Revert "Replace set union_update() references with update()"

This reverts commit 7e3d032cc92af73988f02f281ab354de7fc8e3ab.   the
PR was not provided with a test case or bug report and this is actually
an uncovered code bug.  will re-submit with a proper bug report

diff --git a/alembic/autogenerate/api.py b/alembic/autogenerate/api.py
index 15b5b6b0bcb6e141abb858574c24a511be209f7c..b54e6aa2f87a94f291c9536670cd7fc29dd5cedd 100644 (file)
--- a/alembic/autogenerate/api.py
+++ b/alembic/autogenerate/api.py
@@ -395,7 +395,7 @@ class RevisionContext(object):
             # renders
             autogen_context.imports = set()
             if migration_script.imports:
-                autogen_context.imports.update(migration_script.imports)
+                autogen_context.imports.union_update(migration_script.imports)
             render._render_python_into_templatevars(
                 autogen_context, migration_script, template_args
             )