most (although some are missing) of the examples illustrate it such as
https://docs.sqlalchemy.org/en/14/core/custom_types.html#store-timezone-aware-timestamps-as-timezone-naive-utc
class TZDateTime(TypeDecorator):
impl = DateTime
cache_ok = True
def process_bind_param(self, value, dialect):
if value is not None:
if not value.tzinfo:
raise TypeError("tzinfo is required")
value = value.astimezone(datetime.timezone.utc).replace(
tzinfo=None
)
return value
def process_result_value(self, value, dialect):
if value is not None:
value = value.replace(tzinfo=datetime.timezone.utc)
return value
On Wed, May 26, 2021, at 5:38 PM, Jinghui Niu wrote:
> Thank you. Could you please also give some hint on a case where we must set
> this attribute to `False`?
>
> On Wed, May 26, 2021 at 5:54 AM Mike Bayer <[email protected]> wrote:
>> __
>>
>>
>> On Wed, May 26, 2021, at 3:07 AM, [email protected]
>> <mailto:niuji...%40gmail.com> wrote:
>>> I have consistently receiving the warning:
>>> will not produce a cache key because the ``cache_ok`` flag is not set to
>>> True. Set this flag to True if this type object's state is safe to use in
>>> a cache key, or False to disable this warning.
>>>
>>>
>>> After reading the documentation, I learned that the "cache_ok" class-level
>>> attribute can be set to either True or False. But the documentation is very
>>> abstract on when to use it? What is cache key? Is it for loading and
>>> caching a set of objects from the database? Or caching the query itself?
>>
>> the caching system is used to cache the generated SQL and related parameter
>> and result-handling structures for a statement object and is described at:
>>
>> https://docs.sqlalchemy.org/en/14/core/connections.html#sql-compilation-caching
>>
>> it is always safe to set cache_ok=False on your TypeDecorator class, you
>> will however lose the benefit of SQL statement caching for statements which
>> include use of this datatype.
>>
>>
>>>
>>> If my TypeDecorator class doesn't even have a __init__method, just
>>> "process_bind_param" and "process_result_value" two methods, do I even need
>>> to bother with this "cache_ok" setting at all?
>>
>> If your TypeDecorator is stateless, then it's fine to set cache_ok=True.
>>
>>
>>>
>>> --
>>> SQLAlchemy -
>>> The Python SQL Toolkit and Object Relational Mapper
>>>
>>> http://www.sqlalchemy.org/
>>>
>>> To post example code, please provide an MCVE: Minimal, Complete, and
>>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>>> description.
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>>
>>
>> --
>> SQLAlchemy -
>> The Python SQL Toolkit and Object Relational Mapper
>>
>> http://www.sqlalchemy.org/
>>
>> To post example code, please provide an MCVE: Minimal, Complete, and
>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
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>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
> ---
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>
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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