fix rounding issue in linke turbidity lookup function (pvlib#755)

* Resolve merge conflicts.

The upstream what's new file had been updated.

* Copy whatsnew file from upstream.

* Remove unnecessary np.around().

Instead of rounding to one decimal place, then comparing to .5,
just compare to .55 right away which preserves the same behavior as
before. If you want to stay true to the comment on line 314, use
<= .5 in comparison instead as before, but still drop the np.around().

* Use <= .5 as comparisons in _linearly_scale.

* Correct previous commit, use <= .5.

* Update what's new file.

Closes pvlib#754

* Compare to 0.500001 for margin of error.

* Rename _linearly_scale -> _degrees_to_index, refactor.

This commit reduces inputs to just two, refactors some calculations, and mainly tries
to be much clearer with the documentation.

* Add test for clearsky._degrees_to_index().

Tests that an invalid value for the degree_type argument raises an error.

* Remove try/except/finally.

The exception that is being checked for here is already checked for in the
_degrees_to_index method, so there's no need to do the check again. So this
removes the except portion, and just keeps the try and finally parts.

* Update parameter type description.

* Fix linting errors.

* Fix typo.

* Give more detail in what's new description.

* Rename arg, use context manager, and add more tech. documentation.

* Add missing zero in documentation.

* Break up line so it's < 80 characters.

* Change arg type to float or int.

* Update argument name in test_degrees_to_index_1().

Author: alexandermorgan

docs/sphinx/source/whatsnew/v0.7.0.rst

@@ -20,6 +20,10 @@ Bug fixes
 * Fix handling of keyword arguments in `forecasts.get_processed_data`.
   (:issue:`745`)
 * Fix output as Series feature in :py:func:`pvlib.pvsystem.ashraeiam`.
+* Fix rounding issue in `clearsky._linearly_scale`, a function that converts
+  longitude or latitude degree to an index number in a Linke turbidity lookup
+  table. Also rename the function to `clearsky._degrees_to_index`.
+  (:issue:`754`)
 
 Testing
 ~~~~~~~

pvlib/clearsky.py

@@ -149,9 +149,9 @@ def lookup_linke_turbidity(time, latitude, longitude, filepath=None,
     ----------
     time : pandas.DatetimeIndex
 
-    latitude : float
+    latitude : float or int
 
-    longitude : float
+    longitude : float or int
 
     filepath : None or string, default None
         The path to the ``.h5`` file.
@@ -193,22 +193,12 @@ def lookup_linke_turbidity(time, latitude, longitude, filepath=None,
         pvlib_path = os.path.dirname(os.path.abspath(__file__))
         filepath = os.path.join(pvlib_path, 'data', 'LinkeTurbidities.h5')
 
-    latitude_index = (
-        np.around(_linearly_scale(latitude, 90, -90, 0, 2160))
-        .astype(np.int64))
-    longitude_index = (
-        np.around(_linearly_scale(longitude, -180, 180, 0, 4320))
-        .astype(np.int64))
+    latitude_index = _degrees_to_index(latitude, coordinate='latitude')
+    longitude_index = _degrees_to_index(longitude, coordinate='longitude')
 
-    lt_h5_file = tables.open_file(filepath)
-    try:
+    with tables.open_file(filepath) as lt_h5_file:
         lts = lt_h5_file.root.LinkeTurbidity[latitude_index,
                                              longitude_index, :]
-    except IndexError:
-        raise IndexError('Latitude should be between 90 and -90, '
-                         'longitude between -180 and 180.')
-    finally:
-        lt_h5_file.close()
 
     if interp_turbidity:
         linke_turbidity = _interpolate_turbidity(lts, time)
@@ -299,28 +289,65 @@ def _calendar_month_middles(year):
     return middles
 
 
-def _linearly_scale(inputmatrix, inputmin, inputmax, outputmin, outputmax):
-    """linearly scale input to output, used by Linke turbidity lookup"""
+def _degrees_to_index(degrees, coordinate):
+    """Transform input degrees to an output index integer. The Linke
+    turbidity lookup tables have three dimensions, latitude, longitude, and
+    month. Specify a degree value and either 'latitude' or 'longitude' to get
+    the appropriate index number for the first two of these index numbers.
+
+    Parameters
+    ----------
+    degrees : float or int
+        Degrees of either latitude or longitude.
+    coordinate : string
+        Specify whether degrees arg is latitude or longitude. Must be set to
+        either 'latitude' or 'longitude' or an error will be raised.
+
+    Returns
+    -------
+    index : np.int16
+        The latitude or longitude index number to use when looking up values
+        in the Linke turbidity lookup table.
+    """
+    # Assign inputmin, inputmax, and outputmax based on degree type.
+    if coordinate == 'latitude':
+        inputmin = 90
+        inputmax = -90
+        outputmax = 2160
+    elif coordinate == 'longitude':
+        inputmin = -180
+        inputmax = 180
+        outputmax = 4320
+    else:
+        raise IndexError("coordinate must be 'latitude' or 'longitude'.")
+
     inputrange = inputmax - inputmin
-    outputrange = outputmax - outputmin
-    delta = outputrange/inputrange  # number of indices per input unit
-    inputmin = inputmin + 1.0 / delta / 2.0  # shift to center of index
-    outputmax = outputmax - 1  # shift index to zero indexing
-    outputmatrix = (inputmatrix - inputmin) * delta + outputmin
+    scale = outputmax/inputrange  # number of indices per degree
+    center = inputmin + 1 / scale / 2  # shift to center of index
+    outputmax -= 1  # shift index to zero indexing
+    index = (degrees - center) * scale
     err = IndexError('Input, %g, is out of range (%g, %g).' %
-                     (inputmatrix, inputmax - inputrange, inputmax))
-    # round down if input is within half an index or else raise index error
-    if outputmatrix > outputmax:
-        if np.around(outputmatrix - outputmax, 1) <= 0.5:
-            outputmatrix = outputmax
+                     (degrees, inputmin, inputmax))
+
+    # If the index is still out of bounds after rounding, raise an error.
+    # 0.500001 is used in comparisons instead of 0.5 to allow for a small
+    # margin of error which can occur when dealing with floating point numbers.
+    if index > outputmax:
+        if index - outputmax <= 0.500001:
+            index = outputmax
         else:
             raise err
-    elif outputmatrix < outputmin:
-        if np.around(outputmin - outputmatrix, 1) <= 0.5:
-            outputmatrix = outputmin
+    elif index < 0:
+        if -index <= 0.500001:
+            index = 0
         else:
             raise err
-    return outputmatrix
+    # If the index wasn't set to outputmax or 0, round it and cast it as an
+    # integer so it can be used in integer-based indexing.
+    else:
+        index = int(np.around(index))
+
+    return index
 
 
 def haurwitz(apparent_zenith):

pvlib/test/test_clearsky.py

@@ -519,6 +519,13 @@ def monthly_lt_nointerp(lat, lon, time=months):
         monthly_lt_nointerp(38.2, -181)  # exceeds min longitude
 
 
+def test_degrees_to_index_1():
+    """Test that _degrees_to_index raises an error when something other than
+    'latitude' or 'longitude' is passed."""
+    with pytest.raises(IndexError):  # invalid value for coordinate argument
+        clearsky._degrees_to_index(degrees=22.0, coordinate='width')
+
+
 @pytest.fixture
 def detect_clearsky_data():
     test_dir = os.path.dirname(os.path.abspath(