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80 lines
2.8 KiB
Markdown
80 lines
2.8 KiB
Markdown
# Prime Factors
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Compute the prime factors of a given natural number.
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A prime number is only evenly divisible by itself and 1.
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Note that 1 is not a prime number.
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## Example
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What are the prime factors of 60?
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- Our first divisor is 2. 2 goes into 60, leaving 30.
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- 2 goes into 30, leaving 15.
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- 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
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- 3 goes cleanly into 15, leaving 5.
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- 3 does not go cleanly into 5. The next possible factor is 4.
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- 4 does not go cleanly into 5. The next possible factor is 5.
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- 5 does go cleanly into 5.
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- We're left only with 1, so now, we're done.
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Our successful divisors in that computation represent the list of prime
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factors of 60: 2, 2, 3, and 5.
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You can check this yourself:
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- 2 * 2 * 3 * 5
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- = 4 * 15
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- = 60
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- Success!
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## Exception messages
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Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
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indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
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every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
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a message.
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To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
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`raise Exception`, you should write:
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```python
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raise Exception("Meaningful message indicating the source of the error")
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```
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## Running the tests
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To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):
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- Python 2.7: `py.test prime_factors_test.py`
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- Python 3.4+: `pytest prime_factors_test.py`
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Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
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`python -m pytest prime_factors_test.py`
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### Common `pytest` options
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- `-v` : enable verbose output
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- `-x` : stop running tests on first failure
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- `--ff` : run failures from previous test before running other test cases
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For other options, see `python -m pytest -h`
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## Submitting Exercises
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Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/prime-factors` directory.
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You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.
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For more detailed information about running tests, code style and linting,
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please see [Running the Tests](http://exercism.io/tracks/python/tests).
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## Source
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The Prime Factors Kata by Uncle Bob [http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata](http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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