A solution to the 3rd Euler Challenge

Euler3

"""The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?"""

#I want to define prime(x) which gives x's prime factors

#Define factor(x)

def factor(x):
    array=[]
    i=1
    while (i<=x):
        if x%i==0:
            array.append(i)
        i+=1    
    return array
#Define prime test
def prime(x):
    if factor(x)==[1,x]:
        return True
    else:
        return False

# Define a function nextPrime(x) that returns the least prime greater than x

def nextPrime(x):
    y=x+1
    while prime(y)==False:
        y+=1
    return y

# Define a function giving the prime factorisation of x
def primeFactorisation(x):
    array=[]
    y=x
    while y>1:
        z=2
        while y%z!=0:
            z=nextPrime(z)
        array.append(z)
        y=y/z
    return array

z=primeFactorisation(600851475143)
print(max(z))