Correct day angle in ASCE extraterrestrial irradiance model

[cwhanse]

pvlib python pull request guidelines

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The following items must be addressed before the code can be merged. Please don't hesitate to ask for help if you're unsure of how to accomplish any of the items below:

• Closes Bug in extraterrestrial radiation calculation #211
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):

Change day angle equation to 2 pi / 365 * doy, where doy = 1, …, 365

[mikofski]

LGTM! Can we add the ASCE reference you mentioned in the issue #211 ?

ASCE (2005), THE ASCE STANDARDIZED REFERENCE EVAPOTRANSPIRATION EQUATION, Environmental and Water Resources Institute of the American Civil Engineers, Ed. R. G. Allen et al.

also fix the what's new conflict

also, I like the TODO: to deligate to submethods - for a rainy day but good idea to keep in mind

[wholmgren]

I don't understand the LGTM alert and codecov result. Suggests that the ASCE method is not actually being tested but it looks like it should be. The associated test looks ok though.

[wholmgren]

Ah, the test fails to pass the method kwarg to the function. Lots of failures when fixed, but these can be mitigated by increasing the test tolerance. Suggested code:

@pytest.mark.parametrize('testval, expected', [
    (doy, value),
    (np.float64(doy), value),
    (dt_date, value),
    (dt_datetime, value),
    (dt_np64, value),
    (np.array([doy]), np.array([value])),
    (pd.Series([doy]), np.array([value])),
    (dt_index, pd.Series([value], index=dt_index)),
    (timestamp, value)
])
@pytest.mark.parametrize('method', [
    'asce', 'spencer', 'nrel', pytest.param('pyephem', marks=requires_ephem)])
def test_get_extra_radiation(testval, expected, method):
    out = irradiance.get_extra_radiation(testval, method=method)
    assert_allclose(out, expected, atol=10)

[wholmgren]

Still don't understand the LGTM alert. I confirmed that the kwarg works:

In [5]: import pvlib

In [6]: pvlib.__version__
Out[6]: '0.6.1+23.g12d0678.dirty'

In [7]: pvlib.solarposition._calculate_simple_day_angle?
Signature: pvlib.solarposition._calculate_simple_day_angle(dayofyear, offset=1)
Docstring:
Calculates the day angle for the Earth's orbit around the Sun.

Parameters
----------
dayofyear : numeric
offset : int, default 1
    For the Spencer method, offset=1; for the ASCE method, offset=0

Returns
-------
day_angle : numeric
File:      ~/git_repos/pvlib-python/pvlib/solarposition.py
Type:      function

In [8]: pvlib.solarposition._calculate_simple_day_angle(0)
Out[8]: -0.01721420632103996

In [9]: pvlib.solarposition._calculate_simple_day_angle(0, offset=0)
Out[9]: 0.0

Appveyor failure is unrelated (#713). Merge when ready.

[cwhanse]

Thanks @Schmelzersolar for investigating and reporting the issue.