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tl;dr

Here's how to write a function in Python that calculates the factorial of a given number.

First, start by importing the sys module:

import sys

Next, define the function Factorial, which will take one argument, n, and return the factorial of that number:

def Factorial(n): # return factorial

Inside the function, create a variable called result and set it to 1. This will be used to store the result of the calculation:

result = 1

Now, create a for loop that will iterate from 1 to n. For each iteration, multiply the result by the current value of i and store the result in the result variable:

for i in range (1,n):
    result = result * i

Finally, print the result and return it:

print "factorial is ",result
return result

Now, you can call the function with any number to get the factorial of that number:

print Factorial(10)