Cython Tutorial

Reference:
- Cython Tutorial Release 2011
- Cython Tutorial Release 0.15pre



In [1]:

    
#initialize
%matplotlib inline
%qtconsole

1 Normal python style

Write file gravity.py
cp gravity.py to gravity_cy.pyx



In [16]:

    
%%writefile cython/gravity.py
#gravity.py
from math import sqrt

class Planet(object):
    def __init__(self):
        # Some initial position and velocity
        self.x = 1.0
        self.y = 0.0
        self.z = 0.0
        self.vx = 0.0
        self.vy = 0.5
        self.vz = 0.0
        
        # Some mass
        self.m = 1.0
        
def single_step(planet, dt):
    """ Make a single time step """
    
    # compute Force: gravity towards origin
    distance = sqrt(planet.x ** 2 + planet.y ** 2 + planet.z ** 2)
    Fx = -planet.x / distance ** 3
    Fy = -planet.y / distance ** 3
    Fz = -planet.z / distance ** 3
    
    # Time step position, according to velocity
    planet.x += dt * planet.vx
    planet.y += dt * planet.vy
    planet.z += dt * planet.vz
    
    # Time step velocity, according to force and mass
    planet.vx += dt * Fx / planet.m
    planet.vy += dt * Fy / planet.m
    planet.vz += dt * Fz / planet.m
    
def step_time(planet, time_span, n_steps):
    """Make a number of time steps forward """
    
    dt = time_span / n_steps
    for j in xrange(n_steps):
        single_step(planet, dt)









    



Writing cython/gravity.py



In [28]:

    
!cp cython/gravity.py cython/gravity_cy.pyx

Write setup.py



In [29]:

    
%%writefile cython/setup.py
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

setup(
      cmdclass = {'build_ext': build_ext},
      ext_modules = [Extension("gravity_cy",
                               ["gravity_cy.pyx"],
                    ),
])









    



Writing cython/setup.py



In [37]:

    
#Then run command in normal shell
#   $ python setup.py build
#   or
#   $ python setup.py build_ext -i

2 Type declarations in cython

Syntax
- def some_function (some_type some_parameter):
  - cdef another_type variable
  - ...



In [40]:

    
%load cython/gravity_cy.pyx



In [ ]:

    
#gravity.py
from math import sqrt

class Planet(object):
    def __init__(self):
        # Some initial position and velocity
        self.x = 1.0
        self.y = 0.0
        self.z = 0.0
        self.vx = 0.0
        self.vy = 0.5
        self.vz = 0.0
        
        # Some mass
        self.m = 1.0
        
def single_step(planet, dt):
    """ Make a single time step """
    
    # compute Force: gravity towards origin
    distance = sqrt(planet.x ** 2 + planet.y ** 2 + planet.z ** 2)
    Fx = -planet.x / distance ** 3
    Fy = -planet.y / distance ** 3
    Fz = -planet.z / distance ** 3
    
    # Time step position, according to velocity
    planet.x += dt * planet.vx
    planet.y += dt * planet.vy
    planet.z += dt * planet.vz
    
    # Time step velocity, according to force and mass
    planet.vx += dt * Fx / planet.m
    planet.vy += dt * Fy / planet.m
    planet.vz += dt * Fz / planet.m
    
def step_time(planet, time_span, n_steps):
    """Make a number of time steps forward """
    
    dt = time_span / n_steps
    for j in xrange(n_steps):
        single_step(planet, dt)



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