Attribute error on '_returning' when compiling Executable ClauseElement

[Charles Langlois]

Following the recipe to create an 'explain' clause element on the bitbucket repository, trying to execute a query using `explain` result in an attribute error:

SubTest error: Traceback (most recent call last):
  File "/usr/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/lib/python3.6/unittest/case.py", line 523, in subTest
    yield
  File "/home/charles/PycharmProjects/my_project/models.py", line 39, in test_delete_cascade
    result = self.session.execute(explain_query).fetchall()
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1170, in execute
    bind, close_with_result=True).execute(clause, params or {})
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 269, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1053, in _execute_clauseelement
    if not self.schema_for_object.is_default else None)
  File "<string>", line 1, in <lambda>
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 442, in compile
    return self._compiler(dialect, bind=bind, **kw)
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 448, in _compiler
    return dialect.statement_compiler(dialect, self, **kw)
  File "/home/charles/PycharmProjects/my_project/.env/lib/python3.6/site-packages/sqlalchemy/sql/compiler.py", line 457, in __init__
    ) and statement._returning:
AttributeError: 'explain' object has no attribute '_returning'

Adding 'self._returning = False' in the __init__ of explain solves the issue. Maybe something is missing in the documentation?

[Mike Bayer]

I would assume you're putting an insert(), update() , or delete() into
the explain() ? that codepath wouldn't be called otherwise, or if
you can show me exactly how to reproduce we can update the recipe.
thanks!

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[Charles Langlois]

Yes, that is the case, sorry for the lack of context.

Example:

query = delete(MyMapper.__table__).where(MyMapper.my_column == value)

result = session.execute(explain(query)) # exception occurs

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Charles Langlois

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[Mike Bayer]

I've added https://bitbucket.org/zzzeek/sqlalchemy/issues/4267/explain-example-is-not-working-w-crud to look at this at some point.

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[Charles Langlois]

Ok, thanks! For now, I guess setting self._returning is a reasonable workaround?

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[Mike Bayer]

yup, as long as the recipe is doing what you need it's all good.   not sure if that prevents the RETURNING phrase from emitting, though, that's part of what i want to look into.   might change the query plan.

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