class Variant(TypeDecorator):
A wrapping type that selects among a variety of implementations based on dialect in use.
The .Variant
type is typically constructed
using the .TypeEngine.with_variant
method.
See Also
.TypeEngine.with_variant
for an example of use.
Method | __init__ |
Construct a new .Variant . |
Method | _set_parent |
Support SchemaEventTarget |
Method | _set_parent_with_dispatch |
Support SchemaEventTarget |
Method | coerce_compared_value |
Suggest a type for a 'coerced' Python value in an expression. |
Method | load_dialect_impl |
Return a .TypeEngine object corresponding to a dialect. |
Method | with_variant |
Return a new .Variant which adds the given type + dialect name to the mapping, in addition to the mapping present in this .Variant . |
Class Variable | cache_ok |
Indicate if statements using this .ExternalType are "safe to cache". |
Instance Variable | impl |
Undocumented |
Instance Variable | mapping |
Undocumented |
Property | _static_cache_key |
Undocumented |
Property | comparator_factory |
express comparison behavior in terms of the base type |
Inherited from TypeDecorator
:
Class | Comparator |
A .TypeEngine.Comparator that is specific to .TypeDecorator . |
Method | bind_expression |
Given a bind value (i.e. a .BindParameter instance), return a SQL expression which will typically wrap the given parameter. |
Method | bind_processor |
Provide a bound value processing function for the given .Dialect . |
Method | column_expression |
Given a SELECT column expression, return a wrapping SQL expression. |
Method | compare_values |
Given two values, compare them for equality. |
Method | copy |
Produce a copy of this .TypeDecorator instance. |
Method | get_dbapi_type |
Return the DBAPI type object represented by this .TypeDecorator . |
Method | literal_processor |
Provide a literal processing function for the given .Dialect . |
Method | process_bind_param |
Receive a bound parameter value to be converted. |
Method | process_literal_param |
Receive a literal parameter value to be rendered inline within a statement. |
Method | process_result_value |
Receive a result-row column value to be converted. |
Method | result_processor |
Provide a result value processing function for the given .Dialect . |
Method | type_engine |
Return a dialect-specific .TypeEngine instance for this .TypeDecorator . |
Class Variable | coerce_to_is_types |
Specify those Python types which should be coerced at the expression level to "IS <constant>" when compared using == (and same for IS NOT in conjunction with !=). |
Method | __getattr__ |
Proxy all other undefined accessors to the underlying implementation. |
Method | __repr__ |
Undocumented |
Method | _gen_dialect_impl |
#todo |
Method | _unwrapped_dialect_impl |
Return the 'unwrapped' dialect impl for this type. |
Class Variable | __visit_name__ |
Undocumented |
Class Variable | _is_type_decorator |
Undocumented |
Property | _has_bind_expression |
memoized boolean, check if bind_expression is implemented. |
Property | _has_bind_processor |
memoized boolean, check if process_bind_param is implemented. |
Property | _has_column_expression |
memoized boolean, check if column_expression is implemented. |
Property | _has_literal_processor |
memoized boolean, check if process_literal_param is implemented. |
Property | _has_result_processor |
memoized boolean, check if process_result_value is implemented. |
Property | _type_affinity |
#todo |
Property | sort_key_function |
A sorting function that can be passed as the key to sorted. |
Inherited from TypeEngine
(via TypeDecorator
):
Method | adapt |
Produce an "adapted" form of this type, given an "impl" class to work with. |
Method | as_generic |
Return an instance of the generic type corresponding to this type using heuristic rule. The method may be overridden if this heuristic rule is not sufficient. |
Method | compare_against_backend |
Compare this type against the given backend type. |
Method | compile |
Produce a string-compiled form of this .TypeEngine . |
Method | dialect_impl |
Return a dialect-specific implementation for this .TypeEngine . |
Method | evaluates_none |
Return a copy of this type which has the .should_evaluate_none flag set to True. |
Class Variable | hashable |
Flag, if False, means values from this type aren't hashable. |
Class Variable | should_evaluate_none |
If True, the Python constant None is considered to be handled explicitly by this type. |
Static Method | _to_instance |
Undocumented |
Method | __str__ |
Undocumented |
Method | _cached_bind_processor |
Return a dialect-specific bind processor for this type. |
Method | _cached_custom_processor |
Undocumented |
Method | _cached_literal_processor |
Return a dialect-specific literal processor for this type. |
Method | _cached_result_processor |
Return a dialect-specific result processor for this type. |
Method | _compare_type_affinity |
Undocumented |
Method | _default_dialect |
Undocumented |
Method | _dialect_info |
Return a dialect-specific registry which caches a dialect-specific implementation, bind processing function, and one or more result processing functions. |
Method | _resolve_for_literal |
adjust this type given a literal Python value that will be stored in a bound parameter. |
Method | copy_value |
Undocumented |
Class Variable | _is_array |
Undocumented |
Class Variable | _is_table_value |
Undocumented |
Class Variable | _is_tuple_type |
Undocumented |
Class Variable | _isnull |
Undocumented |
Class Variable | _sqla_type |
Undocumented |
Property | _generic_type_affinity |
Undocumented |
Property | python_type |
Return the Python type object expected to be returned by instances of this type, if known. |
Inherited from Traversible
(via TypeDecorator
, TypeEngine
):
Method | get_children |
Return immediate child .visitors.Traversible elements of this .visitors.Traversible . |
Method | __class_getitem__ |
Undocumented |
sqlalchemy.types.TypeDecorator.__init__
.Variant
.Parameters | |
base | the base 'fallback' type |
mapping | dictionary of string dialect names to
.TypeEngine instances. |
Suggest a type for a 'coerced' Python value in an expression.
By default, returns self. This method is called by the expression system when an object using this type is on the left or right side of an expression against a plain Python object which does not yet have a SQLAlchemy type assigned:
expr = table.c.somecolumn + 35
Where above, if somecolumn uses this type, this method will be called with the value operator.add and 35. The return value is whatever SQLAlchemy type should be used for 35 for this particular operation.
Return a .TypeEngine
object corresponding to a dialect.
This is an end-user override hook that can be used to provide
differing types depending on the given dialect. It is used
by the .TypeDecorator
implementation of type_engine
to help determine what type should ultimately be returned
for a given .TypeDecorator
.
By default returns self.impl.
sqlalchemy.types.TypeEngine.with_variant
.Variant
which adds the given
type + dialect name to the mapping, in addition to the
mapping present in this .Variant
.Parameters | |
type_ | a .TypeEngine that will be selected
as a variant from the originating type, when a dialect
of the given name is in use. |
dialect_name | base name of the dialect which uses this type. (i.e. 'postgresql', 'mysql', etc.) |
bool
=
sqlalchemy.types.ExternalType.cache_ok
Indicate if statements using this .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
.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))))
.TypeDecorator
classes..ExternalType
mixin which
generalizes the cache_ok flag to both the .TypeDecorator
and .UserDefinedType
classes.See Also