Writing code to be optimizable

[snorble]

Sometimes I want to prototype a program in Python, with the idea of
optimizing it later by rewriting parts of it in C or Cython. But I
usually find that in order to rewrite the slow parts, I end up writing
those parts very much like C or C++ anyway, and I end up wondering
what is the point of using Python in such a project.

I liked the idea of Cython, mainly the idea of being able to write in
pure Python, then create a file that modifies the pure Python file
(with type declarations and such), but I ran into the same problems.
In order to be able to modify the code, I can't make a lot of use of
the really helpful things in Python like dicts and such. I have to
dumb down Python to where I'm basically writing C code in Python, so
again I ask myself what is the point?

Is it reasonable to prototype an application in Python that will
require performance? Are there any recommendations on how to write
code in such a way that it can later be optimized or replaced with a
module written in C or Cython? Or any good examples of this being done
that I could learn from?