Base Type API

Base Type API

Object Name	Description
`Concatenable`	A mixin that marks a type as supporting ‘concatenation’, typically strings.
`Indexable`	A mixin that marks a type as supporting indexing operations, such as array or JSON structures.
`NullType`	An unknown type.
`TypeEngine`	The ultimate base class for all SQL datatypes.
`Variant`	A wrapping type that selects among a variety of implementations based on dialect in use.

class sqlalchemy.types.``TypeEngine

The ultimate base class for all SQL datatypes.

Common subclasses of TypeEngine include String, Integer, and Boolean.

For an overview of the SQLAlchemy typing system, see Column and Data Types.

See also

Column and Data Types

Class signature

class sqlalchemy.types.TypeEngine (sqlalchemy.sql.visitors.Traversible)

class Comparator(expr)

Base class for custom comparison operations defined at the type level. See TypeEngine.comparator_factory.

Class signature

class sqlalchemy.types.TypeEngine.Comparator (sqlalchemy.sql.expression.ColumnOperators)
- attribute sqlalchemy.types.TypeEngine.Comparator.default_comparator = None
- method sqlalchemy.types.TypeEngine.Comparator.operate(op, \other, **kwargs*)
  
  Operate on an argument.
  
  This is the lowest level of operation, raises NotImplementedError by default.
  
  Overriding this on a subclass can allow common behavior to be applied to all operations. For example, overriding ColumnOperators to apply func.lower() to the left and right side:
```
class MyComparator(ColumnOperators):
    def operate(self, op, other):
        return op(func.lower(self), func.lower(other))
```
  Parameters
  - op – Operator callable.
  - *other – the ‘other’ side of the operation. Will be a single scalar for most operations.
  - **kwargs – modifiers. These may be passed by special operators such as ColumnOperators.contains().
- method sqlalchemy.types.TypeEngine.Comparator.reverse_operate(op, other, \*kwargs*)
  
  Reverse operate on an argument.
  
  Usage is the same as operate().
method sqlalchemy.types.TypeEngine.adapt(cls, \*kw*)

Produce an “adapted” form of this type, given an “impl” class to work with.

This method is used internally to associate generic types with “implementation” types that are specific to a particular dialect.
method sqlalchemy.types.TypeEngine.as_generic(allow_nulltype=False)

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.
```
>>> from sqlalchemy.dialects.mysql import INTEGER
>>> INTEGER(display_width=4).as_generic()
Integer()
```
```
>>> from sqlalchemy.dialects.mysql import NVARCHAR
>>> NVARCHAR(length=100).as_generic()
Unicode(length=100)
```
New in version 1.4.0b2.

See also

Reflecting with Database-Agnostic Types - describes the use of TypeEngine.as_generic() in conjunction with the DDLEvents.column_reflect() event, which is its intended use.
method sqlalchemy.types.TypeEngine.bind_expression(bindvalue)

Given a bind value (i.e. a BindParameter instance), return a SQL expression in its place.

This is typically a SQL function that wraps the existing bound parameter within the statement. It is used for special data types that require literals being wrapped in some special database function in order to coerce an application-level value into a database-specific format. It is the SQL analogue of the TypeEngine.bind_processor() method.

The method is evaluated at statement compile time, as opposed to statement construction time.

Note that this method, when implemented, should always return the exact same structure, without any conditional logic, as it may be used in an executemany() call against an arbitrary number of bound parameter sets.

See also

Applying SQL-level Bind/Result Processing
method sqlalchemy.types.TypeEngine.bind_processor(dialect)

Return a conversion function for processing bind values.

Returns a callable which will receive a bind parameter value as the sole positional argument and will return a value to send to the DB-API.

If processing is not necessary, the method should return None.
- Parameters
  
  dialect – Dialect instance in use.
method sqlalchemy.types.TypeEngine.coerce_compared_value(op, value)

Suggest a type for a ‘coerced’ Python value in an expression.

Given an operator and value, gives the type a chance to return a type which the value should be coerced into.

The default behavior here is conservative; if the right-hand side is already coerced into a SQL type based on its Python type, it is usually left alone.

End-user functionality extension here should generally be via TypeDecorator, which provides more liberal behavior in that it defaults to coercing the other side of the expression into this type, thus applying special Python conversions above and beyond those needed by the DBAPI to both ides. It also provides the public method TypeDecorator.coerce_compared_value() which is intended for end-user customization of this behavior.
method sqlalchemy.types.TypeEngine.column_expression(colexpr)

Given a SELECT column expression, return a wrapping SQL expression.

This is typically a SQL function that wraps a column expression as rendered in the columns clause of a SELECT statement. It is used for special data types that require columns to be wrapped in some special database function in order to coerce the value before being sent back to the application. It is the SQL analogue of the TypeEngine.result_processor() method.

The method is evaluated at statement compile time, as opposed to statement construction time.

See also

Applying SQL-level Bind/Result Processing
attribute sqlalchemy.types.TypeEngine.comparator_factory

A Comparator class which will apply to operations performed by owning ColumnElement objects.

The comparator_factory attribute is a hook consulted by the core expression system when column and SQL expression operations are performed. When a Comparator class is associated with this attribute, it allows custom re-definition of all existing operators, as well as definition of new operators. Existing operators include those provided by Python operator overloading such as ColumnOperators.__add__() and ColumnOperators.__eq__(), those provided as standard attributes of ColumnOperators such as ColumnOperators.like() and ColumnOperators.in_().

Rudimentary usage of this hook is allowed through simple subclassing of existing types, or alternatively by using TypeDecorator. See the documentation section Redefining and Creating New Operators for examples.

Class signature

class sqlalchemy.types.TypeEngine.Comparator (sqlalchemy.sql.expression.ColumnOperators)

alias of sqlalchemy.sql.type_api.TypeEngine.Comparator
method sqlalchemy.types.TypeEngine.compare_against_backend(dialect, conn_type)

Compare this type against the given backend type.

This function is currently not implemented for SQLAlchemy types, and for all built in types will return None. However, it can be implemented by a user-defined type where it can be consumed by schema comparison tools such as Alembic autogenerate.

A future release of SQLAlchemy will potentially implement this method for builtin types as well.

The function should return True if this type is equivalent to the given type; the type is typically reflected from the database so should be database specific. The dialect in use is also passed. It can also return False to assert that the type is not equivalent.
- Parameters
  - dialect – a Dialect that is involved in the comparison.
  - conn_type – the type object reflected from the backend.

New in version 1.0.3.

method sqlalchemy.types.TypeEngine.compare_values(x, y)

Compare two values for equality.
method sqlalchemy.types.TypeEngine.compile(dialect=None)

Produce a string-compiled form of this TypeEngine.

When called with no arguments, uses a “default” dialect to produce a string result.
- Parameters
  
  dialect – a Dialect instance.
method sqlalchemy.types.TypeEngine.dialect_impl(dialect)

Return a dialect-specific implementation for this TypeEngine.
method sqlalchemy.types.TypeEngine.evaluates_none()

Return a copy of this type which has the should_evaluate_none flag set to True.

E.g.:
```
Table(
    'some_table', metadata,
    Column(
        String(50).evaluates_none(),
        nullable=True,
        server_default='no value')
)
```
The ORM uses this flag to indicate that a positive value of None is passed to the column in an INSERT statement, rather than omitting the column from the INSERT statement which has the effect of firing off column-level defaults. It also allows for types which have special behavior associated with the Python None value to indicate that the value doesn’t necessarily translate into SQL NULL; a prime example of this is a JSON type which may wish to persist the JSON value 'null'.

In all cases, the actual NULL SQL value can be always be persisted in any column by using the null SQL construct in an INSERT statement or associated with an ORM-mapped attribute.

Note

The “evaluates none” flag does not apply to a value of None passed to Column.default or Column.server_default; in these cases, None still means “no default”.

New in version 1.1.

See also

Forcing NULL on a column with a default - in the ORM documentation

JSON.none_as_null - PostgreSQL JSON interaction with this flag.

TypeEngine.should_evaluate_none - class-level flag
method sqlalchemy.types.TypeEngine.get_dbapi_type(dbapi)

Return the corresponding type object from the underlying DB-API, if any.

This can be useful for calling setinputsizes(), for example.
attribute sqlalchemy.types.TypeEngine.hashable = True

Flag, if False, means values from this type aren’t hashable.

Used by the ORM when uniquing result lists.
method sqlalchemy.types.TypeEngine.literal_processor(dialect)

Return a conversion function for processing literal values that are to be rendered directly without using binds.

This function is used when the compiler makes use of the “literal_binds” flag, typically used in DDL generation as well as in certain scenarios where backends don’t accept bound parameters.

New in version 0.9.0.
attribute sqlalchemy.types.TypeEngine.python_type

Return the Python type object expected to be returned by instances of this type, if known.

Basically, for those types which enforce a return type, or are known across the board to do such for all common DBAPIs (like int for example), will return that type.

If a return type is not defined, raises NotImplementedError.

Note that any type also accommodates NULL in SQL which means you can also get back None from any type in practice.
method sqlalchemy.types.TypeEngine.result_processor(dialect, coltype)

Return a conversion function for processing result row values.

Returns a callable which will receive a result row column value as the sole positional argument and will return a value to return to the user.

If processing is not necessary, the method should return None.
- Parameters
  - dialect – Dialect instance in use.
  - coltype – DBAPI coltype argument received in cursor.description.

attribute sqlalchemy.types.TypeEngine.should_evaluate_none = False

If True, the Python constant None is considered to be handled explicitly by this type.

The ORM uses this flag to indicate that a positive value of None is passed to the column in an INSERT statement, rather than omitting the column from the INSERT statement which has the effect of firing off column-level defaults. It also allows types which have special behavior for Python None, such as a JSON type, to indicate that they’d like to handle the None value explicitly.

To set this flag on an existing type, use the TypeEngine.evaluates_none() method.

See also

TypeEngine.evaluates_none()

New in version 1.1.
attribute sqlalchemy.types.TypeEngine.sort_key_function = None

A sorting function that can be passed as the key to sorted.

The default value of None indicates that the values stored by this type are self-sorting.

New in version 1.3.8.
method sqlalchemy.types.TypeEngine.with_variant(type_, dialect_name)

Produce a new type object that will utilize the given type when applied to the dialect of the given name.

e.g.:
```
from sqlalchemy.types import String
from sqlalchemy.dialects import mysql
s = String()
s = s.with_variant(mysql.VARCHAR(collation='foo'), 'mysql')
```
The construction of TypeEngine.with_variant() is always from the “fallback” type to that which is dialect specific. The returned type is an instance of Variant, which itself provides a Variant.with_variant() that can be called repeatedly.
- 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.)

class sqlalchemy.types.``Concatenable

A mixin that marks a type as supporting ‘concatenation’, typically strings.

class Comparator(expr)

Class signature

class sqlalchemy.types.Concatenable.Comparator (sqlalchemy.types.Comparator)
attribute sqlalchemy.types.Concatenable.comparator_factory

alias of sqlalchemy.sql.sqltypes.Concatenable.Comparator

class sqlalchemy.types.``Indexable

A mixin that marks a type as supporting indexing operations, such as array or JSON structures.

New in version 1.1.0.

class Comparator(expr)

Class signature

class sqlalchemy.types.Indexable.Comparator (sqlalchemy.types.Comparator)
attribute sqlalchemy.types.Indexable.comparator_factory

alias of sqlalchemy.sql.sqltypes.Indexable.Comparator

class sqlalchemy.types.``NullType

An unknown type.

NullType is used as a default type for those cases where a type cannot be determined, including:

During table reflection, when the type of a column is not recognized by the Dialect
When constructing SQL expressions using plain Python objects of unknown types (e.g. somecolumn == my_special_object)
When a new Column is created, and the given type is passed as None or is not passed at all.

The NullType can be used within SQL expression invocation without issue, it just has no behavior either at the expression construction level or at the bind-parameter/result processing level. NullType will result in a CompileError if the compiler is asked to render the type itself, such as if it is used in a cast() operation or within a schema creation operation such as that invoked by MetaData.create_all() or the CreateTable construct.

Class signature

class sqlalchemy.types.NullType (sqlalchemy.types.TypeEngine)

class sqlalchemy.types.``Variant(base, mapping)

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.