defer ForecastModel network operations until necessary (#332)

* dont connect to unidata unless its necessary

* add note to whatsnew

Author: wholmgren

docs/sphinx/source/whatsnew/v0.4.5.txt

@@ -26,6 +26,9 @@ Enhancements
 
 * Added irradiance.dni method that determines DNI from GHI and DHI and corrects
   unreasonable DNI values during sunrise/sunset transitions
+* ForecastModel will now only connect to the Unidata server when necessary,
+  rather than when the object is created. This supports offline work
+  and speeds up analysis of previously downloaded data.
 
 
 Contributors

pvlib/forecast.py

@@ -118,12 +118,16 @@ def __init__(self, model_type, model_name, set_type):
         self.model_type = model_type
         self.model_name = model_name
         self.set_type = set_type
+        self.connected = False
+
+    def connect_to_catalog(self):
         self.catalog = TDSCatalog(self.catalog_url)
-        self.fm_models = TDSCatalog(self.catalog.catalog_refs[model_type].href)
+        self.fm_models = TDSCatalog(
+            self.catalog.catalog_refs[self.model_type].href)
         self.fm_models_list = sorted(list(self.fm_models.catalog_refs.keys()))
 
         try:
-            model_url = self.fm_models.catalog_refs[model_name].href
+            model_url = self.fm_models.catalog_refs[self.model_name].href
         except ParseError:
             raise ParseError(self.model_name + ' model may be unavailable.')
 
@@ -137,6 +141,7 @@ def __init__(self, model_type, model_name, set_type):
 
         self.datasets_list = list(self.model.datasets.keys())
         self.set_dataset()
+        self.connected = True
 
     def __repr__(self):
         return '{}, {}'.format(self.model_name, self.set_type)
@@ -224,6 +229,10 @@ def get_data(self, latitude, longitude, start, end,
         forecast_data : DataFrame
             column names are the weather model's variable names.
         """
+
+        if not self.connected:
+            self.connect_to_catalog()
+
         if vert_level is not None:
             self.vert_level = vert_level