Python: TypeError - not all arguments converted during string formatting [duplicate]

Question

I'm writing a basic Python script, the code of which is as followsdef is_prime(n):

def is_prime(n):
    i = 2
    while i<n:
        if(n%i == 0):
            return False
        i+=1
    return True

def truncable(num):
    li = []
    x = str(num)
    if(is_prime(num)):
        n = len(x)
        check = ""
        i = 1
        while(i<n):
            if(is_prime(x[i:])):
                check = "True"
            else:
                check = "False"
                break
        if(check == "True"):
            li.append(num)

print truncable(3797)

However, after running this script, I get the following error:

TypeError: not all arguments converted during string formatting

What's wrong with my code?

Note that if you'd been using Python 3, you would have gotten a much more helpful error message: TypeError: unorderable types: int() < str() which would have pointed out the underlying problem immediately. — DSM
– DSM, Commented Apr 19, 2014 at 15:47
Note that the print will result in None, since you aren't returning anything from truncable. — juanchopanza
– juanchopanza, Commented Apr 19, 2014 at 15:47

Martijn Pieters · Accepted Answer · 2014-04-19 15:45:11Z

6

This happens when n in the expression n%i is a string, not an integer. You made x a string, then passed it to is_prime():

>>> is_prime('5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in is_prime
TypeError: not all arguments converted during string formatting
>>> is_prime(5)
True

Pass integers to is_prime() instead:

is_prime(int(x[i:]))

The % operator on a string is used for string formatting operations.

You appear to have forgotten to return anything from truncable(); perhaps you wanted to add:

return li

at the end?

answered Apr 19, 2014 at 15:45

Martijn Pieters

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