Why when use sys.platform on Mac os it print “darwin”?

Question

In Python, when I type sys.platform on the Mac OS X the output is \"darwin\"? Why is this so?

Answer 1

Because the core of Mac OS X is the Darwin OS.

Quoting from the linked WikiPedia page:

Darwin forms the core set of components upon which Mac OS X and iOS are based.

Even the OS X platform itself reports itself as "Darwin" when you ask it:

$ uname
Darwin

Python merely uses that same platform identifier.