Question

it an answer for Euler project #4.

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99.

Find the largest palindrome made from the product of two 3-digit numbers.

Answer:

906609

code is:

from multiprocessing import Pool
from itertools import product

def sym(lst):
    rst=[]
    for x,y in lst:
        tmp=x*y
        if rec(tmp):
            rst.append(tmp)
    return rst

def rec(num):
    num=str(num)
    if num == "".join(reversed(num)):    return True
    else:    return False

if __name__ == "__main__":
    pool=Pool(processes=8)
    lst=product(xrange(100,1000),repeat=2)
    rst=pool.map(sym,lst)
    #rst=sym(lst)
    print max(rst)

when I run this:

#    TypeError:'int' object is not iterable

but I can’t understand it…isn’t list iterable? or is there an error in my code?

Answer 1

Your problem is with the function sym.

sym is being passed the first element of your product iterable. (e.g. lst = (100,100)). When you get to the for loop, you’re iterating over lst and then trying to unpack it into two numbers — equivalent to:

for x,y in (100,100):
    ...

Which fails for obvious reasons.

I think you probably want to get rid of the for loop all-together — which was probably an artifact of your serial version.

def sym(lst):
    x,y=lst
    tmp=x*y
    if rec(tmp):
        return tmp
    else:
        return None  #max will ignore None values since None > x is always False.

The traceback was somewhat cryptic — Apparently the traceback gets returned to the Pool which then gets re-raised … But the way it is done makes it a little difficult to track.

Sometimes, when debugging these things it is helpful to replace Pool.map() with the regular version of map. Then, any exceptions which get raised are raised on your main “thread” and the tracebacks can be a little easier to follow.