Raise an error if the user tries to use PIP_NO_USE_PEP517.

src/pip/_internal/cli/cmdoptions.py

@@ -9,6 +9,7 @@ pass on state. To be consistent, all options will follow this design.
 """
 from __future__ import absolute_import
 
+import textwrap
 import warnings
 from distutils.util import strtobool
 from functools import partial
@@ -28,6 +29,20 @@ if MYPY_CHECK_RUNNING:
     from pip._internal.cli.parser import ConfigOptionParser  # noqa: F401
 
 
+def raise_option_error(parser, option, msg):
+    """
+    Raise an option parsing error using parser.error().
+
+    Args:
+      parser: an OptionParser instance.
+      option: an Option instance.
+      msg: the error text.
+    """
+    msg = '{} error: {}'.format(option, msg)
+    msg = textwrap.fill(' '.join(msg.split()))
+    parser.error(msg)
+
+
 def make_option_group(group, parser):
     # type: (Dict[str, Any], ConfigOptionParser) -> OptionGroup
     """
@@ -601,6 +616,30 @@ no_build_isolation = partial(
          'if this option is used.'
 )  # type: Callable[..., Option]
 
+
+def no_use_pep517_callback(option, opt, value, parser):
+    """
+    Process a value provided for the --no-use-pep517 option.
+
+    This is an optparse.Option callback for the no_use_pep517 option.
+    """
+    # Since --no-use-pep517 doesn't accept arguments, the value argument
+    # will be None if --no-use-pep517 is passed via the command-line.
+    # However, the value can be non-None if the option is triggered e.g.
+    # by an environment variable, for example "PIP_NO_USE_PEP517=true".
+    if value is not None:
+        msg = """A value was passed for --no-use-pep517,
+        probably using either the PIP_NO_USE_PEP517 environment variable
+        or the "no-use-pep517" config file option. Use an appropriate value
+        of the PIP_USE_PEP517 environment variable or the "use-pep517"
+        config file option instead.
+        """
+        raise_option_error(parser, option=option, msg=msg)
+
+    # Otherwise, --no-use-pep517 was passed via the command-line.
+    parser.values.use_pep517 = False
+
+
 use_pep517 = partial(
     Option,
     '--use-pep517',
@@ -615,7 +654,8 @@ no_use_pep517 = partial(
     Option,
     '--no-use-pep517',
     dest='use_pep517',
-    action='store_false',
+    action='callback',
+    callback=no_use_pep517_callback,
     default=None,
     help=SUPPRESS_HELP
 )  # type: Any

tests/unit/test_options.py

@@ -5,9 +5,27 @@ import pytest
 
 import pip._internal.configuration
 from pip._internal import main
+from pip._internal.commands import DownloadCommand
 from tests.lib.options_helpers import AddFakeCommandMixin
 
 
+@contextmanager
+def temp_environment_variable(name, value):
+    not_set = object()
+    original = os.environ[name] if name in os.environ else not_set
+    os.environ[name] = value
+
+    try:
+        yield
+    finally:
+        # Return the environment variable to its original state.
+        if original is not_set:
+            if name in os.environ:
+                del os.environ[name]
+        else:
+            os.environ[name] = original
+
+
 @contextmanager
 def assert_raises_message(exc_class, expected):
     """
@@ -19,6 +37,22 @@ def assert_raises_message(exc_class, expected):
     assert str(excinfo.value) == expected
 
 
+@contextmanager
+def assert_option_error(capsys, expected):
+    """
+    Assert that a SystemExit occurred because of a parsing error.
+
+    Args:
+      expected: an expected substring of stderr.
+    """
+    with pytest.raises(SystemExit) as excinfo:
+        yield
+
+    assert excinfo.value.code == 2
+    stderr = capsys.readouterr().err
+    assert expected in stderr
+
+
 def assert_is_default_cache_dir(value):
     # This path looks different on different platforms, but the path always
     # has the substring "pip".
@@ -157,6 +191,89 @@ class TestOptionPrecedence(AddFakeCommandMixin):
             main(['--no-cache-dir', 'fake'])
 
 
+class TestUsePEP517Options(object):
+
+    """
+    Test options related to using --use-pep517.
+    """
+
+    def parse_args(self, args):
+        # We use DownloadCommand since that is one of the few Command
+        # classes with the use_pep517 options.
+        command = DownloadCommand()
+        options, args = command.parse_args(args)
+
+        return options
+
+    def test_no_option(self):
+        """
+        Test passing no option.
+        """
+        options = self.parse_args([])
+        assert options.use_pep517 is None
+
+    def test_use_pep517(self):
+        """
+        Test passing --use-pep517.
+        """
+        options = self.parse_args(['--use-pep517'])
+        assert options.use_pep517 is True
+
+    def test_no_use_pep517(self):
+        """
+        Test passing --no-use-pep517.
+        """
+        options = self.parse_args(['--no-use-pep517'])
+        assert options.use_pep517 is False
+
+    def test_PIP_USE_PEP517_true(self):
+        """
+        Test setting PIP_USE_PEP517 to "true".
+        """
+        with temp_environment_variable('PIP_USE_PEP517', 'true'):
+            options = self.parse_args([])
+        # This is an int rather than a boolean because strtobool() in pip's
+        # configuration code returns an int.
+        assert options.use_pep517 == 1
+
+    def test_PIP_USE_PEP517_false(self):
+        """
+        Test setting PIP_USE_PEP517 to "false".
+        """
+        with temp_environment_variable('PIP_USE_PEP517', 'false'):
+            options = self.parse_args([])
+        # This is an int rather than a boolean because strtobool() in pip's
+        # configuration code returns an int.
+        assert options.use_pep517 == 0
+
+    def test_use_pep517_and_PIP_USE_PEP517_false(self):
+        """
+        Test passing --use-pep517 and setting PIP_USE_PEP517 to "false".
+        """
+        with temp_environment_variable('PIP_USE_PEP517', 'false'):
+            options = self.parse_args(['--use-pep517'])
+        assert options.use_pep517 is True
+
+    def test_no_use_pep517_and_PIP_USE_PEP517_true(self):
+        """
+        Test passing --no-use-pep517 and setting PIP_USE_PEP517 to "true".
+        """
+        with temp_environment_variable('PIP_USE_PEP517', 'true'):
+            options = self.parse_args(['--no-use-pep517'])
+        assert options.use_pep517 is False
+
+    def test_PIP_NO_USE_PEP517(self, capsys):
+        """
+        Test setting PIP_NO_USE_PEP517, which isn't allowed.
+        """
+        expected_err = (
+            '--no-use-pep517 error: A value was passed for --no-use-pep517,\n'
+        )
+        with temp_environment_variable('PIP_NO_USE_PEP517', 'true'):
+            with assert_option_error(capsys, expected=expected_err):
+                self.parse_args([])
+
+
 class TestOptionsInterspersed(AddFakeCommandMixin):
 
     def test_general_option_after_subcommand(self):