Just a friendly reminder that two leading underscores wont protect your member functions in C++. Even if people insist that those are totally not supposed to be private in python.
Whenever I say "I'm no longer attached to all that private stuff", people always reply, "wait until you work on a large code base". I work on a million line+ code base. Whatever.
This argument aside, I'm not a total philistine. RAII is awesome but C++ is full to the boot with crusty stuff to keep the compatibility. I always feel there is a language better than anything trying to come out.
These days I'm for minimalism, most of my structs are aggregates of public members, but sometimes you really want to make sure to maintain the invariant that your array pointer and your size field are in sync.
Of course neither double nor single underscore will stop anyone who wants to touch your privates badly enough. Which is big part of the python philosophy: You're not stopped from doing inadvisable things. Instead there's a strong culture around writing "pythonic" code, which largely avoids these pitfalls.
In python, if any of this gives you an trouble you can just replace the stuff in the class dict with your own functions. You don't even need to cast.