Eryk Sun added the comment:

Testing based on integrity level doesn't require creating a child process. I'm 
attaching a ctypes-based example that defines a context manager that 
temporarily sets the integrity level of the current thread's impersonation 
token. 

To get the impersonation token, I initially tried using ImpersonateSelf / 
RevertToSelf, but I was unhappy with how it fails for nested contexts since 
RevertToSelf always switches back to the process token. I opted to instead call 
OpenThreadToken / OpenProcessToken, DuplicateTokenEx, and SetThreadToken. 

I chose to use the WELL_KNOWN_SID_TYPE enum values to get the label SIDs via 
CreateWellKnownSid. Note that I omitted the GetLengthSid call when passing the 
size of the TOKEN_MANDATORY_LABEL to SetTokenInformation. It only needs the 
size of the primary buffer. The SID it points at is a sized structure (i.e. 
SubAuthorityCount). 

Example:

    import winreg

    HKLM = winreg.HKEY_LOCAL_MACHINE
    subkey = r'SOFTWARE\Microsoft\SystemCertificates\CA'
    access = winreg.KEY_ALL_ACCESS

    >>> key = winreg.OpenKey(HKLM, subkey, 0, access)    
    >>> print(key)
    <PyHKEY:0x0000000000000178>
    >>> key.Close()

Repeat with low integrity level:

    >>> with token_integrity_level('low'):
    ...      winreg.OpenKey(HKLM, subkey, 0, access)
    ...
    Traceback (most recent call last):
      File "<stdin>", line 2, in <module>
    PermissionError: [WinError 5] Access is denied 

A context manager like this could be added to the test helper module that was 
proposed in issue 22080. It could also add the ability to impersonate with a 
restricted copy of the process token -- like what UAC creates. psexec -l does 
this by calling CreateRestrictedToken followed by SetInformationToken for the 
TokenIntegrityLevel and TokenDefaultDacl.

----------
Added file: http://bugs.python.org/file41439/integrity_level.py

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<http://bugs.python.org/issue25939>
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