[Sqlalchemy-tickets] Issue #4267: explain example is not working w/ crud (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4267: explain example is not working w/ crud
https://bitbucket.org/zzzeek/sqlalchemy/issues/4267/explain-example-is-not-working-w-crud

Michael Bayer:

I'm not following exactly how this happens:

given https://bitbucket.org/zzzeek/sqlalchemy/wiki/UsageRecipes/Explain

```
#!python


query = delete(MyMapper.__table__).where(MyMapper.my_column == value)
result = session.execute(explain(query)) # exception occurs
```

is said by user to produce:
```
#!python



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'
```

i'm not following how .isinsert/.isdelete/.isupdate is getting set in the first place.   also if these affect the compilation they should be taken into account by the recipe (can add recipe to the tests).