Added unit tests for use_pep517 flag

src/pip/_internal/req/req_install.py

@@ -614,7 +614,10 @@ class InstallRequirement(object):
             if self.use_pep517 is False:
                 raise InstallationError(
                     "Disabling PEP 517 processing is invalid: "
-                    "project specifies a build-backend in pyproject.toml"
+                    "project specifies a build backend of {} "
+                    "in pyproject.toml".format(
+                        build_system["build-backend"]
+                    )
                 )
             self.use_pep517 = True
 

tests/data/src/pep517_pyproject_only/pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["foo"]
+build-backend = "foo"

tests/data/src/pep517_setup_and_pyproject/pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["foo"]
+build-backend = "foo"

tests/data/src/pep517_setup_and_pyproject/setup.py

@@ -0,0 +1,2 @@
+from setuptools import setup
+setup(name="dummy", version="0.1")

tests/data/src/pep517_setup_only/setup.py

@@ -0,0 +1,2 @@
+from setuptools import setup
+setup(name="dummy", version="0.1")

tests/unit/test_pep517.py

@@ -0,0 +1,41 @@
+import pytest
+
+from pip._internal.exceptions import InstallationError
+from pip._internal.req import InstallRequirement
+
+
+@pytest.mark.parametrize(('source', 'expected'), [
+    ("pep517_setup_and_pyproject", True),
+    ("pep517_setup_only", False),
+    ("pep517_pyproject_only", True),
+])
+def test_use_pep517(data, source, expected):
+    """
+    Test that we choose correctly between PEP 517 and legacy code paths
+    """
+    src = data.src.join(source)
+    req = InstallRequirement(None, None, source_dir=src)
+    req.load_pyproject_toml()
+    assert req.use_pep517 is expected
+
+
+@pytest.mark.parametrize(('source', 'msg'), [
+    ("pep517_setup_and_pyproject", "specifies a build backend"),
+    ("pep517_pyproject_only", "does not have a setup.py"),
+])
+def test_disabling_pep517_invalid(data, source, msg):
+    """
+    Test that we fail if we try to disable PEP 517 when it's not acceptable
+    """
+    src = data.src.join(source)
+    req = InstallRequirement(None, None, source_dir=src)
+
+    # Simulate --no-use-pep517
+    req.use_pep517 = False
+
+    with pytest.raises(InstallationError) as e:
+        req.load_pyproject_toml()
+
+    err_msg = e.value.args[0]
+    assert "Disabling PEP 517 processing is invalid" in err_msg
+    assert msg in err_msg