feat(cli): ensure build backend is defined when running poetry build

src/poetry/console/commands/build.py

@@ -114,13 +114,22 @@ def _get_builder(self) -> Callable[..., None]:
 
         return self._build
 
+    def _has_build_backend_defined(self) -> bool:
+        return "build-backend" in self.poetry.file.read().get("build-system", {})
+
     def build(self, options: BuildOptions) -> int:
         if not self.poetry.is_package_mode:
             self.io.write_error_line(
                 "Building a package is not possible in non-package mode."
             )
             return 1
 
+        if not self._has_build_backend_defined():
+            self.io.write_error_line(
+                "No build backend defined. Please define one in pyproject.toml."
+            )
+            return 1
+
         dist_dir = Path(options.output)
         package = self.poetry.package
         self.io.write_line(

tests/console/commands/test_build.py

@@ -300,3 +300,19 @@ def test_build_handler_build_isolated(
     handler.build(BuildOptions(clean=True, formats=["wheel"], output="dist"))
 
     assert mock_builder.build.call_count == 1
+
+
+@pytest.mark.parametrize("project", ["no_build_system", "no_build_backend"])
+def test_build_handler_raise_on_no_build_system(
+    project: str,
+    fixture_dir: FixtureDirGetter,
+    command_tester_factory: CommandTesterFactory,
+) -> None:
+    poetry = Factory().create_poetry(fixture_dir(f"build_systems/{project}"))
+    tester = command_tester_factory("build", poetry)
+
+    assert tester.execute() == 1
+    assert (
+        tester.io.fetch_error()
+        == "No build backend defined. Please define one in pyproject.toml.\n"
+    )

tests/fixtures/build_systems/no_build_backend/README.md

@@ -0,0 +1,2 @@
+My Package
+==========

tests/fixtures/build_systems/no_build_backend/pyproject.toml

@@ -0,0 +1,14 @@
+[project]
+name = "simple-project"
+version = "1.2.3"
+description = "Some description."
+authors = [
+    { name = "Poetry Contributors", email = "[email protected]" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"
+
+[build-system]
+requires = ["poetry-core>=1.1.0a7"]

tests/fixtures/build_systems/no_build_system/README.md

@@ -0,0 +1,2 @@
+My Package
+==========

tests/fixtures/build_systems/no_build_system/pyproject.toml

@@ -0,0 +1,11 @@
+[project]
+name = "simple-project"
+version = "1.2.3"
+description = "Some description."
+authors = [
+    { name = "Poetry Contributors", email = "[email protected]" }
+]
+license = { text = "MIT" }
+readme = "README.md"
+keywords = ["packaging", "dependency", "poetry"]
+requires-python = ">=3.4"