Lambert conformal secant latitudes

lib/iris/coord_systems.py

@@ -776,11 +776,15 @@ def __init__(self, central_lat=39.0, central_lon=-96.0,
         self.false_easting = false_easting
         #: Y offset from planar origin in metres.
         self.false_northing = false_northing
-        #: The two standard parallels of the cone.
+        #: The one or two standard parallels of the cone.
         try:
             self.secant_latitudes = tuple(secant_latitudes)
         except TypeError:
             self.secant_latitudes = (secant_latitudes,)
+        nlats = len(self.secant_latitudes)
+        if nlats == 0 or nlats > 2:
+            emsg = 'Either one or two secant latitudes required, got {}'
+            raise ValueError(emsg.format(nlats))
         #: Ellipsoid definition.
         self.ellipsoid = ellipsoid
 
@@ -796,7 +800,9 @@ def as_cartopy_crs(self):
         # We're either north or south polar. Set a cutoff accordingly.
         if self.secant_latitudes is not None:
             lats = self.secant_latitudes
-            max_lat = lats[0] if abs(lats[0]) > abs(lats[1]) else lats[1]
+            max_lat = lats[0]
+            if len(lats) == 2:
+                max_lat = lats[0] if abs(lats[0]) > abs(lats[1]) else lats[1]
             cutoff = -30 if max_lat > 0 else 30
         else:
             cutoff = None

lib/iris/tests/test_coordsystem.py

@@ -30,11 +30,13 @@
 import iris.coords
 import iris.tests.stock
 
-from iris.coord_systems import *
+from iris.coord_systems import (GeogCS, LambertConformal, RotatedGeogCS,
+                                Stereographic, TransverseMercator)
 
 
 def osgb():
-    return TransverseMercator(latitude_of_projection_origin=49, longitude_of_central_meridian=-2,
+    return TransverseMercator(latitude_of_projection_origin=49,
+                              longitude_of_central_meridian=-2,
                               false_easting=-400, false_northing=100,
                               scale_factor_at_central_meridian=0.9996012717,
                               ellipsoid=GeogCS(6377563.396, 6356256.909))
@@ -310,6 +312,7 @@ def test_as_cartopy_projection(self):
         res = tmerc_cs.as_cartopy_projection()
         self.assertEqual(res, expected)
 
+
 class Test_Stereographic_construction(tests.IrisTest):
     def test_stereo(self):
         st = stereo()
@@ -374,7 +377,32 @@ def test_as_cartopy_projection(self):
         res = st.as_cartopy_projection()
         self.assertEqual(res, expected)
 
+
 class Test_LambertConformal(tests.GraphicsTest):
+    def test_fail_secant_latitudes_none(self):
+        emsg = 'one or two secant latitudes required'
+        with self.assertRaisesRegexp(ValueError, emsg):
+            LambertConformal(secant_latitudes=())
+
+    def test_fail_secant_latitudes_excessive(self):
+        emsg = 'one or two secant latitudes required'
+        with self.assertRaisesRegexp(ValueError, emsg):
+            LambertConformal(secant_latitudes=(1, 2, 3))
+
+    def test_secant_latitudes_single_value(self):
+        lat_1 = 40
+        lcc = LambertConformal(secant_latitudes=lat_1)
+        ccrs = lcc.as_cartopy_crs()
+        self.assertEqual(lat_1, ccrs.proj4_params['lat_1'])
+        self.assertNotIn('lat_2', ccrs.proj4_params)
+
+    def test_secant_latitudes(self):
+        lat_1, lat_2 = 40, 41
+        lcc = LambertConformal(secant_latitudes=(lat_1, lat_2))
+        ccrs = lcc.as_cartopy_crs()
+        self.assertEqual(lat_1, ccrs.proj4_params['lat_1'])
+        self.assertEqual(lat_2, ccrs.proj4_params['lat_2'])
+
     def test_north_cutoff(self):
         lcc = LambertConformal(0, 0, secant_latitudes=(30, 60))
         ccrs = lcc.as_cartopy_crs()