setup.py: optionally build cython extension

If Cython is already pre-installed, we opportunistically try to compile the
cu2qu extension module, and if it fails (e.g. compiler is not available), then
we silently fall back to pure-python mode.

If user explicitly requests --with-cython (or sets 'FONTTOOLS_WITH_CYTHON=1' env
variable), then we add cython to setup_requires, and raise an error if
compilation fails.

setup.py

@@ -6,7 +6,8 @@ import sys
 import os
 from os.path import isfile, join as pjoin
 from glob import glob
-from setuptools import setup, find_packages, Command
+from setuptools import setup, find_packages, Command, Extension
+from setuptools.command.build_ext import build_ext as _build_ext
 from distutils import log
 from distutils.util import convert_path
 import subprocess as sp
@@ -23,10 +24,50 @@ def doraise_py_compile(file, cfile=None, dfile=None, doraise=False):
 
 py_compile.compile = doraise_py_compile
 
-needs_wheel = {'bdist_wheel'}.intersection(sys.argv)
-wheel = ['wheel'] if needs_wheel else []
-needs_bumpversion = {'release'}.intersection(sys.argv)
-bumpversion = ['bump2version'] if needs_bumpversion else []
+setup_requires = []
+
+if {'bdist_wheel'}.intersection(sys.argv):
+	setup_requires.append('wheel')
+
+if {'release'}.intersection(sys.argv):
+	setup_requires.append('bump2version')
+
+try:
+	__import__("cython")
+except ImportError:
+	has_cython = False
+else:
+	has_cython = True
+
+env_with_cython = os.environ.get("FONTTOOLS_WITH_CYTHON")
+with_cython = (
+	True if env_with_cython in {"1", "true", "yes"}
+	else False if env_with_cython in {"0", "false", "no"}
+	else None
+)
+# --with-cython/--without-cython options override environment variables
+opt_with_cython = {'--with-cython'}.intersection(sys.argv)
+opt_without_cython = {'--without-cython'}.intersection(sys.argv)
+if opt_with_cython and opt_without_cython:
+    sys.exit(
+        "error: the options '--with-cython' and '--without-cython' are "
+        "mutually exclusive"
+    )
+elif opt_with_cython:
+    sys.argv.remove("--with-cython")
+    with_cython = True
+elif opt_without_cython:
+    sys.argv.remove("--without-cython")
+    with_cython = False
+
+if with_cython and not has_cython:
+	setup_requires.append("cython")
+
+ext_modules = []
+if with_cython is True or (with_cython is None and has_cython):
+	ext_modules.append(
+		Extension("fontTools.cu2qu.cu2qu", ["Lib/fontTools/cu2qu/cu2qu.py"]),
+	)
 
 extras_require = {
 	# for fontTools.ufoLib: to read/write UFO fonts
@@ -343,7 +384,53 @@ def find_data_files(manpath="share/man"):
 	return data_files
 
 
-setup(
+class cython_build_ext(_build_ext):
+	"""Compile *.pyx source files to *.c using cythonize if Cython is
+	installed and there is a working C compiler, else fall back to pure python dist.
+	"""
+
+	def finalize_options(self):
+		from Cython.Build import cythonize
+
+		# optionally enable line tracing for test coverage support
+		linetrace = os.environ.get("CYTHON_TRACE") == "1"
+
+		self.distribution.ext_modules[:] = cythonize(
+			self.distribution.ext_modules,
+			force=linetrace or self.force,
+			annotate=os.environ.get("CYTHON_ANNOTATE") == "1",
+			quiet=not self.verbose,
+			compiler_directives={
+				"linetrace": linetrace,
+				"language_level": 3,
+				"embedsignature": True,
+			},
+		)
+
+		_build_ext.finalize_options(self)
+
+	def build_extensions(self):
+		try:
+			_build_ext.build_extensions(self)
+		except Exception as e:
+			if with_cython:
+				raise
+			from distutils.errors import DistutilsModuleError
+
+			# optional compilation failed: we delete 'ext_modules' and make sure
+			# the generated wheel is 'pure'
+			del self.distribution.ext_modules[:]
+			try:
+				bdist_wheel = self.get_finalized_command("bdist_wheel")
+			except DistutilsModuleError:
+				# 'bdist_wheel' command not available as wheel is not installed
+				pass
+			else:
+				bdist_wheel.root_is_pure = True
+			log.error('error: building extensions failed: %s' % e)
+
+
+setup_params = dict(
 	name="fonttools",
 	version="4.6.1.dev0",
 	description="Tools to manipulate font files",
@@ -360,7 +447,8 @@ setup(
 	packages=find_packages("Lib"),
 	include_package_data=True,
 	data_files=find_data_files(),
-	setup_requires=wheel + bumpversion,
+	ext_modules=ext_modules,
+	setup_requires=setup_requires,
 	extras_require=extras_require,
 	entry_points={
 		'console_scripts': [
@@ -372,6 +460,11 @@ setup(
 	},
 	cmdclass={
 		"release": release,
+		"build_ext": cython_build_ext,
 	},
 	**classifiers
 )
+
+
+if __name__ == "__main__":
+	setup(**setup_params)