I rewrote the same program (compare two images, generate a third that shows the diff) in a number of languages. Considering CPython as the reference implementation (1x), I got 100x in Rust, 60x in Go, 12x in Node.js and 10-11x in PyPy.
Initially I got 4x with PyPy but I did a light refactor, removing some map()s and zip()s that were gratitious (3-element lists) and then PyPy went real fast.
Here's the OpenCV way for a pair of 2048x2048 images:
import cv2
import time
t = time.clock()
a = cv2.imread("./image_1.tiff", cv2.IMREAD_GRAYSCALE);
b = cv2.imread("./image_0.tiff", cv2.IMREAD_GRAYSCALE);
c = a-b
cv2.imwrite("out.tiff", c);
print time.clock()-t
#include <opencv2/opencv.hpp>
#include <ctime>
using namespace cv;
using namespace std;
int main(void){
clock_t begin = clock();
Mat a = imread("./image_1.tiff", IMREAD_GRAYSCALE);
Mat b = imread("./image_0.tiff", IMREAD_GRAYSCALE);
Mat c = a - b;
imwrite("out.tiff", c);
clock_t end = clock();
double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
cout << elapsed_secs << endl;
}
I've had a few cases where you can't, and have become a big fan of Cython for that. It lets you add C typedefs to Python code, and then compile the module at import time. Example here: http://pastebin.com/sF8KmyiU
All of pure Python is still allowed in these modules, but the typedeffed variables become pure C variables instead of objects, and loops become pure C loops. For this particular function, I got a 1000x speedup compared to the original Python code.
In the end, this isn't Python any more - but it's close enough, and only needed for loops that run over millions of items.
Also, are you sure that doesn't measure disk speed?
I agree that in pure Python it'd be slower, but realistically why would you do that? Unless you work somewhere where you're forced to write your own libraries... but even then you could still implement your own.
And you would want to do it in pure Python if you want to answer the question "how fast can we make interpreted Python?". Using C extensions for that is cheating, as it isn't Python and it isn't interpreted. You don't answer the question "how fast can you run?" With "30 km an hour, using a bicycle", either.
If you make those images large enough (and I guess 2k x 2k is large enough), any language that uses OpenCV to do the job will give results in the same ballpark. For example, you can make the difference between Python implementations that can call OpenCV as small as you want it.
I agree that the question is valid - making vanilla Python faster is cool. My point was that this particular example (image processing) was flawed, because it's not something a sane person would ever do in pure Python.
Of course it is about the same. Except for function entry and function exit, which should be a few thousand instructions, at the most, it runs the exact same instruction sequence (if you are using identical versions, compiler and compiler flags)
If you want an easily measurable difference, use way smaller images, and make a few thousand or even a few million calls, or look at the python sources to see how efficiently it calls into C.
Python comprises less than 50% of the code in both of those repositories, but they are certainly great for learning the CPython API.
But I think this is the great strength of Python. It's a glue language. If you need speed, you can always write a wrapper around a C/C++ library.
OpenCV is written in C++, it's going to be fairly efficient to call out to it in any language.
For me, most of the time spent in our code is in 'business logic' which necessarily must be in the main language of the codebase.
That's where PyPy gets its wins.
I think the only thing I could say would be that it would be safer? And possible terser and possibly easy to understand.
Note: The first thing anybody will ask when you complain about Rust being slow is whether you compiled with optimizations turned on (`cargo build --release`) since it tends to make a 10-15x difference.