Release: 1.0.17 | Release Date: January 17, 2017

SQLAlchemy 1.0 Documentation

Base Type API

class sqlalchemy.types.TypeEngine

Bases: sqlalchemy.sql.visitors.Visitable

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.

class Comparator(expr)

Bases: sqlalchemy.sql.operators.ColumnOperators

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

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.


“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


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.
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.


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


Bases: sqlalchemy.sql.operators.ColumnOperators

A TypeEngine.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 TypeEngine.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 operators.ColumnOperators.__add__() and operators.ColumnOperators.__eq__(), those provided as standard attributes of operators.ColumnOperators such as and operators.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.

New in version 0.8: The expression system was enhanced to support customization of operators on a per-type level.

alias of Comparator

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 impement 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.

  • dialect – a Dialect that is involved in the comparison.
  • conn_type – the type object reflected from the backend.

New in version 1.0.3.

TypeEngine.compare_values(x, y)

Compare two values for equality.


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.

Return a dialect-specific implementation for this TypeEngine.


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

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

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

Used by the ORM when uniquing result lists.


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.


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.

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.

  • dialect – Dialect instance in use.
  • coltype – DBAPI coltype argument received in cursor.description.
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.


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.

  • 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.)

New in version 0.7.2.

class sqlalchemy.types.Concatenable

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

inherited from the __init__ attribute of object

x.__init__(...) initializes x; see help(type(x)) for signature

class sqlalchemy.types.NullType

Bases: sqlalchemy.types.TypeEngine

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 sqlalchemy.types.Variant(base, mapping)

Bases: sqlalchemy.types.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.

New in version 0.7.2.

See also

TypeEngine.with_variant() for an example of use.

Members:with_variant, __init__
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