 # rootulp's solution

## to Prime Factors in the Python Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

## Example

What are the prime factors of 60?

• Our first divisor is 2. 2 goes into 60, leaving 30.
• 2 goes into 30, leaving 15.
• 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
• 3 goes cleanly into 15, leaving 5.
• 3 does not go cleanly into 5. The next possible factor is 4.
• 4 does not go cleanly into 5. The next possible factor is 5.
• 5 does go cleanly into 5.
• We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

• 2 * 2 * 3 * 5
• = 4 * 15
• = 60
• Success!

## Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of `raise Exception`, you should write:

``````raise Exception("Meaningful message indicating the source of the error")
``````

## Running the tests

To run the tests, run the appropriate command below (why they are different):

• Python 2.7: `py.test prime_factors_test.py`
• Python 3.4+: `pytest prime_factors_test.py`

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): `python -m pytest prime_factors_test.py`

### Common `pytest` options

• `-v` : enable verbose output
• `-x` : stop running tests on first failure
• `--ff` : run failures from previous test before running other test cases

For other options, see `python -m pytest -h`

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `\$EXERCISM_WORKSPACE/python/prime-factors` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.

For more detailed information about running tests, code style and linting, please see the help page.

## Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### prime_factors_test.py

``````import unittest

from prime_factors import prime_factors

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.1.0

class PrimeFactorsTest(unittest.TestCase):
def test_no_factors(self):
self.assertEqual(prime_factors(1), [])

def test_prime_number(self):
self.assertEqual(prime_factors(2), )

def test_square_of_a_prime(self):
self.assertEqual(prime_factors(9), [3, 3])

def test_cube_of_a_prime(self):
self.assertEqual(prime_factors(8), [2, 2, 2])

def test_product_of_primes_and_non_primes(self):
self.assertEqual(prime_factors(12), [2, 2, 3])

def test_product_of_primes(self):
self.assertEqual(prime_factors(901255), [5, 17, 23, 461])

def test_factors_include_a_large_prime(self):
self.assertEqual(prime_factors(93819012551), [11, 9539, 894119])

if __name__ == '__main__':
unittest.main()``````
``````def prime_factors(num):
factors = []
i = 2
while i <= num:
if num % i == 0:
factors.append(i)
num /= i
else:
i += 1
return factors``````