init: reuse existing pyproject.toml when safe (#2448)

At the moment a `poetry init` will fail, if a pyproject.toml already
exists. This change ensures pyproject.toml is reused if no build-system
has already been defined or poetry is not already configured.

Resolves: #1639

poetry/console/commands/init.py

@@ -13,6 +13,7 @@
 from cleo import option
 from tomlkit import inline_table
 
+from poetry.core.pyproject.toml import PyProjectTOML
 from poetry.utils._compat import OrderedDict
 from poetry.utils._compat import Path
 from poetry.utils._compat import urlparse
@@ -22,7 +23,6 @@
 
 
 class InitCommand(Command):
-
     name = "init"
     description = (
         "Creates a basic <comment>pyproject.toml</> file in the current directory."
@@ -67,9 +67,20 @@ def handle(self):
         from poetry.utils._compat import Path
         from poetry.utils.env import SystemEnv
 
-        if (Path.cwd() / "pyproject.toml").exists():
-            self.line("<error>A pyproject.toml file already exists.</error>")
-            return 1
+        pyproject = PyProjectTOML(Path.cwd() / "pyproject.toml")
+
+        if pyproject.file.exists():
+            if pyproject.is_poetry_project():
+                self.line(
+                    "<error>A pyproject.toml file with a poetry section already exists.</error>"
+                )
+                return 1
+
+            if pyproject.data.get("build-system"):
+                self.line(
+                    "<error>A pyproject.toml file with a defined build-system already exists.</error>"
+                )
+                return 1
 
         vcs_config = GitConfig()
 
@@ -198,7 +209,7 @@ def handle(self):
             dev_dependencies=dev_requirements,
         )
 
-        content = layout_.generate_poetry_content()
+        content = layout_.generate_poetry_content(pyproject)
         if self.io.is_interactive():
             self.line("<info>Generated file</info>")
             self.line("")

poetry/layouts/layout.py

@@ -1,10 +1,15 @@
+from typing import TYPE_CHECKING
+
 from tomlkit import dumps
 from tomlkit import loads
 from tomlkit import table
 
 from poetry.utils.helpers import module_name
 
 
+if TYPE_CHECKING:
+    from poetry.core.pyproject.toml import PyProjectTOML
+
 TESTS_DEFAULT = u"""from {package_name} import __version__
 
 
@@ -81,7 +86,7 @@ def create(self, path, with_tests=True):
 
         self._write_poetry(path)
 
-    def generate_poetry_content(self):
+    def generate_poetry_content(self, original_toml):  # type: ("PyProjectTOML") -> str
         template = POETRY_DEFAULT
         if self._license:
             template = POETRY_WITH_LICENSE
@@ -114,7 +119,12 @@ def generate_poetry_content(self):
 
         content.add("build-system", build_system)
 
-        return dumps(content)
+        content = dumps(content)
+
+        if original_toml.file.exists():
+            content = dumps(original_toml.data) + "\n" + content
+
+        return content
 
     def _create_default(self, path, src=True):
         raise NotImplementedError()

tests/console/commands/test_init.py

@@ -3,36 +3,46 @@
 import pytest
 
 from poetry.utils._compat import Path
+from poetry.utils._compat import decode
 from tests.helpers import get_package
 
 
+@pytest.fixture
+def source_dir(tmp_path):  # type: (Path) -> Path
+    yield Path(tmp_path.as_posix())
+
+
 @pytest.fixture(autouse=True)
-def patches(mocker, mocked_open_files):
-    mocked_open_files.append("pyproject.toml")
+def patches(mocker, source_dir):
     patch = mocker.patch("poetry.utils._compat.Path.cwd")
-    patch.return_value = Path(__file__).parent
+    patch.return_value = source_dir
 
 
 @pytest.fixture
 def tester(command_tester_factory):
     return command_tester_factory("init")
 
 
-def test_basic_interactive(tester):
-    inputs = [
-        "my-package",  # Package name
-        "1.2.3",  # Version
-        "This is a description",  # Description
-        "n",  # Author
-        "MIT",  # License
-        "~2.7 || ^3.6",  # Python
-        "n",  # Interactive packages
-        "n",  # Interactive dev packages
-        "\n",  # Generate
-    ]
-    tester.execute(inputs="\n".join(inputs))
+@pytest.fixture
+def init_basic_inputs():
+    return "\n".join(
+        [
+            "my-package",  # Package name
+            "1.2.3",  # Version
+            "This is a description",  # Description
+            "n",  # Author
+            "MIT",  # License
+            "~2.7 || ^3.6",  # Python
+            "n",  # Interactive packages
+            "n",  # Interactive dev packages
+            "\n",  # Generate
+        ]
+    )
 
-    expected = """\
+
+@pytest.fixture()
+def init_basic_toml():
+    return """\
 [tool.poetry]
 name = "my-package"
 version = "1.2.3"
@@ -46,7 +56,10 @@ def test_basic_interactive(tester):
 [tool.poetry.dev-dependencies]
 """
 
-    assert expected in tester.io.fetch_output()
+
+def test_basic_interactive(tester, init_basic_inputs, init_basic_toml):
+    tester.execute(inputs=init_basic_inputs)
+    assert init_basic_toml in tester.io.fetch_output()
 
 
 def test_interactive_with_dependencies(tester, repo):
@@ -565,3 +578,36 @@ def test_add_package_with_extras_and_whitespace(tester):
     assert len(result[0]["extras"]) == 2
     assert "postgresql" in result[0]["extras"]
     assert "sqlite" in result[0]["extras"]
+
+
+def test_init_existing_pyproject_simple(
+    tester, source_dir, init_basic_inputs, init_basic_toml
+):
+    pyproject_file = source_dir / "pyproject.toml"
+    existing_section = """
+[tool.black]
+line-length = 88
+"""
+    pyproject_file.write_text(decode(existing_section))
+    tester.execute(inputs=init_basic_inputs)
+    assert (
+        "{}\n{}".format(existing_section, init_basic_toml) in pyproject_file.read_text()
+    )
+
+
+def test_init_existing_pyproject_with_build_system_fails(
+    tester, source_dir, init_basic_inputs
+):
+    pyproject_file = source_dir / "pyproject.toml"
+    existing_section = """
+[build-system]
+requires = ["setuptools >= 40.6.0", "wheel"]
+build-backend = "setuptools.build_meta"
+"""
+    pyproject_file.write_text(decode(existing_section))
+    tester.execute(inputs=init_basic_inputs)
+    assert (
+        tester.io.fetch_output().strip()
+        == "A pyproject.toml file with a defined build-system already exists."
+    )
+    assert "{}".format(existing_section) in pyproject_file.read_text()