sqlalchemy.sql.type_api.ExternalType

Indicate if statements using this :class:`.ExternalType` are "safe to cache". The default value ``None`` will emit a warning and then not allow caching of a statement which includes this type. Set to ``False`` to disable statements using this type from being cached at all without a warning. When set to ``True``, the object's class and selected elements from its state will be used as part of the cache key. For example, using a :class:`.TypeDecorator`:: class MyType(TypeDecorator): impl = String cache_ok = True def __init__(self, choices): self.choices = tuple(choices) self.internal_only = True The cache key for the above type would be equivalent to:: >>> MyType(["a", "b", "c"])._static_cache_key (<class '__main__.MyType'>, ('choices', ('a', 'b', 'c'))) The caching scheme will extract attributes from the type that correspond to the names of parameters in the ``__init__()`` method. Above, the "choices" attribute becomes part of the cache key but "internal_only" does not, because there is no parameter named "internal_only". The requirements for cacheable elements is that they are hashable and also that they indicate the same SQL rendered for expressions using this type every time for a given cache value. To accommodate for datatypes that refer to unhashable structures such as dictionaries, sets and lists, these objects can be made "cacheable" by assigning hashable structures to the attributes whose names correspond with the names of the arguments. For example, a datatype which accepts a dictionary of lookup values may publish this as a sorted series of tuples. Given a previously un-cacheable type as:: class LookupType(UserDefinedType): '''a custom type that accepts a dictionary as a parameter. this is the non-cacheable version, as "self.lookup" is not hashable. ''' def __init__(self, lookup): self.lookup = lookup def get_col_spec(self, **kw): return "VARCHAR(255)" def bind_processor(self, dialect): # ... works with "self.lookup" ... Where "lookup" is a dictionary. The type will not be able to generate a cache key:: >>> type_ = LookupType({"a": 10, "b": 20}) >>> type_._static_cache_key <stdin>:1: SAWarning: UserDefinedType LookupType({'a': 10, 'b': 20}) 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. symbol('no_cache') If we **did** set up such a cache key, it wouldn't be usable. We would get a tuple structure that contains a dictionary inside of it, which cannot itself be used as a key in a "cache dictionary" such as SQLAlchemy's statement cache, since Python dictionaries aren't hashable:: >>> # set cache_ok = True >>> type_.cache_ok = True >>> # this is the cache key it would generate >>> key = type_._static_cache_key >>> key (<class '__main__.LookupType'>, ('lookup', {'a': 10, 'b': 20})) >>> # however this key is not hashable, will fail when used with >>> # SQLAlchemy statement cache >>> some_cache = {key: "some sql value"} Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict' The type may be made cacheable by assigning a sorted tuple of tuples to the ".lookup" attribute:: class LookupType(UserDefinedType): '''a custom type that accepts a dictionary as a parameter. The dictionary is stored both as itself in a private variable, and published in a public variable as a sorted tuple of tuples, which is hashable and will also return the same value for any two equivalent dictionaries. Note it assumes the keys and values of the dictionary are themselves hashable. ''' cache_ok = True def __init__(self, lookup): self._lookup = lookup # assume keys/values of "lookup" are hashable; otherwise # they would also need to be converted in some way here self.lookup = tuple( (key, lookup[key]) for key in sorted(lookup) ) def get_col_spec(self, **kw): return "VARCHAR(255)" def bind_processor(self, dialect): # ... works with "self._lookup" ... Where above, the cache key for ``LookupType({"a": 10, "b": 20})`` will be:: >>> LookupType({"a": 10, "b": 20})._static_cache_key (<class '__main__.LookupType'>, ('lookup', (('a', 10), ('b', 20)))) .. versionadded:: 1.4.14 - added the ``cache_ok`` flag to allow some configurability of caching for :class:`.TypeDecorator` classes. .. versionadded:: 1.4.28 - added the :class:`.ExternalType` mixin which generalizes the ``cache_ok`` flag to both the :class:`.TypeDecorator` and :class:`.UserDefinedType` classes. .. seealso:: :ref:`sql_caching`