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Mac mini late 2014, upgraded to El capitan.
Out of the box python 2.7 reports the wrong system info.
platform.system() should report El Capitan
platform.release() should report 10.11.5

Python's platform module has both platform-independent and platform-dependent functions as noted in its documentation. While it isn't as clearly documented as perhaps it should be, on most "Unix-y" platforms, like OS X, much of the platform-independent system information comes from the same source as the platform's "uname" command. You can see that, if you use platform.uname(), it matches the output of OS X's uname(1) command:
>>> platform.uname()
('Darwin', 'kitt.local', '15.5.0', 'Darwin Kernel Version 15.5.0: Tue Apr 19 18:36:36 PDT 2016; root:xnu-32485021~8/RELEASE_X86_64', 'x86_64', 'i386')
>>> platform.system()
'Darwin'
>>> platform.release()
'15.5.0'
$ uname -a
Darwin kitt.local 15.5.0 Darwin Kernel Version 15.5.0: Tue Apr 19 18:36:36 PDT 2016; root:xnu-32485021~8/RELEASE_X86_64 x86_64
Somewhat confusingly, "Darwin" is how the OS X kernel identifies itself and "15.5.0" is the Darwin version number that corresponds to OS X 10.11.5. And the uname output is what the platform module uses.
For OS X, the platform module does provide a Mac-specific function, platform.mac_ver. From it, you can get the OS X version information, rather than the Darwin kernel information.
>>> platform.mac_ver()
('10.11.5', ('', '', ''), 'x86_64')
Suggestions on how to improve the documentation are welcome! But changing the results from the platform-independent functions after all these years is not likely to happen.
https://docs.python.org/2.7/library/platform.html#cross-platform
https://en.wikipedia.org/wiki/Darwin_(operating_system)#Release_history 

Hello Ned,
That is a brilliant answer!
After reporting this initially, I tried with 3.5.1, and uname... Convinced it was a 2.7.1, I could not be more wrong! What a stupid and confusing output does the OS report.
platform.mac_ver() does the trick!
thank you!