纯Python比Numpy更快？ 我可以更快地制作这个numpy代码吗？

Question

I need to compute the min, max, and mean from a specific list of faces/vertices. I tried to optimize this computing with the use of Numpy but without success.

Here is my test case:

#!/usr/bin/python
# -*- coding: iso-8859-15 -*-
'''
Module Started 22 févr. 2013
@note: test case comparaison numpy vs python
@author: Python4D/damien
'''

import numpy as np
import time


def Fnumpy(vertices):
  np_vertices=np.array(vertices)
  _x=np_vertices[:,:,0]
  _y=np_vertices[:,:,1]
  _z=np_vertices[:,:,2]
  _min=[np.min(_x),np.min(_y),np.min(_z)]
  _max=[np.max(_x),np.max(_y),np.max(_z)]
  _mean=[np.mean(_x),np.mean(_y),np.mean(_z)]
  return _mean,_max,_min

def Fpython(vertices):
  list_x=[item[0] for sublist in vertices for item in sublist]
  list_y=[item[1] for sublist in vertices for item in sublist]
  list_z=[item[2] for sublist in vertices for item in sublist]
  taille=len(list_x)
  _mean=[sum(list_x)/taille,sum(list_y)/taille,sum(list_z)/taille]
  _max=[max(list_x),max(list_y),max(list_z)]
  _min=[min(list_x),min(list_y),min(list_z)]    
  return _mean,_max,_min

if __name__=="__main__":
  vertices=[[[1.1,2.2,3.3,4.4]]*4]*1000000
  _t=time.clock()
  print ">>NUMPY >>{} for {}s.".format(Fnumpy(vertices),time.clock()-_t)
  _t=time.clock()
  print ">>PYTHON>>{} for {}s.".format(Fpython(vertices),time.clock()-_t)

The results are:

Numpy:

([1.1000000000452519, 2.2000000000905038, 3.3000000001880174], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998]) for 27.327068618s.

Python:

([1.100000000045252, 2.200000000090504, 3.3000000001880174], [1.1, 2.2, 3.3], [1.1, 2.2, 3.3]) for 1.81366938593s.

Pure Python is 15x faster than Numpy!

Answer 1

The reason your Fnumpy is slower is that it contains an additional step not done by Fpython : the creation of a numpy array in memory. If you move the line np_verticies=np.array(verticies) outside of Fnumpy and the timed section your results will be very different:

>>NUMPY >>([1.1000000000452519, 2.2000000000905038, 3.3000000001880174], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998]) for 0.500802s.
>>PYTHON>>([1.100000000045252, 2.200000000090504, 3.3000000001880174], [1.1, 2.2, 3.3], [1.1, 2.2, 3.3]) for 2.182239s.

You can also speed up the allocation step significantly by providing a datatype hint to numpy when you create it. If you tell Numpy you have an array of floats, then even if you leave the np.array() call in the timing loop it will beat the pure python version.

If I change np_vertices=np.array(vertices) to np_vertices=np.array(vertices, dtype=np.float_) and keep it in Fnumpy , the Fnumpy version will beat Fpython even though it has to do a lot more work:

>>NUMPY >>([1.1000000000452519, 2.2000000000905038, 3.3000000001880174], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998], [1.1000000000000001, 2.2000000000000002, 3.2999999999999998]) for 1.586066s.
>>PYTHON>>([1.100000000045252, 2.200000000090504, 3.3000000001880174], [1.1, 2.2, 3.3], [1.1, 2.2, 3.3]) for 2.196787s.

Answer 2

As already pointed out by others, your problem is the conversion from list to array. By using the appropriate numpy functions for that, you will beat Python. I modified the main part of your program:

if __name__=="__main__":
  _t = time.clock()
  vertices_np = np.resize(np.array([ 1.1, 2.2, 3.3, 4.4 ], dtype=np.float64), 
                          (1000000, 4, 4))
  print "Creating numpy vertices: {}".format(time.clock() - _t)
  _t = time.clock()
  vertices=[[[1.1,2.2,3.3,4.4]]*4]*1000000
  print "Creating python vertices: {}".format(time.clock() - _t)
  _t=time.clock()
  print ">>NUMPY >>{} for {}s.".format(Fnumpy(vertices_np),time.clock()-_t)
  _t=time.clock()
  print ">>PYTHON>>{} for {}s.".format(Fpython(vertices),time.clock()-_t)

Running your code with the modifed main part results on my machine in:

Creating numpy vertices: 0.6
Creating python vertices: 0.01
>>NUMPY >>([1.1000000000452519, 2.2000000000905038, 3.3000000001880174], 
[1.1000000000000001, 2.2000000000000002, 3.2999999999999998], [1.1000000000000001, 
2.2000000000000002, 3.2999999999999998]) for 0.5s.
>>PYTHON>>([1.100000000045252, 2.200000000090504, 3.3000000001880174], [1.1, 2.2, 3.3], 
[1.1, 2.2, 3.3]) for 1.91s.

Although the array creation is still somewhat longer with Numpy tools as the creation of the nested lists with python's list multiplication operator (0.6s versus 0.01s), you gain a factor of ca. 4 for the run-time relevant part of your code. If I replace the line:

np_vertices=np.array(vertices)

with

np_vertices = np.asarray(vertices)

to avoid the copying of a big array, the running time of the numpy function even goes down to 0.37s on my machine, being more than 5 times faster then the pure python version.

In your real code, if you know the number of vertices in advance, you can preallocate the appropriate array via np.empty() , then fill it with the appropriate data, and pass it to the numpy-version of your function.