Force Step in is not enabled

I don't understand step in functionality while debugging python. Pycharm pro, v 2016.1.2

I want to step into to a call into a site library. But the force stepin icon is greyed out (inactive) and the 'smart step in' just skips over.

If I command-B on the function to see the library source code and put a breakpoint there, pycharm will break there. But why can't I step in to library code?

 

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