MAINT: Move possible side assignment to insert metadata method.

The previous workflow was:

- insert basic metadata for all assets

- iterate over all metadata entries. Create a sid if one does not
exist

- call insert_metadata again to update the assigned sid value.

Instead, create a sid, if missing on the initial metadata assignment and
remove the second pass of calling insert_metadata.

Also, since the sid assignment code is only done in this one context,
inline the sid assignment code so that there is less code step through
while looking for where sid assignment is done.

zipline/assets/assets.py

@@ -85,19 +85,6 @@ class AssetFinder(object):
 
         self.populate_cache()
 
-    def _next_free_sid(self):
-        if len(self.cache) > 0:
-            return max(self.cache.keys()) + 1
-        return 0
-
-    def _assign_sid(self, identifier):
-        if hasattr(identifier, '__int__'):
-            return identifier.__int__()
-        if not self.allow_sid_assignment:
-            raise SidAssignmentError(identifier=identifier)
-        if isinstance(identifier, string_types):
-            return self._next_free_sid()
-
     def retrieve_asset(self, sid, default_none=False):
         if isinstance(sid, Asset):
             return sid
@@ -290,16 +277,6 @@ class AssetFinder(object):
 
     def _spawn_asset(self, identifier, **kwargs):
 
-        # Check if the sid is declared
-        try:
-            kwargs['sid']
-            pass
-        except KeyError:
-            # If the identifier is not a sid, assign one
-            kwargs['sid'] = self._assign_sid(identifier)
-            # Update the metadata object with the new sid
-            self.insert_metadata(identifier=identifier, sid=kwargs['sid'])
-
         # If the file_name is in the kwargs, it will be used as the symbol
         try:
             kwargs['symbol'] = kwargs.pop('file_name')
@@ -545,6 +522,21 @@ class AssetFinder(object):
                 continue
             entry[key] = value
 
+        # Check if the sid is declared
+        try:
+            entry['sid']
+        except KeyError:
+            # If the identifier is not a sid, assign one
+            if hasattr(identifier, '__int__'):
+                entry['sid'] = identifier.__int__()
+            else:
+                if self.allow_sid_assignment:
+                    # Assign the sid the value of its insertion order.
+                    # This assumes that we are assigning values to all assets.
+                    entry['sid'] = len(self.metadata_cache)
+                else:
+                    raise SidAssignmentError(identifier=identifier)
+
         self.metadata_cache[identifier] = entry
 
     def consume_identifiers(self, identifiers):