The "is not None" is important when checking a variable that may
contain a ClauseElement, precisely because ClauseElement defines that
__bool__ method.
However, in Session.get_bind(), "mapper" is not supposed to contain a
ClauseElement. It should either be an instance of
sqlalchemy.orm.mapper.Mapper, or None, in which case "if mapper:" is a
valid and concise way to write it. Comparing to None might be strictly
more accurate, but it shouldn't be necessary.
Simon
On Fri, Oct 20, 2017 at 3:17 PM, Антонио Антуан <a.ch....@gmail.com> wrote:
> Really, `mapper` contains this: <sqlalchemy.sql.selectable.CTE at
> 0x7fe673fb0a50; group_getter>. But does it changes something? I already
> encountered this problem in my code, when I checked sqla-objects existance
> without "is not None", project was broken. Is it normal to omit that
> condition in sqlalchemy code?
>
> I use:
>>>> sqlalchemy.__version__
> '1.0.19'
>
> пятница, 20 октября 2017 г., 16:42:23 UTC+3 пользователь Simon King написал:
>>
>> On Fri, Oct 20, 2017 at 2:15 PM, Антонио Антуан <a.ch...@gmail.com> wrote:
>> > Hi.
>> > I use my own `RoutingSession` and `RoutingQuery` implementation, most of
>> > it
>> > inspired by `sqlalchemy.ext.horizontal_shard`:
>> >
>> > class RoutingSession(Session):
>> > def get_bind(self, mapper=None, clause=None, shard_id=None,
>> > **kwargs):
>> > original_bind = None
>> > try:
>> > original_bind = super(RoutingSession, self).get_bind(mapper,
>> > clause)
>> > except UnboundExecutionError:
>> > # may not be bound
>> > pass
>> > if shard_id is None:
>> > shard_id = TenantIDStorage.get_shard_id() # just global
>> > storage
>> > bind_for_shard = self.__binds[shard_id]
>> > if original_bind is not None and original_bind.url ==
>> > bind_for_shard.url:
>> > return original_bind
>> > else:
>> > return bind_for_shard
>> >
>> > def __init__(self, shards=None, query_cls=CachingQuery,
>> > engines_factory=None, **kwargs):
>> > super(RoutingSession, self).__init__(query_cls=query_cls,
>> > **kwargs)
>> > self.__binds = {}
>> > self.engines_factory = engines_factory
>> > if shards is not None:
>> > self.update_shards(**shards)
>> >
>> > def _add_bind(self, key, bind):
>> > self.__binds[key] = bind
>> >
>> >
>> >
>> >
>> > class RoutingQuery(Query):
>> > def __init__(self, *args, **kwargs):
>> > super(RoutingQuery, self).__init__(*args, **kwargs)
>> > self._shard_id = TenantIDStorage.get_shard_id()
>> >
>> > def get(self, ident):
>> > self._check_bound_to_shard()
>> > return super(CachingQuery, self).get(ident)
>> >
>> > def _check_bound_to_shard(self):
>> > if self._shard_id is None:
>> > raise ValueError('query not bound to any shard')
>> >
>> > def _execute_and_instances(self, querycontext):
>> > self._check_bound_to_shard()
>> > querycontext.attributes['shard_id'] = self._shard_id
>> > result = self._connection_from_session(
>> > mapper=self._mapper_zero(),
>> > shard_id=self._shard_id).execute(
>> > querycontext.statement,
>> > self._params
>> > )
>> > return self.instances(result, querycontext)
>> >
>> >
>> >
>> >
>> >
>> >
>> > Sometimes I got this error:
>> >
>> > File "/home/anton/Projects/proj/admin/proj/admin/views/stats.py", line
>> > 898, in _get_filters_from_request
>> > control_groups = [g.id for g in
>> > User.get_own_groups_query(current_user.id)]
>> > File
>> >
>> > "/home/anton/Projects/proj/.venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py",
>> > line 2802, in __iter__
>> > return self._execute_and_instances(context)
>> > File
>> >
>> > "/home/anton/Projects/proj/core/proj/core/orm_extensions/rouing_session.py",
>> > line 105, in _execute_and_instances
>> > shard_id=self._shard_id).execute(
>> > File
>> >
>> > "/home/anton/Projects/proj/.venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py",
>> > line 2806, in _connection_from_session
>> > **kw)
>> > File
>> >
>> > "/home/anton/Projects/proj/.venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py",
>> > line 984, in connection
>> > bind = self.get_bind(mapper, clause=clause, **kw)
>> > File
>> >
>> > "/home/anton/Projects/proj/core/proj/core/orm_extensions/cachingquery.py",
>> > line 279, in get_bind
>> > original_bind = super(RoutingSession, self).get_bind(mapper, clause)
>> > File
>> >
>> > "/home/anton/Projects/proj/.venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py",
>> > line 1336, in get_bind
>> > if mapper and mapper.mapped_table.bind:
>> > File
>> >
>> > "/home/anton/Projects/proj/.venv/lib/python2.7/site-packages/sqlalchemy/sql/elements.py",
>> > line 539, in __bool__
>> > raise TypeError("Boolean value of this clause is not defined")
>> > TypeError: Boolean value of this clause is not defined
>> >
>> >
>> >
>> >
>> > I found that `mapper` in original `get_bind` always checks with `is not
>> > None` condition. There is only one place where condition omitted: `if
>> > mapper
>> > and mapper.mapped_table.bind...`.
>> > Possibly it is not correct? Or my code does not do something important?
>>
>> Are you able to catch this error in a debugger?
>>
>> I'm going to make a wild guess that "mapper" at this point is some
>> sort of SQL construct (such as a column object or mapped property),
>> rather than an actual mapper.
>>
>> You don't say which version of SQLAlchemy you are using, but since
>> you've overridden _execute_and_instances, I wonder if you need to use
>> something like query._bind_mapper() as used by _get_bind_args:
>>
>>
>> <https://bitbucket.org/zzzeek/sqlalchemy/src/4b22b01ade6f94cc4db2f2040c802067a25d29a4/lib/sqlalchemy/orm/query.py?at=rel_1_1_14&fileviewer=file-view-default#query.py-2872>
>>
>> Hope that helps,
>>
>> Simon
>
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> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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http://www.sqlalchemy.org/
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