Add a filter_by_keys kwargs to open_dataset to filter the GRIB file i…

…ndex before building the cube. See #2.

cfgrib/dataset.py

@@ -357,8 +357,9 @@ def dict_merge(master, update):
 def build_dataset_components(
         stream,
         encode_parameter=False, encode_time=False, encode_vertical=False, encode_geography=False,
+        filter_by_keys={},
 ):
-    index = stream.index(ALL_KEYS)
+    index = stream.index(ALL_KEYS).subindex(filter_by_keys)
     param_ids = index['paramId']
     dimensions = collections.OrderedDict()
     variables = collections.OrderedDict()
@@ -387,6 +388,7 @@ class Dataset(object):
     encode_time = attr.attrib(default=True)
     encode_vertical = attr.attrib(default=True)
     encode_geography = attr.attrib(default=True)
+    filter_by_keys = attr.attrib(default={}, type=T.Dict[str, T.Any])
 
     @classmethod
     def frompath(cls, path, mode='r', **kwargs):

cfgrib/messages.py

@@ -227,8 +227,8 @@ def getone(self, item):
             raise ValueError("not one value for %r: %r" % (item, len(values)))
         return values[0]
 
-    def subindex(self, dict_query={}, **query):
-        query.update(dict_query)
+    def subindex(self, filter_by_keys={}, **query):
+        query.update(filter_by_keys)
         raw_query = [(self.index_keys.index(k), v) for k, v in query.items()]
         offsets = collections.OrderedDict()
         for header_values in self.offsets:

cfgrib/xarray_store.py

@@ -151,12 +151,15 @@ def get_encoding(self):
         return encoding
 
 
-def open_dataset(path, flavour_name='ecmwf', **kwargs):
-    overrides = {}
+def open_dataset(path, flavour_name='ecmwf', filter_by_keys={}, **kwargs):
+    overrides = {
+        'flavour_name': flavour_name,
+        'filter_by_keys': filter_by_keys,
+    }
     for k in list(kwargs):  # copy to allow the .pop()
         if k.startswith('encode_'):
             overrides[k] = kwargs.pop(k)
-    store = GribDataStore.frompath(path, flavour_name=flavour_name, **overrides)
+    store = GribDataStore.frompath(path, **overrides)
     return _open_dataset(store, **kwargs)