Release: 1.1.0b2 | Release Date: July 1, 2016

SQLAlchemy 1.1 Documentation

Custom Types

A variety of methods exist to redefine the behavior of existing types as well as to provide new ones.

Overriding Type Compilation

A frequent need is to force the “string” version of a type, that is the one rendered in a CREATE TABLE statement or other SQL function like CAST, to be changed. For example, an application may want to force the rendering of BINARY for all platforms except for one, in which is wants BLOB to be rendered. Usage of an existing generic type, in this case LargeBinary, is preferred for most use cases. But to control types more accurately, a compilation directive that is per-dialect can be associated with any type:

from sqlalchemy.ext.compiler import compiles
from sqlalchemy.types import BINARY

@compiles(BINARY, "sqlite")
def compile_binary_sqlite(type_, compiler, **kw):
    return "BLOB"

The above code allows the usage of types.BINARY, which will produce the string BINARY against all backends except SQLite, in which case it will produce BLOB.

See the section Changing Compilation of Types, a subsection of Custom SQL Constructs and Compilation Extension, for additional examples.

Augmenting Existing Types

The TypeDecorator allows the creation of custom types which add bind-parameter and result-processing behavior to an existing type object. It is used when additional in-Python marshaling of data to and from the database is required.


The bind- and result-processing of TypeDecorator is in addition to the processing already performed by the hosted type, which is customized by SQLAlchemy on a per-DBAPI basis to perform processing specific to that DBAPI. To change the DBAPI-level processing for an existing type, see the section Replacing the Bind/Result Processing of Existing Types.

class sqlalchemy.types.TypeDecorator(*args, **kwargs)

Bases: sqlalchemy.sql.expression.SchemaEventTarget, sqlalchemy.types.TypeEngine

Allows the creation of types which add additional functionality to an existing type.

This method is preferred to direct subclassing of SQLAlchemy’s built-in types as it ensures that all required functionality of the underlying type is kept in place.

Typical usage:

import sqlalchemy.types as types

class MyType(types.TypeDecorator):
    '''Prefixes Unicode values with "PREFIX:" on the way in and
    strips it off on the way out.

    impl = types.Unicode

    def process_bind_param(self, value, dialect):
        return "PREFIX:" + value

    def process_result_value(self, value, dialect):
        return value[7:]

    def copy(self, **kw):
        return MyType(self.impl.length)

The class-level “impl” attribute is required, and can reference any TypeEngine class. Alternatively, the load_dialect_impl() method can be used to provide different type classes based on the dialect given; in this case, the “impl” variable can reference TypeEngine as a placeholder.

Types that receive a Python type that isn’t similar to the ultimate type used may want to define the TypeDecorator.coerce_compared_value() method. This is used to give the expression system a hint when coercing Python objects into bind parameters within expressions. Consider this expression:

mytable.c.somecol +, 5, 15)

Above, if “somecol” is an Integer variant, it makes sense that we’re doing date arithmetic, where above is usually interpreted by databases as adding a number of days to the given date. The expression system does the right thing by not attempting to coerce the “date()” value into an integer-oriented bind parameter.

However, in the case of TypeDecorator, we are usually changing an incoming Python type to something new - TypeDecorator by default will “coerce” the non-typed side to be the same type as itself. Such as below, we define an “epoch” type that stores a date value as an integer:

class MyEpochType(types.TypeDecorator):
    impl = types.Integer

    epoch =, 1, 1)

    def process_bind_param(self, value, dialect):
        return (value - self.epoch).days

    def process_result_value(self, value, dialect):
        return self.epoch + timedelta(days=value)

Our expression of somecol + date with the above type will coerce the “date” on the right side to also be treated as MyEpochType.

This behavior can be overridden via the coerce_compared_value() method, which returns a type that should be used for the value of the expression. Below we set it such that an integer value will be treated as an Integer, and any other value is assumed to be a date and will be treated as a MyEpochType:

def coerce_compared_value(self, op, value):
    if isinstance(value, int):
        return Integer()
        return self


Note that the behavior of coerce_compared_value is not inherited by default from that of the base type. If the TypeDecorator is augmenting a type that requires special logic for certain types of operators, this method must be overridden. A key example is when decorating the postgresql.JSON and postgresql.JSONB types; the default rules of TypeEngine.coerce_compared_value() should be used in order to deal with operators like index operations:

class MyJsonType(TypeDecorator):
    impl = postgresql.JSON

    def coerce_compared_value(self, op, value):
        return self.impl.coerce_compared_value(op, value)

Without the above step, index operations such as mycol['foo'] will cause the index value 'foo' to be JSON encoded.

__init__(*args, **kwargs)

Construct a TypeDecorator.

Arguments sent here are passed to the constructor of the class assigned to the impl class level attribute, assuming the impl is a callable, and the resulting object is assigned to the self.impl instance attribute (thus overriding the class attribute of the same name).

If the class level impl is not a callable (the unusual case), it will be assigned to the same instance attribute ‘as-is’, ignoring those arguments passed to the constructor.

Subclasses can override this to customize the generation of self.impl entirely.

adapt(cls, **kw)
inherited from the adapt() method of TypeEngine

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.

inherited from the bind_expression() method of TypeEngine

“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


Provide a bound value processing function for the given Dialect.

This is the method that fulfills the TypeEngine contract for bound value conversion. TypeDecorator will wrap a user-defined implementation of process_bind_param() here.

User-defined code can override this method directly, though its likely best to use process_bind_param() so that the processing provided by self.impl is maintained.

Parameters:dialect – Dialect instance in use.

This method is the reverse counterpart to the result_processor() method of this class.

coerce_compared_value(op, value)

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.

coerce_to_is_types = (<type 'NoneType'>,)

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 !=.

For most SQLAlchemy types, this includes NoneType, as well as bool.

TypeDecorator modifies this list to only include NoneType, as typedecorator implementations that deal with boolean types are common.

Custom TypeDecorator classes can override this attribute to return an empty tuple, in which case no values will be coerced to constants.

..versionadded:: 0.8.2
Added TypeDecorator.coerce_to_is_types to allow for easier control of __eq__() __ne__() operations.
inherited from the column_expression() method of TypeEngine

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

compare_against_backend(dialect, conn_type)
inherited from the compare_against_backend() method of TypeEngine

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.

compare_values(x, y)

Given two values, compare them for equality.

By default this calls upon TypeEngine.compare_values() of the underlying “impl”, which in turn usually uses the Python equals operator ==.

This function is used by the ORM to compare an original-loaded value with an intercepted “changed” value, to determine if a net change has occurred.

inherited from the compile() method of TypeEngine

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.

Produce a copy of this TypeDecorator instance.

This is a shallow copy and is provided to fulfill part of the TypeEngine contract. It usually does not need to be overridden unless the user-defined TypeDecorator has local state that should be deep-copied.

inherited from the dialect_impl() method of TypeEngine

Return a dialect-specific implementation for this TypeEngine.

inherited from the evaluates_none() method of TypeEngine

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


    'some_table', metadata,
        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.

New in version 1.1.

See also

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

postgresql.JSON.none_as_null - Postgresql JSON interaction with this flag.

TypeEngine.should_evaluate_none - class-level flag


Return the DBAPI type object represented by this TypeDecorator.

By default this calls upon TypeEngine.get_dbapi_type() of the underlying “impl”.


Provide a literal processing function for the given Dialect.

Subclasses here will typically override TypeDecorator.process_literal_param() instead of this method directly.

By default, this method makes use of TypeDecorator.process_bind_param() if that method is implemented, where TypeDecorator.process_literal_param() is not. The rationale here is that TypeDecorator typically deals with Python conversions of data that are above the layer of database presentation. With the value converted by TypeDecorator.process_bind_param(), the underlying type will then handle whether it needs to be presented to the DBAPI as a bound parameter or to the database as an inline SQL value.

New in version 0.9.0.


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.

process_bind_param(value, dialect)

Receive a bound parameter value to be converted.

Subclasses override this method to return the value that should be passed along to the underlying TypeEngine object, and from there to the DBAPI execute() method.

The operation could be anything desired to perform custom behavior, such as transforming or serializing data. This could also be used as a hook for validating logic.

This operation should be designed with the reverse operation in mind, which would be the process_result_value method of this class.

  • value – Data to operate upon, of any type expected by this method in the subclass. Can be None.
  • dialect – the Dialect in use.
process_literal_param(value, dialect)

Receive a literal parameter value to be rendered inline within a statement.

This method is used when the compiler renders a literal value without using binds, typically within DDL such as in the “server default” of a column or an expression within a CHECK constraint.

The returned string will be rendered into the output string.

New in version 0.9.0.

process_result_value(value, dialect)

Receive a result-row column value to be converted.

Subclasses should implement this method to operate on data fetched from the database.

Subclasses override this method to return the value that should be passed back to the application, given a value that is already processed by the underlying TypeEngine object, originally from the DBAPI cursor method fetchone() or similar.

The operation could be anything desired to perform custom behavior, such as transforming or serializing data. This could also be used as a hook for validating logic.

  • value – Data to operate upon, of any type expected by this method in the subclass. Can be None.
  • dialect – the Dialect in use.

This operation should be designed to be reversible by the “process_bind_param” method of this class.

inherited from the python_type attribute of TypeEngine

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.

result_processor(dialect, coltype)

Provide a result value processing function for the given Dialect.

This is the method that fulfills the TypeEngine contract for result value conversion. TypeDecorator will wrap a user-defined implementation of process_result_value() here.

User-defined code can override this method directly, though its likely best to use process_result_value() so that the processing provided by self.impl is maintained.

  • dialect – Dialect instance in use.
  • coltype – A SQLAlchemy data type

This method is the reverse counterpart to the bind_processor() method of this class.


Return a dialect-specific TypeEngine instance for this TypeDecorator.

In most cases this returns a dialect-adapted form of the TypeEngine type represented by self.impl. Makes usage of dialect_impl() but also traverses into wrapped TypeDecorator instances. Behavior can be customized here by overriding load_dialect_impl().

with_variant(type_, dialect_name)
inherited from the with_variant() method of TypeEngine

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.

TypeDecorator Recipes

A few key TypeDecorator recipes follow.

Coercing Encoded Strings to Unicode

A common source of confusion regarding the Unicode type is that it is intended to deal only with Python unicode objects on the Python side, meaning values passed to it as bind parameters must be of the form u'some string' if using Python 2 and not 3. The encoding/decoding functions it performs are only to suit what the DBAPI in use requires, and are primarily a private implementation detail.

The use case of a type that can safely receive Python bytestrings, that is strings that contain non-ASCII characters and are not u'' objects in Python 2, can be achieved using a TypeDecorator which coerces as needed:

from sqlalchemy.types import TypeDecorator, Unicode

class CoerceUTF8(TypeDecorator):
    """Safely coerce Python bytestrings to Unicode
    before passing off to the database."""

    impl = Unicode

    def process_bind_param(self, value, dialect):
        if isinstance(value, str):
            value = value.decode('utf-8')
        return value

Rounding Numerics

Some database connectors like those of SQL Server choke if a Decimal is passed with too many decimal places. Here’s a recipe that rounds them down:

from sqlalchemy.types import TypeDecorator, Numeric
from decimal import Decimal

class SafeNumeric(TypeDecorator):
    """Adds quantization to Numeric."""

    impl = Numeric

    def __init__(self, *arg, **kw):
        TypeDecorator.__init__(self, *arg, **kw)
        self.quantize_int = - self.impl.scale
        self.quantize = Decimal(10) ** self.quantize_int

    def process_bind_param(self, value, dialect):
        if isinstance(value, Decimal) and \
            value.as_tuple()[2] < self.quantize_int:
            value = value.quantize(self.quantize)
        return value

Backend-agnostic GUID Type

Receives and returns Python uuid() objects. Uses the PG UUID type when using Postgresql, CHAR(32) on other backends, storing them in stringified hex format. Can be modified to store binary in CHAR(16) if desired:

from sqlalchemy.types import TypeDecorator, CHAR
from sqlalchemy.dialects.postgresql import UUID
import uuid

class GUID(TypeDecorator):
    """Platform-independent GUID type.

    Uses Postgresql's UUID type, otherwise uses
    CHAR(32), storing as stringified hex values.

    impl = CHAR

    def load_dialect_impl(self, dialect):
        if == 'postgresql':
            return dialect.type_descriptor(UUID())
            return dialect.type_descriptor(CHAR(32))

    def process_bind_param(self, value, dialect):
        if value is None:
            return value
        elif == 'postgresql':
            return str(value)
            if not isinstance(value, uuid.UUID):
                return "%.32x" % uuid.UUID(value).int
                # hexstring
                return "%.32x" %

    def process_result_value(self, value, dialect):
        if value is None:
            return value
            return uuid.UUID(value)

Marshal JSON Strings

This type uses simplejson to marshal Python data structures to/from JSON. Can be modified to use Python’s builtin json encoder:

from sqlalchemy.types import TypeDecorator, VARCHAR
import json

class JSONEncodedDict(TypeDecorator):
    """Represents an immutable structure as a json-encoded string.




    impl = VARCHAR

    def process_bind_param(self, value, dialect):
        if value is not None:
            value = json.dumps(value)

        return value

    def process_result_value(self, value, dialect):
        if value is not None:
            value = json.loads(value)
        return value

Note that the ORM by default will not detect “mutability” on such a type - meaning, in-place changes to values will not be detected and will not be flushed. Without further steps, you instead would need to replace the existing value with a new one on each parent object to detect changes. Note that there’s nothing wrong with this, as many applications may not require that the values are ever mutated once created. For those which do have this requirement, support for mutability is best applied using the sqlalchemy.ext.mutable extension - see the example in Mutation Tracking.

Replacing the Bind/Result Processing of Existing Types

Most augmentation of type behavior at the bind/result level is achieved using TypeDecorator. For the rare scenario where the specific processing applied by SQLAlchemy at the DBAPI level needs to be replaced, the SQLAlchemy type can be subclassed directly, and the bind_processor() or result_processor() methods can be overridden. Doing so requires that the adapt() method also be overridden. This method is the mechanism by which SQLAlchemy produces DBAPI-specific type behavior during statement execution. Overriding it allows a copy of the custom type to be used in lieu of a DBAPI-specific type. Below we subclass the types.TIME type to have custom result processing behavior. The process() function will receive value from the DBAPI cursor directly:

class MySpecialTime(TIME):
    def __init__(self, special_argument):
        super(MySpecialTime, self).__init__()
        self.special_argument = special_argument

    def result_processor(self, dialect, coltype):
        import datetime
        time = datetime.time
        def process(value):
            if value is not None:
                microseconds = value.microseconds
                seconds = value.seconds
                minutes = seconds / 60
                return time(
                          minutes / 60,
                          minutes % 60,
                          seconds - minutes * 60,
                return None
        return process

    def adapt(self, impltype):
        return MySpecialTime(self.special_argument)

Applying SQL-level Bind/Result Processing

As seen in the sections Augmenting Existing Types and Replacing the Bind/Result Processing of Existing Types, SQLAlchemy allows Python functions to be invoked both when parameters are sent to a statement, as well as when result rows are loaded from the database, to apply transformations to the values as they are sent to or from the database. It is also possible to define SQL-level transformations as well. The rationale here is when only the relational database contains a particular series of functions that are necessary to coerce incoming and outgoing data between an application and persistence format. Examples include using database-defined encryption/decryption functions, as well as stored procedures that handle geographic data. The Postgis extension to Postgresql includes an extensive array of SQL functions that are necessary for coercing data into particular formats.

Any TypeEngine, UserDefinedType or TypeDecorator subclass can include implementations of TypeEngine.bind_expression() and/or TypeEngine.column_expression(), which when defined to return a non-None value should return a ColumnElement expression to be injected into the SQL statement, either surrounding bound parameters or a column expression. For example, to build a Geometry type which will apply the Postgis function ST_GeomFromText to all outgoing values and the function ST_AsText to all incoming data, we can create our own subclass of UserDefinedType which provides these methods in conjunction with func:

from sqlalchemy import func
from sqlalchemy.types import UserDefinedType

class Geometry(UserDefinedType):
    def get_col_spec(self):
        return "GEOMETRY"

    def bind_expression(self, bindvalue):
        return func.ST_GeomFromText(bindvalue, type_=self)

    def column_expression(self, col):
        return func.ST_AsText(col, type_=self)

We can apply the Geometry type into Table metadata and use it in a select() construct:

geometry = Table('geometry', metadata,
              Column('geom_id', Integer, primary_key=True),
              Column('geom_data', Geometry)

  geometry.c.geom_data == 'LINESTRING(189412 252431,189631 259122)'))

The resulting SQL embeds both functions as appropriate. ST_AsText is applied to the columns clause so that the return value is run through the function before passing into a result set, and ST_GeomFromText is run on the bound parameter so that the passed-in value is converted:

SELECT geometry.geom_id, ST_AsText(geometry.geom_data) AS geom_data_1
FROM geometry
WHERE geometry.geom_data = ST_GeomFromText(:geom_data_2)

The TypeEngine.column_expression() method interacts with the mechanics of the compiler such that the SQL expression does not interfere with the labeling of the wrapped expression. Such as, if we rendered a select() against a label() of our expression, the string label is moved to the outside of the wrapped expression:



SELECT ST_AsText(geometry.geom_data) AS my_data
FROM geometry

For an example of subclassing a built in type directly, we subclass postgresql.BYTEA to provide a PGPString, which will make use of the Postgresql pgcrypto extension to encrpyt/decrypt values transparently:

from sqlalchemy import create_engine, String, select, func, \
        MetaData, Table, Column, type_coerce

from sqlalchemy.dialects.postgresql import BYTEA

class PGPString(BYTEA):
    def __init__(self, passphrase, length=None):
        super(PGPString, self).__init__(length)
        self.passphrase = passphrase

    def bind_expression(self, bindvalue):
        # convert the bind's type from PGPString to
        # String, so that it's passed to psycopg2 as is without
        # a dbapi.Binary wrapper
        bindvalue = type_coerce(bindvalue, String)
        return func.pgp_sym_encrypt(bindvalue, self.passphrase)

    def column_expression(self, col):
        return func.pgp_sym_decrypt(col, self.passphrase)

metadata = MetaData()
message = Table('message', metadata,
                Column('username', String(50)),
                    PGPString("this is my passphrase", length=1000)),

engine = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
with engine.begin() as conn:

    conn.execute(message.insert(), username="some user",
                                message="this is my message")

                where(message.c.username == "some user")

The pgp_sym_encrypt and pgp_sym_decrypt functions are applied to the INSERT and SELECT statements:

INSERT INTO message (username, message)
  VALUES (%(username)s, pgp_sym_encrypt(%(message)s, %(pgp_sym_encrypt_1)s))
  {'username': 'some user', 'message': 'this is my message',
    'pgp_sym_encrypt_1': 'this is my passphrase'}

SELECT pgp_sym_decrypt(message.message, %(pgp_sym_decrypt_1)s) AS message_1
  FROM message
  WHERE message.username = %(username_1)s
  {'pgp_sym_decrypt_1': 'this is my passphrase', 'username_1': 'some user'}

New in version 0.8: Added the TypeEngine.bind_expression() and TypeEngine.column_expression() methods.

See also:

PostGIS Integration

Redefining and Creating New Operators

SQLAlchemy Core defines a fixed set of expression operators available to all column expressions. Some of these operations have the effect of overloading Python’s built in operators; examples of such operators include ColumnOperators.__eq__() (table.c.somecolumn == 'foo'), ColumnOperators.__invert__() (~table.c.flag), and ColumnOperators.__add__() (table.c.x + table.c.y). Other operators are exposed as explicit methods on column expressions, such as ColumnOperators.in_() (table.c.value.in_(['x', 'y'])) and ('%ed%')).

The Core expression constructs in all cases consult the type of the expression in order to determine the behavior of existing operators, as well as to locate additional operators that aren’t part of the built in set. The TypeEngine base class defines a root “comparison” implementation TypeEngine.Comparator, and many specific types provide their own sub-implementations of this class. User-defined TypeEngine.Comparator implementations can be built directly into a simple subclass of a particular type in order to override or define new operations. Below, we create a Integer subclass which overrides the ColumnOperators.__add__() operator:

from sqlalchemy import Integer

class MyInt(Integer):
    class comparator_factory(Integer.Comparator):
        def __add__(self, other):
            return self.op("goofy")(other)

The above configuration creates a new class MyInt, which establishes the TypeEngine.comparator_factory attribute as referring to a new class, subclassing the TypeEngine.Comparator class associated with the Integer type.


>>> sometable = Table("sometable", metadata, Column("data", MyInt))
>>> print( + 5) goofy :data_1

The implementation for ColumnOperators.__add__() is consulted by an owning SQL expression, by instantiating the TypeEngine.Comparator with itself as the expr attribute. The mechanics of the expression system are such that operations continue recursively until an expression object produces a new SQL expression construct. Above, we could just as well have said self.expr.op("goofy")(other) instead of self.op("goofy")(other).

New methods added to a TypeEngine.Comparator are exposed on an owning SQL expression using a __getattr__ scheme, which exposes methods added to TypeEngine.Comparator onto the owning ColumnElement. For example, to add a log() function to integers:

from sqlalchemy import Integer, func

class MyInt(Integer):
    class comparator_factory(Integer.Comparator):
        def log(self, other):
            return func.log(self.expr, other)

Using the above type:

>>> print(
log(:log_1, :log_2)

Unary operations are also possible. For example, to add an implementation of the Postgresql factorial operator, we combine the UnaryExpression construct along with a custom_op to produce the factorial expression:

from sqlalchemy import Integer
from sqlalchemy.sql.expression import UnaryExpression
from sqlalchemy.sql import operators

class MyInteger(Integer):
    class comparator_factory(Integer.Comparator):
        def factorial(self):
            return UnaryExpression(self.expr,

Using the above type:

>>> from sqlalchemy.sql import column
>>> print(column('x', MyInteger).factorial())
x !

See also:


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

Creating New Types

The UserDefinedType class is provided as a simple base class for defining entirely new database types. Use this to represent native database types not known by SQLAlchemy. If only Python translation behavior is needed, use TypeDecorator instead.

class sqlalchemy.types.UserDefinedType

Bases: sqlalchemy.types.TypeEngine

Base for user defined types.

This should be the base of new types. Note that for most cases, TypeDecorator is probably more appropriate:

import sqlalchemy.types as types

class MyType(types.UserDefinedType):
    def __init__(self, precision = 8):
        self.precision = precision

    def get_col_spec(self, **kw):
        return "MYTYPE(%s)" % self.precision

    def bind_processor(self, dialect):
        def process(value):
            return value
        return process

    def result_processor(self, dialect, coltype):
        def process(value):
            return value
        return process

Once the type is made, it’s immediately usable:

table = Table('foo', meta,
    Column('id', Integer, primary_key=True),
    Column('data', MyType(16))

The get_col_spec() method will in most cases receive a keyword argument type_expression which refers to the owning expression of the type as being compiled, such as a Column or cast() construct. This keyword is only sent if the method accepts keyword arguments (e.g. **kw) in its argument signature; introspection is used to check for this in order to support legacy forms of this function.

New in version 1.0.0: the owning expression is passed to the get_col_spec() method via the keyword argument type_expression, if it receives **kw in its signature.

coerce_compared_value(op, value)

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

Default behavior for UserDefinedType is the same as that of TypeDecorator; by default it returns self, assuming the compared value should be coerced into the same type as this one. See TypeDecorator.coerce_compared_value() for more detail.

Changed in version 0.8: UserDefinedType.coerce_compared_value() now returns self by default, rather than falling onto the more fundamental behavior of TypeEngine.coerce_compared_value().