corrected as per notations from @YannickJadoul on #113

test/03_before_build/setup.py

@@ -1,6 +1,23 @@
 from setuptools import setup, Extension
 import sys, os
 
+# assert that the Python version as written to pythonversion.txt in the CIBW_BEFORE_BUILD step
+# is the same one as is currently running.
+version_file = 'c:\\pythonversion.txt' if sys.platform == 'win32' else '/tmp/pythonversion.txt'
+with open(version_file) as f:
+    stored_version = f.read()
+print('stored_version', stored_version)
+print('sys.version', sys.version)
+assert stored_version == sys.version
+# check that the executable also was written
+executable_file = 'c:\\pythonexecutable.txt' if sys.platform == 'win32' else '/tmp/pythonexecutable.txt'
+with open(executable_file) as f:
+    stored_executable = f.read()
+print('stored_executable', stored_executable)
+print('sys.executable', sys.executable)
+# windows/mac are case insensitive
+assert os.path.realpath(stored_executable).lower() == os.path.realpath(sys.executable).lower()
+
 setup(
     name="spam",
     ext_modules=[Extension('spam', sources=['spam.c'])],