Work around for side effects in setup.py script

setup.py

@@ -122,6 +122,132 @@ def run_tests(self):
         sys.exit(errno)
 
 
+def keywords_with_side_effects(argv):
+    """
+    Get a dictionary with setup keywords that (can) have side effects.
+
+    :param argv: A list of strings with command line arguments.
+    :returns: A dictionary with keyword arguments for the ``setup()`` function.
+
+    This setup.py script uses the setuptools 'setup_requires' feature because
+    this is required by the cffi package to compile extension modules. The
+    purpose of ``keywords_with_side_effects()`` is to avoid triggering the cffi
+    build process as a result of setup.py invocations that don't need the cffi
+    module to be built (setup.py serves the dual purpose of exposing package
+    metadata).
+
+    All of the options listed by ``python setup.py --help`` that print
+    information should be recognized here. The commands ``clean``,
+    ``egg_info``, ``register``, ``sdist`` and ``upload`` are also recognized.
+    Any combination of these options and commands is also supported.
+
+    This function was originally based on the `setup.py script`_ of SciPy (see
+    also the discussion in `pip issue #25`_).
+
+    .. _pip issue #25: https://github.com/pypa/pip/issues/25
+    .. _setup.py script: https://github.com/scipy/scipy/blob/master/setup.py
+    """
+    no_setup_requires_arguments = (
+        '-h', '--help',
+        '-n', '--dry-run',
+        '-q', '--quiet',
+        '-v', '--verbose',
+        '-V', '--version',
+        '--author',
+        '--author-email',
+        '--classifiers',
+        '--contact',
+        '--contact-email',
+        '--description',
+        '--fullname',
+        '--help-commands',
+        '--keywords',
+        '--licence',
+        '--license',
+        '--long-description',
+        '--maintainer',
+        '--maintainer-email',
+        '--name',
+        '--no-user-cfg',
+        '--obsoletes',
+        '--platforms',
+        '--provides',
+        '--requires',
+        '--url',
+        'clean',
+        'egg_info',
+        'register',
+        'sdist',
+        'upload',
+    )
+    if all((arg in no_setup_requires_arguments) or
+           all(('-' + char) in no_setup_requires_arguments for char in arg[1:])
+           for arg in argv[1:]):
+        return {
+            "cmdclass": {
+                "build": DummyCFFIBuild,
+                "install": DummyCFFIInstall,
+                "test": DummyPyTest,
+            }
+        }
+    else:
+        return {
+            "setup_requires": requirements,
+            "cmdclass": {
+                "build": CFFIBuild,
+                "install": CFFIInstall,
+                "test": PyTest,
+            }
+        }
+
+
+setup_requires_error = ("Requested setup command that needs 'setup_requires' "
+                        "while command line arguments implied a side effect "
+                        "free command or option.")
+
+
+class DummyCFFIBuild(CFFIBuild):
+    """
+    This class makes it very obvious when ``keywords_with_side_effects()`` has
+    incorrectly interpreted the command line arguments to ``setup.py build`` as
+    one of the 'side effect free' commands or options.
+    """
+
+    def finalize_options(self):
+        build.finalize_options(self)
+
+    def run(self):
+        raise RuntimeError(setup_requires_error)
+
+
+class DummyCFFIInstall(CFFIInstall):
+    """
+    This class makes it very obvious when ``keywords_with_side_effects()`` has
+    incorrectly interpreted the command line arguments to ``setup.py install``
+    as one of the 'side effect free' commands or options.
+    """
+
+    def finalize_options(self):
+        install.finalize_options(self)
+
+    def run(self):
+        raise RuntimeError(setup_requires_error)
+
+
+class DummyPyTest(PyTest):
+    """
+    This class makes it very obvious when ``keywords_with_side_effects()`` has
+    incorrectly interpreted the command line arguments to ``setup.py test`` as
+    one of the 'side effect free' commands or options.
+    """
+
+    def finalize_options(self):
+        test.finalize_options(self)
+
+    def run_tests(self):
+        raise RuntimeError(setup_requires_error)
+
+
 with open(os.path.join(base_dir, "README.rst")) as f:
     long_description = f.read()
 
@@ -163,15 +289,11 @@ def run_tests(self):
     packages=find_packages(exclude=["tests", "tests.*"]),
 
     install_requires=requirements,
-    setup_requires=requirements,
     tests_require=test_requirements,
 
     # for cffi
     zip_safe=False,
     ext_package="cryptography",
-    cmdclass={
-        "build": CFFIBuild,
-        "install": CFFIInstall,
-        "test": PyTest,
-    }
+
+    **keywords_with_side_effects(sys.argv)
 )