make URL immutable

it's not really correct that URL is mutable and doesn't do
any argument checking.   propose replacing it with an immutable
named tuple with rich copy-and-mutate methods.

At the moment this makes a hard change to the CreateEnginePlugin
docs that previously recommended url.query.pop().  I can't find
any plugins on github other than my own that are using this
feature, so see if we can just make a hard change on this one.

Fixes: #5526
Change-Id: I28a0a471d80792fa8c28f4fa573d6352966a4a79

1 files changed, 4 insertions, 7 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/provision.py b/lib/sqlalchemy/dialects/mysql/provision.py
index bbe752d78..a1d82222d 100644
--- a/lib/sqlalchemy/dialects/mysql/provision.py
+++ b/lib/sqlalchemy/dialects/mysql/provision.py
@@ -1,5 +1,3 @@
-import copy
-
 from ... import exc
 from ...testing.provision import configure_follower
 from ...testing.provision import create_db
@@ -9,7 +7,7 @@ from ...testing.provision import temp_table_keyword_args
 
 
 @generate_driver_url.for_db("mysql", "mariadb")
-def generate_driver_url(url, driver, query):
+def generate_driver_url(url, driver, query_str):
     backend = url.get_backend_name()
 
     if backend == "mysql":
@@ -17,10 +15,9 @@ def generate_driver_url(url, driver, query):
         if dialect_cls._is_mariadb_from_url(url):
             backend = "mariadb"
 
-    new_url = copy.copy(url)
-    new_url.query = dict(new_url.query)
-    new_url.drivername = "%s+%s" % (backend, driver)
-    new_url.query.update(query)
+    new_url = url.set(
+        drivername="%s+%s" % (backend, driver)
+    ).update_query_string(query_str)
 
     try:
         new_url.get_dialect()