exposing if_not_exists argument in create_index

Author: dantownsend

piccolo/apps/migrations/auto/migration_manager.py

@@ -415,7 +415,7 @@ async def _run_alter_columns(self, backwards=False):
                         column._meta._name = column_name
                         await _Table.drop_index([column]).run()
                         await _Table.create_index(
-                            [column], method=index_method
+                            [column], method=index_method, if_not_exists=True
                         ).run()
                 else:
                     # If the index value has changed, then we are either
@@ -427,7 +427,9 @@ async def _run_alter_columns(self, backwards=False):
                         kwargs = (
                             {"method": index_method} if index_method else {}
                         )
-                        await _Table.create_index([column], **kwargs).run()
+                        await _Table.create_index(
+                            [column], if_not_exists=True, **kwargs
+                        ).run()
                     else:
                         await _Table.drop_index([column]).run()
 

piccolo/table.py

@@ -623,14 +623,20 @@ def create_index(
         cls,
         columns: t.List[t.Union[Column, str]],
         method: IndexMethod = IndexMethod.btree,
+        if_not_exists: bool = False,
     ) -> CreateIndex:
         """
         Create a table index. If multiple columns are specified, this refers
         to a multicolumn index, rather than multiple single column indexes.
 
         await Band.create_index([Band.name]).run()
         """
-        return CreateIndex(table=cls, columns=columns, method=method)
+        return CreateIndex(
+            table=cls,
+            columns=columns,
+            method=method,
+            if_not_exists=if_not_exists,
+        )
 
     @classmethod
     def drop_index(