Raise errors for invalid rolling windows arguments (#280)

Raise a specific error if no spacing or shape is passed to
rolling_window(). Raise a specific error if window size is larger than
the smaller dimension of the region. Add tests for these new methods
that include matching error messages to identify which error has been
raised.

verde/coordinates.py

@@ -1015,11 +1015,21 @@ def rolling_window(
     [20. 20. 20. 20. 20. 20. 20. 20. 20.]
 
     """
+    # Check if shape or spacing were passed
+    if shape is None and spacing is None:
+        raise ValueError("Either a shape or a spacing must be provided.")
     # Select the coordinates after checking to make sure indexing will still
     # work on the ignored coordinates.
     coordinates = check_coordinates(coordinates)[:2]
     if region is None:
         region = get_region(coordinates)
+    # Check if window size is bigger than the minimum dimension of the region
+    region_min_width = min(region[1] - region[0], region[3] - region[2])
+    if region_min_width < size:
+        raise ValueError(
+            "Window size '{}' is larger ".format(size)
+            + "than dimensions of the region '{}'.".format(region)
+        )
     # Calculate the region spanning the centers of the rolling windows
     window_region = [
         dimension + (-1) ** (i % 2) * size / 2 for i, dimension in enumerate(region)

verde/tests/test_coordinates.py

@@ -58,6 +58,40 @@ def test_rolling_window_warnings():
             assert str(userwarnings[-1].message).split()[0] == "Rolling"
 
 
+def test_rolling_window_no_shape_or_spacing():
+    """
+    Check if error is raise if no shape or spacing is passed
+    """
+    coords = grid_coordinates((-5, -1, 6, 10), spacing=1)
+    err_msg = "Either a shape or a spacing must be provided."
+    with pytest.raises(ValueError, match=err_msg):
+        rolling_window(coords, size=2)
+
+
+def test_rolling_window_oversized_window():
+    """
+    Check if error is raised if size larger than region is passed
+    """
+    oversize = 5
+    regions = [
+        (-5, -1, 6, 20),  # window larger than west-east
+        (-20, -1, 6, 10),  # window larger than south-north
+        (-5, -1, 6, 10),  # window larger than both dims
+    ]
+    for region in regions:
+        coords = grid_coordinates(region, spacing=1)
+        # The expected error message with regex
+        # (the long expression intends to capture floats and ints)
+        float_regex = r"[+-]?([0-9]*[.])?[0-9]+"
+        err_msg = (
+            r"Window size '{}' is larger ".format(float_regex)
+            + r"than dimensions of the region "
+            + r"'\({0}, {0}, {0}, {0}\)'.".format(float_regex)
+        )
+        with pytest.raises(ValueError, match=err_msg):
+            rolling_window(coords, size=oversize, spacing=2)
+
+
 def test_spacing_to_shape():
     "Check that correct spacing and region are returned"
     region = (-10, 0, 0, 5)