Set CIBUILDWHEEL environment variable to 1 during build

README.md

@@ -31,6 +31,17 @@ Usage
 
 `cibuildwheel` is not intended to run on your development machine. It will try to install packages globally; this is no good. Travis CI and Appveyor run their builds in isolated environments, so are ideal for this kind of script.
 
+`cibuildwheel` defines the environment variable `CIBUILDWHEEL` to the value `1` allowing projects for which the C extension is optional to make it mandatory when building wheels.
+An easy way to do it in Python 3 is through the `optional` named argument of `Extension` constructor in your `setup.py`:
+```python
+myextension = Extension(
+    "myextension",
+    ["myextension.c"],
+    optional=os.environ.get('CIBUILDWHEEL', '0') != '1',
+)
+```
+
+
 ### Minimal setup
 
 - Create a `.travis.yml` file in your repo.

cibuildwheel/__main__.py

@@ -75,6 +75,10 @@ def main():
 
     skip = BuildSkipper(skip_config)
 
+    # Add CIBUILDWHEEL environment variable
+    # This needs to be passed on to the docker container in linux.py
+    os.environ['CIBUILDWHEEL'] = '1'
+
     try:
         project_setup_py = os.path.join(project_dir, 'setup.py')
         name_output = subprocess.check_output([sys.executable, project_setup_py, '--name'],

cibuildwheel/linux.py

@@ -111,6 +111,8 @@ def build(project_dir, package_name, output_dir, test_command, test_requires, be
         docker_process = subprocess.Popen([
                 'docker',
                 'run',
+                '--env',
+                'CIBUILDWHEEL',
                 '--rm',
                 '-i',
                 '-v', '%s:/project' % os.path.abspath(project_dir),

test/01_basic/setup.py

@@ -1,5 +1,10 @@
+import os
+
 from setuptools import setup, Extension
 
+if os.environ.get('CIBUILDWHEEL', '0') != '1':
+    raise Exception('CIBUILDWHEEL environment variable is not set to 1')
+
 setup(
     name="spam",
     ext_modules=[Extension('spam', sources=['spam.c'])],