Sage vs pure python

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The thousands of lines of math-specific code ?

http://www.sagemath.org/doc/reference/

none of this belongs to Python.

Sage also has pretty robust interfaces to a wide variety of very discipline-specific outside packages, as well as uses them (to varying degrees) "natively" in its Pythonesque syntax. See the list of standard packages. Much of this would be very difficult to use in Python otherwise.