Credit to https://vim.wikia.com/wiki/Scientific_calculator for the idea.

Note: There are no default mappings.

:PySolve <args> - Insert solved python expresion, args, into buffer; don't store.
:PySolveSave <args> - Save solved python expresion, args, into a register.
:PySolveView <args> - Print solved python expression, args, as a message; dont't store.

PySolve → <YOUR_KEY> <C-\><C-O>:call PySolve(0)<CR>
PySolveSave → <YOUR_KEY> <C-\><C-O>:call PySolve(1)<CR>
PySolveView → <YOUR_KEY> <C-\><C-O>:call PySolveView<CR>