在这种情况下，python比C慢得多的原因是什么？

Question

I was solving some problems on project euler and I wrote identical functions for problem 10...

The thing that amazes me is that the C solution runs in about 4 seconds while the python solution takes about 283 seconds. I am struggling to explain to myself why the C implementation is so much faster than the python implementation, what is actually happening to make it so?

C:

#include <stdio.h>
#include <time.h>
#include <math.h>

int is_prime(int num)
{
    int sqrtDiv = lround(sqrt(num));
    while (sqrtDiv > 1) {
        if (num % sqrtDiv == 0) {
            return(0);
        } else {
            sqrtDiv--;
        }
    }
    return(1);
}

int main () 
{
    clock_t start = clock();

    long sum = 0;
    for ( int i = 2; i < 2000000; i++ ) {
        if (is_prime(i)) {
            sum += i;
        }
    }
    printf("Sum of primes below 2,000,000 is: %ld\n", sum);

    clock_t end = clock();
    double time_elapsed_in_seconds = (end - start)/(double)CLOCKS_PER_SEC;
    printf("Finished in %f seconds.\n", time_elapsed_in_seconds);   
}

Python:

from math import sqrt
import time


def is_prime(num):
    div = round(sqrt(num))
    while div > 1:
        if num % div == 0:
            return False
        div -= 1
    return True

start_time = time.clock()

tsum = 0
for i in range(2, 2000000):
    if is_prime(i):
        tsum += i

print tsum
print('finished in:', time.clock() - start_time, 'seconds')

Answer 1

It's CPython (the implementation) that's slow in this case, not Python necessarily. CPython needs to interpret the bytecode which will almost always be slower than compiled C code. It simply does more work than the equivalent C code. In theory each call to sqrt for example requires looking up that function, rather than just a call to a known address.

If you want comparable speed from Python, you could either annotate the source with types and compile with Cython, or try running with Pypy for some JIT performance.