Author: Thomas Severin
Date: 22.Oct.2020
With Cython you are able to write C and C++ modules for Python. It implements a superset of the Python language.
You are also able to call C-functions and realize declaration of variables and functions like in C (e.g. float x;or int y;).
.
Here is an example:
def primes(int kmax):
"""Calculate all prime numbers up to kmax
(max. value: 1000) cdef variable declaration """
cdef int n, k, i
cdef int p[1000]
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
This implementation of an algorithm to find prime numbers has some additional keywords instead of the next one, which is implemented in pure Python:
def primes(kmax):
#Calculate all prime numbers up to kmax (max. value: 1000)
p= range(1000)
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
The only difference between the both algorithm is this part:
#primes function with additional Cython code:
def primes(int kmax):
cdef int n, k, i
cdef int p[1000]
result = []
#primes in standard Python syntax:
def primes(kmax):
p= range(1000)
result = []
What is the difference? In the upper Cython version you can see the declaration of the variable types and the integer array like in standard C.
For example cdef int n,k,i in line 5. This additional type definition (e.g. integer) allows the Cython compiler to generate
more efficient C code from the second code. While standard Python code is saved in *.py files,Cython code is saved in *.pyx files.
And what is with the speed? So lets try it!
import time
#activate pyx compiler
import pyximport
pyximport.install()
#primes implemented with Cython
import primesCy
#primes implemented with Python
import primes
print("Cython:")
t1= time.time()
print( primesCy.primes(500))
t2= time.time()
print("Cython time: %s" %(t2-t1))
print("")
print("Python")
t1= time.time()
print(primes.primes(500))
t2= time.time()
print( "Python time: %s" %(t2-t1))
These lines are special:
import pyximport
pyximport.install()
The pyximport module allows you to import pyx files (e.g., primesCy.pyx) with the Cython-compiled version of the primes function.
The pyximport.install() command allows the Python interpreter to start the Cython compiler directly to generate C-code,
which is automatically compiled to a *.so C-library. Cython is able to import this library for you in your Python-code.
Very easy and very efficient. With the time.time() function you are able to compare the time between this 2 different calls to find 500 prime numbers.
On a standard notebook (dualcore AMD E-450 1,6 GHz) the measured values are:
Cython time: 0.0054 seconds
Python time: 0.0566 seconds
And here the output of an embedded ARM beaglebone machine:
Cython time: 0.0196 seconds
Python time: 0.3302 seconds
That is a really good result...
(I contributed this article also to the famous kennethreitz/python-guide.)