improve addition for snowpack and atmosphere. The order of the additi…

…on is important and checked more carefully.

smrt/core/atmosphere.py

@@ -1,8 +1,29 @@
 
 
+from .error import SMRTError
+from .snowpack import Snowpack
+
+
 # this is a temporary solution to deal with Atmosphere.
 # user should not rely on this object. For temporary internal use only !!
 
 class AtmosphereBase(object):
     # has no special properties yet, we just use the type.
-    pass
+
+    def __add__(self, other):
+        """Return a new snowpack made by setting the atmosphere
+
+        :param other: the snowpack to add.
+"""
+
+        if not isinstance(other, Snowpack):
+            raise SMRTError("Attempt to add an incorrect object to an atmopsher. Only adding an atmosphere and a snowpack (in that order)"
+                            " is a valid operation.")
+
+        return Snowpack(layers=other.layers,
+                        interfaces=other.interfaces,
+                        substrate=other.substrate,
+                        atmosphere=self)
+
+    def __iadd__(self, other):
+        raise SMRTError("Inplace addition with an atmosphere is not a valid operation.")

smrt/core/snowpack.py

@@ -25,7 +25,6 @@
 from ..interface.flat import Flat  # core should not depend on something defined in interface...
 from .layer import Layer
 from .interface import SubstrateBase
-from .atmosphere import AtmosphereBase
 
 
 class Snowpack(object):
@@ -117,12 +116,15 @@ def basic_check(self):
 
     def check_addition_validity(self, other):
 
+        # import here to avoid circular reference
+        from .atmosphere import AtmosphereBase
+
         if isinstance(other, SubstrateBase):
             if self.substrate is not None:
-                raise SMRTError("Adding a substrate to a snowpack that already has one is not valid. Unset the substrate first")            
+                raise SMRTError("Adding a substrate to a snowpack that already has a substrate set is not valid."
+                                " Unset the substrate first.")
         elif isinstance(other, AtmosphereBase):
-            if self.atmosphere is not None:
-                raise SMRTError("Adding an atmosphere to a snowpack that already has one is not valid. Unset the atmosphere first")            
+            raise SMRTError("Adding an atmosphere to a snowpack is not allowed. Add an atmosphere and a snowpack.")
         elif not (hasattr(other, "layers") and hasattr(other, "interfaces") and hasattr(other, "substrate")):
             raise SMRTError("Addition of snowpacks requires two instances of class Snowpack or equivalent compatible objects")
 
@@ -150,11 +152,6 @@ def __add__(self, other):
                             interfaces=self.interfaces,
                             atmosphere=self.atmosphere,
                             substrate=other)
-        if isinstance(other, AtmosphereBase):
-            return Snowpack(layers=self.layers,
-                            interfaces=self.interfaces,
-                            substrate=self.substrate,
-                            atmosphere=other)
         else:
             return Snowpack(layers=self.layers + other.layers,
                             interfaces=self.interfaces + other.interfaces,
@@ -167,6 +164,7 @@ def __radd__(self, other):
             return self
         else:
             # should never be called
+            raise SMRTError("The addition operator is not commutative for snowpacks")
             return other.__add__(self)
 
     def __iadd__(self, other):  # just for optimization
@@ -177,8 +175,6 @@ def __iadd__(self, other):  # just for optimization
 
         if isinstance(other, SubstrateBase):
             self.substrate = other
-        elif isinstance(other, AtmosphereBase):
-            self.atmosphere = other
         else:
             self.layers += other.layers
             self.interfaces += other.interfaces