WIP: Slicing and refactoring offline

Closes #37 (closed)

km3io/offline.py

@@ -137,6 +137,8 @@ class Usr:
                     basketcache=BASKET_CACHE)[0]
             ]
         except (KeyError, IndexError):  # e.g. old aanet files
+            print("The `usr` fields could not be parsed for the '{}' branch."
+                  .format(name))
             self._usr_names = []
         else:
             self._usr_idx_lookup = {
@@ -264,14 +266,17 @@ class Branch:
             return object.__getattribute__(self, attr)
 
         if attr in self._keymap.keys():  # intercept branch key lookups
-            out = self._branch[self._keymap[attr]].lazyarray(
-                basketcache=BASKET_CACHE)
-            if self._index is not None:
-                out = out[self._index]
-            return out
+            return self.__getkey__(attr)
 
         return object.__getattribute__(self, attr)
 
+    def __getkey__(self, key):
+        out = self._branch[self._keymap[key]].lazyarray(
+            basketcache=BASKET_CACHE)
+        if self._index is not None:
+            out = out[self._index]
+        return out
+
     def __getitem__(self, item):
         """Slicing magic"""
         if isinstance(item, (int, slice)):
@@ -284,6 +289,9 @@ class Branch:
         if isinstance(item, tuple):
             return self[item[0]][item[1]]
 
+        if isinstance(item, str):
+            return self.__getkey__(item)
+
         return self.__class__(self._tree,
                               self._mapper,
                               index=np.array(item),