Pyrex is a language for writing Python extension modules. It’s pretty interesting—the syntax looks very similar to Python (the authors claim you can write C extension modules without knowing anything about the Python/C API) but uses additional type hints to compile down to ultra efficient C code, ready to be imported in to your Python applications. The prime numbers example maakes things a lot more clear:
# # Calculate prime numbers # def primes(int kmax): cdef int n, k, i cdef int p result =  if kmax > 1000: kmax = 1000 k = 0 n = 2 while k < kmax: i = 0 while i < k and n % p[i] <> 0: i = i + 1 if i == k: p[k] = n k = k + 1 result.append(n) n = n + 1 return result bash$ python pyrexc primes.pyx bash$ gcc -shared primes.o -lxosd -o primes.so >>> import primes >>> primes.primes(10) [2, 3, 5, 7, 11, 13, 17, 19, 23, 29] >>>
I imagine there’s a slight performance impact from using Python’s list data structures instead of a more low level C array, but I doubt it’s significant. In any case, the real promise of Pyrex lies in making it easier to write Python wrappers for existing C libraries—a topic touched on by the Pyrex Documentation.