Refactored PatternLookup to improve readability.

Author: timgraham

django/db/models/lookups.py

@@ -578,32 +578,41 @@ class PatternLookup(BuiltinLookup):
     prepare_rhs = False
 
     def get_rhs_op(self, connection, rhs):
-        # Assume we are in startswith. We need to produce SQL like:
-        #     col LIKE %s, ['thevalue%']
-        # For python values we can (and should) do that directly in Python,
-        # but if the value is for example reference to other column, then
-        # we need to add the % pattern match to the lookup by something like
+        # If the lookup value is a reference to another column or a transform,
+        # then the SQL is something like:
         #     col LIKE othercol || '%%'
-        # So, for Python values we don't need any special pattern, but for
-        # SQL reference values or SQL transformations we need the correct
-        # pattern added.
-        if hasattr(self.rhs, "as_sql") or self.bilateral_transforms:
+        if not self.is_simple_lookup:
+            # In that case, prepare the LIKE clause using
+            # DatabaseWrapper.pattern_ops.
             pattern = connection.pattern_ops[self.lookup_name].format(
                 connection.pattern_esc
             )
             return pattern.format(rhs)
         else:
+            # Otherwise, use the LIKE in DatabaseWrapper.operators.
             return super().get_rhs_op(connection, rhs)
 
     def process_rhs(self, qn, connection):
         rhs, params = super().process_rhs(qn, connection)
-        if self.rhs_is_direct_value() and params and not self.bilateral_transforms:
+        # Assume the lookup is startswith. For simple lookups involving a
+        # Python value like "thevalue", the (SQL, params) is something like:
+        #     ("col LIKE %s", ['thevalue%'])
+        if self.is_simple_lookup:
+            # Prepare the lookup parameter, a Python value, for use in a LIKE
+            # clause, usually by adding % signs to the beginning and/or end of
+            # the value.
             param = connection.ops.prep_for_like_query(params[0])
             if connection.features.pattern_lookup_needs_param_pattern:
                 param = self.param_pattern % param
             params = (param, *params[1:])
         return rhs, params
 
+    @property
+    def is_simple_lookup(self):
+        # A "simple lookup" is a Python value (as long as it's not a bilateral
+        # transform).
+        return self.rhs_is_direct_value() and not self.bilateral_transforms
+
 
 @Field.register_lookup
 class Contains(PatternLookup):