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On Windows, if Python is started with an empty environment (no environment variable at all), Python fails with:
"Fatal Python error: Failed to initialize Windows random API (CryptoGen)"
It's the Windows error 0x80090006 which is not displayed in the error message :-(
In fact, it's because Python 3 cannot be started without SYSTEMROOT environment variable. Otherwise, CryptAcquireContext() is unable to find some DLL.
IMO the script_helper should be fixed to copy the variable.
Recent example of this issue:
http://bugs.python.org/issue20599#msg211111 

cleanenv.patch: untested test.

When Python is executed in an empty environement, not only it displays "Fatal Python error: Failed to initialize Windows random API (CryptoGen)", but it opens also the fatal error popup on Windows.
The issue #19983 proposes a patch to not abort() at exit, but exit with an non-zero exit code instead. The same should be done for this error.
By the way, we can maybe help the use to fix this issue with a better error message. For example, add in the message " (check if SYSTEMROOT environment variable is correctly set)", maybe only if the environment variable is not set.

Oh, another problem: if Python is compiled in shared module, libpython3.4.so cannot be found. LD_LIBRARY_PATH must also be copied if __cleanenv=True. Py_ENABLE_SHARED can be checked in sysconfig to decide if the environment variable should be copied or not.

Interesting, how isolation mode works on Windows or if Python is compiled in shared module? Should we keep these environment variables if the __isolated keyword is used?

> Oh, another problem: if Python is compiled in shared module, libpython3.4.so cannot be found.
> LD_LIBRARY_PATH must also be copied if __cleanenv=True. Py_ENABLE_SHARED can be checked in
> sysconfig to decide if the environment variable should be copied or not.
The name of variable is specific to operating system.
Potentially incomplete list based on value of RUNSHARED variable in configure.ac:
 AIX: LIBPATH
 HP-UX: SHLIB_PATH
 Linux, GNU Hurd, *BSD, SunOS: LD_LIBRARY_PATH
 Mac OS X (with --enable-shared): DYLD_LIBRARY_PATH
 Mac OS X (with --disable-shared and --enable-framework): DYLD_FRAMEWORK_PATH

Serhiy> Interesting, how isolation mode works on Windows or if Python is compiled in shared module? Should we keep these environment variables if the __isolated keyword is used?
The __isolated parameter of script_helper._assert_python() don't start Python with an empty environment, it only adds -I to the command line.
There is a single test which uses __cleanenv=True: test_hash.
Arfrever> The name of variable is specific to operating system.
Ah yes, right. It's probably safe to copy all these environment variables without taking care of the operating system.

> There is a single test which uses __cleanenv=True: test_hash.
The test pass on Windows and this issue is old. I prefer to simplify close it.