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# Calculate Today's Moon Phase Using Python, ephem, and unittest - A Comprehensive Guide

Theo van der Sluijs
·May 3, 2023·

Have you ever wondered how to calculate the moon phase for today or any specific date using Python? In this article, I will guide you through the process of fetching the moon phase using the `ephem` library and creating a well-structured Python project using `poetry` for dependency management and packaging. I will also write tests for our code using the `unittest` library.

## Project Setup

First, let's set up a new Python project using `poetry`. Follow these steps:

1. Install Poetry if you haven't already:
``````curl -sSL https://install.python-poetry.org | python3 -
``````
1. Create a new project and navigate to the project directory:
``````poetry new moon_phase
cd moon_phase
``````
1. Add the `ephem` library as a dependency:
``````poetry add ephem
``````

Now that we have our project set up, let's dive into writing the code.

## Calculating Moon Phase

We'll create a function called `get_moon_phase()` in a new Python file named `moon_phase.py`. This function takes an optional date parameter; if no date is provided, it will use the current date.

``````import ephem
import datetime

def get_moon_phase(date=None):
"""
This function returns the moon phase for a given date.
If no date is provided, the current date is used.

:param date: The date for which the moon phase is fetched.
:type date: datetime.date, optional
:return: The moon phase (0: New Moon, 0.5: First Quarter, 1.0: Full Moon)
:rtype: float
"""
if date is None:
date = datetime.datetime.now()

moon = ephem.Moon(date)
phase = moon.moon_phase
return phase
``````

Here's a breakdown of what the code does:

1. Import the required libraries - `ephem` for moon phase calculations and `datetime` to handle dates.

2. Define the `get_moon_phase()` function, which takes an optional `date` parameter.

3. If no date is provided, use the current date.

4. Create a `ephem.Moon` object for the given date.

5. Retrieve the `moon_phase` attribute of the `Moon` object, which is a float between 0 and 1.

6. Return the moon phase.

Now, if you want to fetch the moon phase for today, you can simply call the `get_moon_phase()` function:

``````moon_phase = get_moon_phase()
print(f"Today's moon phase is: {moon_phase:.2f}")
``````

## Writing Unit Tests

We'll write unit tests for our `get_moon_phase()` function using Python's built-in `unittest` library. Create a new Python file named `test_moon_phase.py` and add the following code:

``````import unittest
from moon_phase import get_moon_phase
import datetime

class TestMoonPhase(unittest.TestCase):

def test_get_moon_phase_known_date(self):
# Test with a known date, January 1, 2000
test_date = datetime.date(2000, 1, 1)
expected_phase = 0.04  # Known moon phase for this date
actual_phase = get_moon_phase_phase(test_date)
self.assertAlmostEqual(expected_phase, actual_phase, places=2)
def test_get_moon_phase_today(self):
# Test with the current date
test_date = datetime.datetime.now()
actual_phase = get_moon_phase(test_date)
self.assertIsInstance(actual_phase, float)
self.assertTrue(0 <= actual_phase <= 1)

if name == 'main':
unittest.main()
``````

In our test suite, we have two test cases:

1. `test_get_moon_phase_known_date`: This test checks if the function returns the correct moon phase for a known date (January 1, 2000, in this case). We compare the expected moon phase with the actual moon phase returned by the function using `assertAlmostEqual` with a precision of 2 decimal places.

2. `test_get_moon_phase_today`: This test checks if the function returns a valid moon phase for the current date. We first assert that the returned value is a float, and then check that the value is between 0 and 1 (inclusive), as expected for a moon phase.

To run the tests, use `poetry`:

``````poetry run python -m unittest test_moon_phase.py
``````

If everything is implemented correctly, all tests should pass. You can now run the `moon_phase.py` script to fetch the current moon phase using `poetry run python moon_phase.py`.

## Conclusion

In this article, we demonstrated how to calculate the moon phase for any given date using Python and the `ephem` library. We also used `poetry` for dependency management and packaging, and wrote unit tests with Python's built-in `unittest` library.

Now you have a solid foundation for expanding your project, such as creating a web application or a command-line tool to fetch the moon phase for any date. Happy coding!