        if existing_packages:
+            self.notify_about_existing_packages(existing_packages)
+
+        packages = [name for name in packages if name not in existing_packages]
+
+        if not packages:
+            self.poetry.file.write(content)
+            self.line("Nothing to add.")
+            return 0
 
         requirements = self._determine_requirements(
             packages,
@@ -184,3 +189,28 @@ class AddCommand(InstallerCommand, InitCommand):
             self.poetry.file.write(original_content)
 
         return status
+
+    def get_existing_packages_from_input(
+        self, packages, poetry_content, target_section
+    ):  # type: (List[str], Dict, str) -> List[str]
+        existing_packages = []
+
+        for name in packages:
+            for key in poetry_content[target_section]:
+                if key.lower() == name.lower():
+                    existing_packages.append(name)
+
+        return existing_packages
+
+    def notify_about_existing_packages(
+        self, existing_packages
+    ):  # type: (List[str]) -> None
+        self.line(
+            "The following packages are already present in the pyproject.toml and will be skipped:\n"
+        )
+        for name in existing_packages:
+            self.line("  • <c1>{name}</c1>".format(name=name))
+        self.line(
+            "\nIf you want to update it to the latest compatible version, you can use `poetry update package`.\n"
+            "If you prefer to upgrade it to the latest available version, you can use `poetry add package@latest`.\n"
+        )

tests/console/commands/test_add.py

@@ -714,7 +714,7 @@ Package operations: 1 install, 0 updates, 0 removals
     assert content["dependencies"]["pyyaml"] == "^3.13"
 
 
-def test_add_should_display_an_error_when_adding_existing_package_with_no_constraint(
+def test_add_should_skip_when_adding_existing_package_with_no_constraint(
     app, repo, tester
 ):
     content = app.poetry.file.read()
@@ -722,11 +722,18 @@ def test_add_should_display_an_error_when_adding_existing_package_with_no_constr
     app.poetry.file.write(content)
 
     repo.add_package(get_package("foo", "1.1.2"))
+    tester.execute("foo")
 
-    with pytest.raises(ValueError) as e:
-        tester.execute("foo")
+    expected = """\
+The following packages are already present in the pyproject.toml and will be skipped:
+
+  • foo
 
-    assert "Package foo is already present" == str(e.value)
+If you want to update it to the latest compatible version, you can use `poetry update package`.
+If you prefer to upgrade it to the latest available version, you can use `poetry add package@latest`.
+"""
+
+    assert expected in tester.io.fetch_output()
 
 
 def test_add_should_work_when_adding_existing_package_with_latest_constraint(
@@ -1527,7 +1534,7 @@ Package operations: 1 install, 0 updates, 0 removals
     assert content["dependencies"]["pyyaml"] == "^3.13"
 
 
-def test_add_should_display_an_error_when_adding_existing_package_with_no_constraint_old_installer(
+def test_add_should_skip_when_adding_existing_package_with_no_constraint_old_installer(
     app, repo, installer, old_tester
 ):
     content = app.poetry.file.read()
@@ -1536,10 +1543,18 @@ def test_add_should_display_an_error_when_adding_existing_package_with_no_constr
 
     repo.add_package(get_package("foo", "1.1.2"))
 
-    with pytest.raises(ValueError) as e:
-        old_tester.execute("foo")
+    old_tester.execute("foo")
+
+    expected = """\
+The following packages are already present in the pyproject.toml and will be skipped:
+
+  • foo
 
-    assert "Package foo is already present" == str(e.value)
+If you want to update it to the latest compatible version, you can use `poetry update package`.
+If you prefer to upgrade it to the latest available version, you can use `poetry add package@latest`.
+"""
+
+    assert expected in old_tester.io.fetch_output()
 
 
 def test_add_should_work_when_adding_existing_package_with_latest_constraint_old_installer(