Cleanup based on review comments

django/contrib/gis/db/models/constants.py

@@ -1,6 +1,6 @@
 from django.db.models.sql.constants import QUERY_TERMS
 
-ALL_TERMS = set([
+GIS_LOOKUPS = {
     'bbcontains', 'bboverlaps', 'contained', 'contains',
     'contains_properly', 'coveredby', 'covers', 'crosses', 'disjoint',
     'distance_gt', 'distance_gte', 'distance_lt', 'distance_lte',
@@ -9,8 +9,7 @@ ALL_TERMS = set([
     'left', 'right', 'overlaps_left', 'overlaps_right',
     'overlaps_above', 'overlaps_below',
     'strictly_above', 'strictly_below'
-])
-GIS_LOOKUPS = ALL_TERMS.copy()
-ALL_TERMS.update(QUERY_TERMS)
+}
+ALL_TERMS = GIS_LOOKUPS | QUERY_TERMS
 
 __all__ = ['ALL_TERMS', 'GIS_LOOKUPS']

django/db/backends/utils.py

@@ -201,16 +201,20 @@ compile_implementations = defaultdict(dict)
 
 
 def get_implementations(vendor):
-    try:
-        implementation = compile_implementations[vendor]
-    except KeyError:
-        # TODO: do we need thread safety here? We could easily use an lock...
-        implementation = {}
-        compile_implementations[vendor] = implementation
-    return implementation
+    return compile_implementations[vendor]
 
 
 class add_implementation(object):
+    """
+    A decorator to allow customised implementations for query expressions.
+    For example:
+        @add_implementation(Exact, 'mysql')
+        def mysql_exact(node, qn, connection):
+            # Play with the node here.
+            return somesql, list_of_params
+    Now Exact nodes are compiled to SQL using mysql_exact instead of
+    Exact.as_sql() when using MySQL backend.
+    """
     def __init__(self, klass, vendor):
         self.klass = klass
         self.vendor = vendor